1//===- JumpThreading.cpp - Thread control through conditional blocks ------===//
2//
3// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
4// See https://llvm.org/LICENSE.txt for license information.
5// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
6//
7//===----------------------------------------------------------------------===//
8//
9// This file implements the Jump Threading pass.
10//
11//===----------------------------------------------------------------------===//
12
13#include "llvm/Transforms/Scalar/JumpThreading.h"
14#include "llvm/ADT/DenseMap.h"
15#include "llvm/ADT/MapVector.h"
16#include "llvm/ADT/STLExtras.h"
17#include "llvm/ADT/ScopeExit.h"
18#include "llvm/ADT/SmallPtrSet.h"
19#include "llvm/ADT/SmallVector.h"
20#include "llvm/ADT/Statistic.h"
21#include "llvm/Analysis/AliasAnalysis.h"
22#include "llvm/Analysis/BlockFrequencyInfo.h"
23#include "llvm/Analysis/BranchProbabilityInfo.h"
24#include "llvm/Analysis/CFG.h"
25#include "llvm/Analysis/ConstantFolding.h"
26#include "llvm/Analysis/GlobalsModRef.h"
27#include "llvm/Analysis/GuardUtils.h"
28#include "llvm/Analysis/InstructionSimplify.h"
29#include "llvm/Analysis/LazyValueInfo.h"
30#include "llvm/Analysis/Loads.h"
31#include "llvm/Analysis/LoopInfo.h"
32#include "llvm/Analysis/MemoryLocation.h"
33#include "llvm/Analysis/PostDominators.h"
34#include "llvm/Analysis/TargetLibraryInfo.h"
35#include "llvm/Analysis/TargetTransformInfo.h"
36#include "llvm/Analysis/ValueTracking.h"
37#include "llvm/IR/BasicBlock.h"
38#include "llvm/IR/CFG.h"
39#include "llvm/IR/Constant.h"
40#include "llvm/IR/ConstantRange.h"
41#include "llvm/IR/Constants.h"
42#include "llvm/IR/DataLayout.h"
43#include "llvm/IR/DebugInfo.h"
44#include "llvm/IR/Dominators.h"
45#include "llvm/IR/Function.h"
46#include "llvm/IR/InstrTypes.h"
47#include "llvm/IR/Instruction.h"
48#include "llvm/IR/Instructions.h"
49#include "llvm/IR/IntrinsicInst.h"
50#include "llvm/IR/Intrinsics.h"
51#include "llvm/IR/LLVMContext.h"
52#include "llvm/IR/MDBuilder.h"
53#include "llvm/IR/Metadata.h"
54#include "llvm/IR/Module.h"
55#include "llvm/IR/PassManager.h"
56#include "llvm/IR/PatternMatch.h"
57#include "llvm/IR/ProfDataUtils.h"
58#include "llvm/IR/Type.h"
59#include "llvm/IR/Use.h"
60#include "llvm/IR/Value.h"
61#include "llvm/Support/BlockFrequency.h"
62#include "llvm/Support/BranchProbability.h"
63#include "llvm/Support/Casting.h"
64#include "llvm/Support/CommandLine.h"
65#include "llvm/Support/Debug.h"
66#include "llvm/Support/raw_ostream.h"
67#include "llvm/Transforms/Utils/BasicBlockUtils.h"
68#include "llvm/Transforms/Utils/Cloning.h"
69#include "llvm/Transforms/Utils/Local.h"
70#include "llvm/Transforms/Utils/SSAUpdater.h"
71#include "llvm/Transforms/Utils/ValueMapper.h"
72#include <cassert>
73#include <cstdint>
74#include <iterator>
75#include <memory>
76#include <utility>
77
78using namespace llvm;
79using namespace jumpthreading;
80
81#define DEBUG_TYPE "jump-threading"
82
83STATISTIC(NumThreads, "Number of jumps threaded");
84STATISTIC(NumFolds, "Number of terminators folded");
85STATISTIC(NumDupes, "Number of branch blocks duplicated to eliminate phi");
86
87static cl::opt<unsigned>
88BBDuplicateThreshold("jump-threading-threshold",
89 cl::desc("Max block size to duplicate for jump threading"),
90 cl::init(Val: 6), cl::Hidden);
91
92static cl::opt<unsigned>
93ImplicationSearchThreshold(
94 "jump-threading-implication-search-threshold",
95 cl::desc("The number of predecessors to search for a stronger "
96 "condition to use to thread over a weaker condition"),
97 cl::init(Val: 3), cl::Hidden);
98
99static cl::opt<unsigned> PhiDuplicateThreshold(
100 "jump-threading-phi-threshold",
101 cl::desc("Max PHIs in BB to duplicate for jump threading"), cl::init(Val: 76),
102 cl::Hidden);
103
104static cl::opt<bool> ThreadAcrossLoopHeaders(
105 "jump-threading-across-loop-headers",
106 cl::desc("Allow JumpThreading to thread across loop headers, for testing"),
107 cl::init(Val: false), cl::Hidden);
108
109namespace llvm {
110extern cl::opt<bool> ProfcheckDisableMetadataFixes;
111}
112
113JumpThreadingPass::JumpThreadingPass(int T) {
114 DefaultBBDupThreshold = (T == -1) ? BBDuplicateThreshold : unsigned(T);
115}
116
117// Update branch probability information according to conditional
118// branch probability. This is usually made possible for cloned branches
119// in inline instances by the context specific profile in the caller.
120// For instance,
121//
122// [Block PredBB]
123// [Branch PredBr]
124// if (t) {
125// Block A;
126// } else {
127// Block B;
128// }
129//
130// [Block BB]
131// cond = PN([true, %A], [..., %B]); // PHI node
132// [Branch CondBr]
133// if (cond) {
134// ... // P(cond == true) = 1%
135// }
136//
137// Here we know that when block A is taken, cond must be true, which means
138// P(cond == true | A) = 1
139//
140// Given that P(cond == true) = P(cond == true | A) * P(A) +
141// P(cond == true | B) * P(B)
142// we get:
143// P(cond == true ) = P(A) + P(cond == true | B) * P(B)
144//
145// which gives us:
146// P(A) is less than P(cond == true), i.e.
147// P(t == true) <= P(cond == true)
148//
149// In other words, if we know P(cond == true) is unlikely, we know
150// that P(t == true) is also unlikely.
151//
152static void updatePredecessorProfileMetadata(PHINode *PN, BasicBlock *BB) {
153 CondBrInst *CondBr = dyn_cast<CondBrInst>(Val: BB->getTerminator());
154 if (!CondBr)
155 return;
156
157 uint64_t TrueWeight, FalseWeight;
158 if (!extractBranchWeights(I: *CondBr, TrueVal&: TrueWeight, FalseVal&: FalseWeight))
159 return;
160
161 if (TrueWeight + FalseWeight == 0)
162 // Zero branch_weights do not give a hint for getting branch probabilities.
163 // Technically it would result in division by zero denominator, which is
164 // TrueWeight + FalseWeight.
165 return;
166
167 // Returns the outgoing edge of the dominating predecessor block
168 // that leads to the PhiNode's incoming block:
169 auto GetPredOutEdge =
170 [](BasicBlock *IncomingBB,
171 BasicBlock *PhiBB) -> std::pair<BasicBlock *, BasicBlock *> {
172 auto *PredBB = IncomingBB;
173 auto *SuccBB = PhiBB;
174 SmallPtrSet<BasicBlock *, 16> Visited;
175 while (true) {
176 if (isa<CondBrInst>(Val: PredBB->getTerminator()))
177 return {PredBB, SuccBB};
178 Visited.insert(Ptr: PredBB);
179 auto *SinglePredBB = PredBB->getSinglePredecessor();
180 if (!SinglePredBB)
181 return {nullptr, nullptr};
182
183 // Stop searching when SinglePredBB has been visited. It means we see
184 // an unreachable loop.
185 if (Visited.count(Ptr: SinglePredBB))
186 return {nullptr, nullptr};
187
188 SuccBB = PredBB;
189 PredBB = SinglePredBB;
190 }
191 };
192
193 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
194 Value *PhiOpnd = PN->getIncomingValue(i);
195 ConstantInt *CI = dyn_cast<ConstantInt>(Val: PhiOpnd);
196
197 if (!CI || !CI->getType()->isIntegerTy(Bitwidth: 1))
198 continue;
199
200 BranchProbability BP =
201 (CI->isOne() ? BranchProbability::getBranchProbability(
202 Numerator: TrueWeight, Denominator: TrueWeight + FalseWeight)
203 : BranchProbability::getBranchProbability(
204 Numerator: FalseWeight, Denominator: TrueWeight + FalseWeight));
205
206 auto PredOutEdge = GetPredOutEdge(PN->getIncomingBlock(i), BB);
207 if (!PredOutEdge.first)
208 return;
209
210 BasicBlock *PredBB = PredOutEdge.first;
211 CondBrInst *PredBr = dyn_cast<CondBrInst>(Val: PredBB->getTerminator());
212 if (!PredBr)
213 return;
214
215 uint64_t PredTrueWeight, PredFalseWeight;
216 // FIXME: We currently only set the profile data when it is missing.
217 // With PGO, this can be used to refine even existing profile data with
218 // context information. This needs to be done after more performance
219 // testing.
220 if (extractBranchWeights(I: *PredBr, TrueVal&: PredTrueWeight, FalseVal&: PredFalseWeight))
221 continue;
222
223 // We can not infer anything useful when BP >= 50%, because BP is the
224 // upper bound probability value.
225 if (BP >= BranchProbability(50, 100))
226 continue;
227
228 uint32_t Weights[2];
229 if (PredBr->getSuccessor(i: 0) == PredOutEdge.second) {
230 Weights[0] = BP.getNumerator();
231 Weights[1] = BP.getCompl().getNumerator();
232 } else {
233 Weights[0] = BP.getCompl().getNumerator();
234 Weights[1] = BP.getNumerator();
235 }
236 setBranchWeights(I&: *PredBr, Weights, IsExpected: hasBranchWeightOrigin(I: *PredBr));
237 }
238}
239
240PreservedAnalyses JumpThreadingPass::run(Function &F,
241 FunctionAnalysisManager &AM) {
242 auto &TTI = AM.getResult<TargetIRAnalysis>(IR&: F);
243 // Jump Threading has no sense for the targets with divergent CF
244 if (TTI.hasBranchDivergence(F: &F))
245 return PreservedAnalyses::all();
246 auto &TLI = AM.getResult<TargetLibraryAnalysis>(IR&: F);
247 auto &LVI = AM.getResult<LazyValueAnalysis>(IR&: F);
248 auto &AA = AM.getResult<AAManager>(IR&: F);
249 auto &DT = AM.getResult<DominatorTreeAnalysis>(IR&: F);
250
251 bool Changed =
252 runImpl(F, FAM: &AM, TLI: &TLI, TTI: &TTI, LVI: &LVI, AA: &AA,
253 DTU: std::make_unique<DomTreeUpdater>(
254 args: &DT, args: nullptr, args: DomTreeUpdater::UpdateStrategy::Lazy),
255 BFI: nullptr, BPI: nullptr);
256
257 if (!Changed)
258 return PreservedAnalyses::all();
259
260
261 getDomTreeUpdater()->flush();
262
263#if defined(EXPENSIVE_CHECKS)
264 assert(getDomTreeUpdater()->getDomTree().verify(
265 DominatorTree::VerificationLevel::Full) &&
266 "DT broken after JumpThreading");
267 assert((!getDomTreeUpdater()->hasPostDomTree() ||
268 getDomTreeUpdater()->getPostDomTree().verify(
269 PostDominatorTree::VerificationLevel::Full)) &&
270 "PDT broken after JumpThreading");
271#else
272 assert(getDomTreeUpdater()->getDomTree().verify(
273 DominatorTree::VerificationLevel::Fast) &&
274 "DT broken after JumpThreading");
275 assert((!getDomTreeUpdater()->hasPostDomTree() ||
276 getDomTreeUpdater()->getPostDomTree().verify(
277 PostDominatorTree::VerificationLevel::Fast)) &&
278 "PDT broken after JumpThreading");
279#endif
280
281 return getPreservedAnalysis();
282}
283
284bool JumpThreadingPass::runImpl(Function &F_, FunctionAnalysisManager *FAM_,
285 TargetLibraryInfo *TLI_,
286 TargetTransformInfo *TTI_, LazyValueInfo *LVI_,
287 AliasAnalysis *AA_,
288 std::unique_ptr<DomTreeUpdater> DTU_,
289 BlockFrequencyInfo *BFI_,
290 BranchProbabilityInfo *BPI_) {
291 LLVM_DEBUG(dbgs() << "Jump threading on function '" << F_.getName() << "'\n");
292 F = &F_;
293 FAM = FAM_;
294 TLI = TLI_;
295 TTI = TTI_;
296 LVI = LVI_;
297 AA = AA_;
298 DTU = std::move(DTU_);
299 BFI = BFI_;
300 BPI = BPI_;
301 auto *GuardDecl = Intrinsic::getDeclarationIfExists(
302 M: F->getParent(), id: Intrinsic::experimental_guard);
303 HasGuards = GuardDecl && !GuardDecl->use_empty();
304
305 // Reduce the number of instructions duplicated when optimizing strictly for
306 // size.
307 if (BBDuplicateThreshold.getNumOccurrences())
308 BBDupThreshold = BBDuplicateThreshold;
309 else if (F->hasMinSize())
310 BBDupThreshold = 3;
311 else
312 BBDupThreshold = DefaultBBDupThreshold;
313
314 assert(DTU && "DTU isn't passed into JumpThreading before using it.");
315 assert(DTU->hasDomTree() && "JumpThreading relies on DomTree to proceed.");
316 DominatorTree &DT = DTU->getDomTree();
317
318 Unreachable.clear();
319 for (auto &BB : *F)
320 if (!DT.isReachableFromEntry(A: &BB))
321 Unreachable.insert(Ptr: &BB);
322
323 if (!ThreadAcrossLoopHeaders)
324 findLoopHeaders(F&: *F);
325
326 bool EverChanged = false;
327 bool Changed;
328 do {
329 Changed = false;
330 for (auto &BB : *F) {
331 if (Unreachable.count(Ptr: &BB))
332 continue;
333 while (processBlock(BB: &BB)) // Thread all of the branches we can over BB.
334 Changed = ChangedSinceLastAnalysisUpdate = true;
335
336 // Stop processing BB if it's the entry or is now deleted. The following
337 // routines attempt to eliminate BB and locating a suitable replacement
338 // for the entry is non-trivial.
339 if (&BB == &F->getEntryBlock() || DTU->isBBPendingDeletion(DelBB: &BB))
340 continue;
341
342 if (pred_empty(BB: &BB)) {
343 // When processBlock makes BB unreachable it doesn't bother to fix up
344 // the instructions in it. We must remove BB to prevent invalid IR.
345 LLVM_DEBUG(dbgs() << " JT: Deleting dead block '" << BB.getName()
346 << "' with terminator: " << *BB.getTerminator()
347 << '\n');
348 LoopHeaders.erase(Ptr: &BB);
349 LVI->eraseBlock(BB: &BB);
350 DeleteDeadBlock(BB: &BB, DTU: DTU.get());
351 Changed = ChangedSinceLastAnalysisUpdate = true;
352 continue;
353 }
354
355 // processBlock doesn't thread BBs with unconditional TIs. However, if BB
356 // is "almost empty", we attempt to merge BB with its sole successor.
357 if (auto *BI = dyn_cast<UncondBrInst>(Val: BB.getTerminator())) {
358 BasicBlock *Succ = BI->getSuccessor();
359 if (
360 // The terminator must be the only non-phi instruction in BB.
361 BB.getFirstNonPHIOrDbg(SkipPseudoOp: true)->isTerminator() &&
362 // Don't alter Loop headers and latches to ensure another pass can
363 // detect and transform nested loops later.
364 !LoopHeaders.count(Ptr: &BB) && !LoopHeaders.count(Ptr: Succ) &&
365 TryToSimplifyUncondBranchFromEmptyBlock(BB: &BB, DTU: DTU.get())) {
366 // BB is valid for cleanup here because we passed in DTU. F remains
367 // BB's parent until a DTU->getDomTree() event.
368 LVI->eraseBlock(BB: &BB);
369 Changed = ChangedSinceLastAnalysisUpdate = true;
370 }
371 }
372 }
373 EverChanged |= Changed;
374 } while (Changed);
375
376 // Jump threading may have introduced redundant debug values into F which
377 // should be removed.
378 if (EverChanged)
379 for (auto &BB : *F) {
380 RemoveRedundantDbgInstrs(BB: &BB);
381 }
382
383 LoopHeaders.clear();
384 return EverChanged;
385}
386
387// Replace uses of Cond with ToVal when safe to do so. If all uses are
388// replaced, we can remove Cond. We cannot blindly replace all uses of Cond
389// because we may incorrectly replace uses when guards/assumes are uses of
390// of `Cond` and we used the guards/assume to reason about the `Cond` value
391// at the end of block. RAUW unconditionally replaces all uses
392// including the guards/assumes themselves and the uses before the
393// guard/assume.
394static bool replaceFoldableUses(Instruction *Cond, Value *ToVal,
395 BasicBlock *KnownAtEndOfBB) {
396 bool Changed = false;
397 assert(Cond->getType() == ToVal->getType());
398 // We can unconditionally replace all uses in non-local blocks (i.e. uses
399 // strictly dominated by BB), since LVI information is true from the
400 // terminator of BB.
401 if (Cond->getParent() == KnownAtEndOfBB)
402 Changed |= replaceNonLocalUsesWith(From: Cond, To: ToVal);
403 for (Instruction &I : reverse(C&: *KnownAtEndOfBB)) {
404 // Replace any debug-info record users of Cond with ToVal.
405 for (DbgVariableRecord &DVR : filterDbgVars(R: I.getDbgRecordRange()))
406 DVR.replaceVariableLocationOp(OldValue: Cond, NewValue: ToVal, AllowEmpty: true);
407
408 // Reached the Cond whose uses we are trying to replace, so there are no
409 // more uses.
410 if (&I == Cond)
411 break;
412 // We only replace uses in instructions that are guaranteed to reach the end
413 // of BB, where we know Cond is ToVal.
414 if (!isGuaranteedToTransferExecutionToSuccessor(I: &I))
415 break;
416 Changed |= I.replaceUsesOfWith(From: Cond, To: ToVal);
417 }
418 if (Cond->use_empty() && !Cond->mayHaveSideEffects()) {
419 Cond->eraseFromParent();
420 Changed = true;
421 }
422 return Changed;
423}
424
425/// Return the cost of duplicating a piece of this block from first non-phi
426/// and before StopAt instruction to thread across it. Stop scanning the block
427/// when exceeding the threshold. If duplication is impossible, returns ~0U.
428static unsigned getJumpThreadDuplicationCost(const TargetTransformInfo *TTI,
429 BasicBlock *BB,
430 Instruction *StopAt,
431 unsigned Threshold) {
432 assert(StopAt->getParent() == BB && "Not an instruction from proper BB?");
433
434 // Do not duplicate the BB if it has a lot of PHI nodes.
435 // If a threadable chain is too long then the number of PHI nodes can add up,
436 // leading to a substantial increase in compile time when rewriting the SSA.
437 unsigned PhiCount = 0;
438 Instruction *FirstNonPHI = nullptr;
439 for (Instruction &I : *BB) {
440 if (!isa<PHINode>(Val: &I)) {
441 FirstNonPHI = &I;
442 break;
443 }
444 if (++PhiCount > PhiDuplicateThreshold)
445 return ~0U;
446 }
447
448 /// Ignore PHI nodes, these will be flattened when duplication happens.
449 BasicBlock::const_iterator I(FirstNonPHI);
450
451 // FIXME: THREADING will delete values that are just used to compute the
452 // branch, so they shouldn't count against the duplication cost.
453
454 unsigned Bonus = 0;
455 if (BB->getTerminator() == StopAt) {
456 // Threading through a switch statement is particularly profitable. If this
457 // block ends in a switch, decrease its cost to make it more likely to
458 // happen.
459 if (isa<SwitchInst>(Val: StopAt))
460 Bonus = 6;
461
462 // The same holds for indirect branches, but slightly more so.
463 if (isa<IndirectBrInst>(Val: StopAt))
464 Bonus = 8;
465 }
466
467 // Bump the threshold up so the early exit from the loop doesn't skip the
468 // terminator-based Size adjustment at the end.
469 Threshold += Bonus;
470
471 // Sum up the cost of each instruction until we get to the terminator. Don't
472 // include the terminator because the copy won't include it.
473 unsigned Size = 0;
474 for (; &*I != StopAt; ++I) {
475
476 // Stop scanning the block if we've reached the threshold.
477 if (Size > Threshold)
478 return Size;
479
480 // Bail out if this instruction gives back a token type, it is not possible
481 // to duplicate it if it is used outside this BB.
482 if (I->getType()->isTokenTy() && I->isUsedOutsideOfBlock(BB))
483 return ~0U;
484
485 // Blocks with NoDuplicate are modelled as having infinite cost, so they
486 // are never duplicated.
487 if (const CallInst *CI = dyn_cast<CallInst>(Val&: I))
488 if (CI->cannotDuplicate() || CI->isConvergent())
489 return ~0U;
490
491 if (TTI->getInstructionCost(U: &*I, CostKind: TargetTransformInfo::TCK_SizeAndLatency) ==
492 TargetTransformInfo::TCC_Free)
493 continue;
494
495 // All other instructions count for at least one unit.
496 ++Size;
497
498 // Calls are more expensive. If they are non-intrinsic calls, we model them
499 // as having cost of 4. If they are a non-vector intrinsic, we model them
500 // as having cost of 2 total, and if they are a vector intrinsic, we model
501 // them as having cost 1.
502 if (const CallInst *CI = dyn_cast<CallInst>(Val&: I)) {
503 if (!isa<IntrinsicInst>(Val: CI))
504 Size += 3;
505 else if (!CI->getType()->isVectorTy())
506 Size += 1;
507 }
508 }
509
510 return Size > Bonus ? Size - Bonus : 0;
511}
512
513/// findLoopHeaders - We do not want jump threading to turn proper loop
514/// structures into irreducible loops. Doing this breaks up the loop nesting
515/// hierarchy and pessimizes later transformations. To prevent this from
516/// happening, we first have to find the loop headers. Here we approximate this
517/// by finding targets of backedges in the CFG.
518///
519/// Note that there definitely are cases when we want to allow threading of
520/// edges across a loop header. For example, threading a jump from outside the
521/// loop (the preheader) to an exit block of the loop is definitely profitable.
522/// It is also almost always profitable to thread backedges from within the loop
523/// to exit blocks, and is often profitable to thread backedges to other blocks
524/// within the loop (forming a nested loop). This simple analysis is not rich
525/// enough to track all of these properties and keep it up-to-date as the CFG
526/// mutates, so we don't allow any of these transformations.
527void JumpThreadingPass::findLoopHeaders(Function &F) {
528 SmallVector<std::pair<const BasicBlock*,const BasicBlock*>, 32> Edges;
529 FindFunctionBackedges(F, Result&: Edges);
530 LoopHeaders.insert_range(R: llvm::make_second_range(c&: Edges));
531}
532
533/// getKnownConstant - Helper method to determine if we can thread over a
534/// terminator with the given value as its condition, and if so what value to
535/// use for that. What kind of value this is depends on whether we want an
536/// integer or a block address, but an undef is always accepted.
537/// Returns null if Val is null or not an appropriate constant.
538static Constant *getKnownConstant(Value *Val, ConstantPreference Preference) {
539 if (!Val)
540 return nullptr;
541
542 // Undef is "known" enough.
543 if (UndefValue *U = dyn_cast<UndefValue>(Val))
544 return U;
545
546 if (Preference == WantBlockAddress)
547 return dyn_cast<BlockAddress>(Val: Val->stripPointerCasts());
548
549 return dyn_cast<ConstantInt>(Val);
550}
551
552/// computeValueKnownInPredecessors - Given a basic block BB and a value V, see
553/// if we can infer that the value is a known ConstantInt/BlockAddress or undef
554/// in any of our predecessors. If so, return the known list of value and pred
555/// BB in the result vector.
556///
557/// This returns true if there were any known values.
558bool JumpThreadingPass::computeValueKnownInPredecessorsImpl(
559 Value *V, BasicBlock *BB, PredValueInfo &Result,
560 ConstantPreference Preference, SmallPtrSet<Value *, 4> &RecursionSet,
561 Instruction *CxtI) {
562 const DataLayout &DL = BB->getDataLayout();
563
564 // This method walks up use-def chains recursively. Because of this, we could
565 // get into an infinite loop going around loops in the use-def chain. To
566 // prevent this, keep track of what (value, block) pairs we've already visited
567 // and terminate the search if we loop back to them
568 if (!RecursionSet.insert(Ptr: V).second)
569 return false;
570
571 // If V is a constant, then it is known in all predecessors.
572 if (Constant *KC = getKnownConstant(Val: V, Preference)) {
573 for (BasicBlock *Pred : predecessors(BB))
574 Result.emplace_back(Args&: KC, Args&: Pred);
575
576 return !Result.empty();
577 }
578
579 // If V is a non-instruction value, or an instruction in a different block,
580 // then it can't be derived from a PHI.
581 Instruction *I = dyn_cast<Instruction>(Val: V);
582 if (!I || I->getParent() != BB) {
583
584 // Okay, if this is a live-in value, see if it has a known value at the any
585 // edge from our predecessors.
586 for (BasicBlock *P : predecessors(BB)) {
587 using namespace PatternMatch;
588 // If the value is known by LazyValueInfo to be a constant in a
589 // predecessor, use that information to try to thread this block.
590 Constant *PredCst = LVI->getConstantOnEdge(V, FromBB: P, ToBB: BB, CxtI);
591 // If I is a non-local compare-with-constant instruction, use more-rich
592 // 'getPredicateOnEdge' method. This would be able to handle value
593 // inequalities better, for example if the compare is "X < 4" and "X < 3"
594 // is known true but "X < 4" itself is not available.
595 CmpPredicate Pred;
596 Value *Val;
597 Constant *Cst;
598 if (!PredCst && match(V, P: m_Cmp(Pred, L: m_Value(V&: Val), R: m_Constant(C&: Cst))))
599 PredCst = LVI->getPredicateOnEdge(Pred, V: Val, C: Cst, FromBB: P, ToBB: BB, CxtI);
600 if (Constant *KC = getKnownConstant(Val: PredCst, Preference))
601 Result.emplace_back(Args&: KC, Args&: P);
602 }
603
604 return !Result.empty();
605 }
606
607 /// If I is a PHI node, then we know the incoming values for any constants.
608 if (PHINode *PN = dyn_cast<PHINode>(Val: I)) {
609 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
610 Value *InVal = PN->getIncomingValue(i);
611 if (Constant *KC = getKnownConstant(Val: InVal, Preference)) {
612 Result.emplace_back(Args&: KC, Args: PN->getIncomingBlock(i));
613 } else {
614 Constant *CI = LVI->getConstantOnEdge(V: InVal,
615 FromBB: PN->getIncomingBlock(i),
616 ToBB: BB, CxtI);
617 if (Constant *KC = getKnownConstant(Val: CI, Preference))
618 Result.emplace_back(Args&: KC, Args: PN->getIncomingBlock(i));
619 }
620 }
621
622 return !Result.empty();
623 }
624
625 // Handle Cast instructions.
626 if (CastInst *CI = dyn_cast<CastInst>(Val: I)) {
627 Value *Source = CI->getOperand(i_nocapture: 0);
628 PredValueInfoTy Vals;
629 computeValueKnownInPredecessorsImpl(V: Source, BB, Result&: Vals, Preference,
630 RecursionSet, CxtI);
631 if (Vals.empty())
632 return false;
633
634 // Convert the known values.
635 for (auto &Val : Vals)
636 if (Constant *Folded = ConstantFoldCastOperand(Opcode: CI->getOpcode(), C: Val.first,
637 DestTy: CI->getType(), DL))
638 Result.emplace_back(Args&: Folded, Args&: Val.second);
639
640 return !Result.empty();
641 }
642
643 if (FreezeInst *FI = dyn_cast<FreezeInst>(Val: I)) {
644 Value *Source = FI->getOperand(i_nocapture: 0);
645 computeValueKnownInPredecessorsImpl(V: Source, BB, Result, Preference,
646 RecursionSet, CxtI);
647
648 erase_if(C&: Result, P: [](auto &Pair) {
649 return !isGuaranteedNotToBeUndefOrPoison(Pair.first);
650 });
651
652 return !Result.empty();
653 }
654
655 // Handle some boolean conditions.
656 if (I->getType()->getPrimitiveSizeInBits() == 1) {
657 using namespace PatternMatch;
658 if (Preference != WantInteger)
659 return false;
660 // X | true -> true
661 // X & false -> false
662 Value *Op0, *Op1;
663 if (match(V: I, P: m_LogicalOr(L: m_Value(V&: Op0), R: m_Value(V&: Op1))) ||
664 match(V: I, P: m_LogicalAnd(L: m_Value(V&: Op0), R: m_Value(V&: Op1)))) {
665 PredValueInfoTy LHSVals, RHSVals;
666
667 computeValueKnownInPredecessorsImpl(V: Op0, BB, Result&: LHSVals, Preference: WantInteger,
668 RecursionSet, CxtI);
669 computeValueKnownInPredecessorsImpl(V: Op1, BB, Result&: RHSVals, Preference: WantInteger,
670 RecursionSet, CxtI);
671
672 if (LHSVals.empty() && RHSVals.empty())
673 return false;
674
675 ConstantInt *InterestingVal;
676 if (match(V: I, P: m_LogicalOr()))
677 InterestingVal = ConstantInt::getTrue(Context&: I->getContext());
678 else
679 InterestingVal = ConstantInt::getFalse(Context&: I->getContext());
680
681 SmallPtrSet<BasicBlock*, 4> LHSKnownBBs;
682
683 // Scan for the sentinel. If we find an undef, force it to the
684 // interesting value: x|undef -> true and x&undef -> false.
685 for (const auto &LHSVal : LHSVals)
686 if (LHSVal.first == InterestingVal || isa<UndefValue>(Val: LHSVal.first)) {
687 Result.emplace_back(Args&: InterestingVal, Args: LHSVal.second);
688 LHSKnownBBs.insert(Ptr: LHSVal.second);
689 }
690 for (const auto &RHSVal : RHSVals)
691 if (RHSVal.first == InterestingVal || isa<UndefValue>(Val: RHSVal.first)) {
692 // If we already inferred a value for this block on the LHS, don't
693 // re-add it.
694 if (!LHSKnownBBs.count(Ptr: RHSVal.second))
695 Result.emplace_back(Args&: InterestingVal, Args: RHSVal.second);
696 }
697
698 return !Result.empty();
699 }
700
701 // Handle the NOT form of XOR.
702 if (I->getOpcode() == Instruction::Xor &&
703 isa<ConstantInt>(Val: I->getOperand(i: 1)) &&
704 cast<ConstantInt>(Val: I->getOperand(i: 1))->isOne()) {
705 computeValueKnownInPredecessorsImpl(V: I->getOperand(i: 0), BB, Result,
706 Preference: WantInteger, RecursionSet, CxtI);
707 if (Result.empty())
708 return false;
709
710 // Invert the known values.
711 for (auto &R : Result)
712 R.first = ConstantExpr::getNot(C: R.first);
713
714 return true;
715 }
716
717 // Try to simplify some other binary operator values.
718 } else if (BinaryOperator *BO = dyn_cast<BinaryOperator>(Val: I)) {
719 if (Preference != WantInteger)
720 return false;
721 if (ConstantInt *CI = dyn_cast<ConstantInt>(Val: BO->getOperand(i_nocapture: 1))) {
722 PredValueInfoTy LHSVals;
723 computeValueKnownInPredecessorsImpl(V: BO->getOperand(i_nocapture: 0), BB, Result&: LHSVals,
724 Preference: WantInteger, RecursionSet, CxtI);
725
726 // Try to use constant folding to simplify the binary operator.
727 for (const auto &LHSVal : LHSVals) {
728 Constant *V = LHSVal.first;
729 Constant *Folded =
730 ConstantFoldBinaryOpOperands(Opcode: BO->getOpcode(), LHS: V, RHS: CI, DL);
731
732 if (Constant *KC = getKnownConstant(Val: Folded, Preference: WantInteger))
733 Result.emplace_back(Args&: KC, Args: LHSVal.second);
734 }
735 }
736
737 return !Result.empty();
738 }
739
740 // Handle compare with phi operand, where the PHI is defined in this block.
741 if (CmpInst *Cmp = dyn_cast<CmpInst>(Val: I)) {
742 if (Preference != WantInteger)
743 return false;
744 Type *CmpType = Cmp->getType();
745 Value *CmpLHS = Cmp->getOperand(i_nocapture: 0);
746 Value *CmpRHS = Cmp->getOperand(i_nocapture: 1);
747 CmpInst::Predicate Pred = Cmp->getPredicate();
748
749 PHINode *PN = dyn_cast<PHINode>(Val: CmpLHS);
750 if (!PN)
751 PN = dyn_cast<PHINode>(Val: CmpRHS);
752 // Do not perform phi translation across a loop header phi, because this
753 // may result in comparison of values from two different loop iterations.
754 // FIXME: This check is broken if LoopHeaders is not populated.
755 if (PN && PN->getParent() == BB && !LoopHeaders.contains(Ptr: BB)) {
756 const DataLayout &DL = PN->getDataLayout();
757 // We can do this simplification if any comparisons fold to true or false.
758 // See if any do.
759 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
760 BasicBlock *PredBB = PN->getIncomingBlock(i);
761 Value *LHS, *RHS;
762 if (PN == CmpLHS) {
763 LHS = PN->getIncomingValue(i);
764 RHS = CmpRHS->DoPHITranslation(CurBB: BB, PredBB);
765 } else {
766 LHS = CmpLHS->DoPHITranslation(CurBB: BB, PredBB);
767 RHS = PN->getIncomingValue(i);
768 }
769 Value *Res = simplifyCmpInst(Predicate: Pred, LHS, RHS, Q: {DL});
770 if (!Res) {
771 if (!isa<Constant>(Val: RHS))
772 continue;
773
774 // getPredicateOnEdge call will make no sense if LHS is defined in BB.
775 auto LHSInst = dyn_cast<Instruction>(Val: LHS);
776 if (LHSInst && LHSInst->getParent() == BB)
777 continue;
778
779 Res = LVI->getPredicateOnEdge(Pred, V: LHS, C: cast<Constant>(Val: RHS), FromBB: PredBB,
780 ToBB: BB, CxtI: CxtI ? CxtI : Cmp);
781 }
782
783 if (Constant *KC = getKnownConstant(Val: Res, Preference: WantInteger))
784 Result.emplace_back(Args&: KC, Args&: PredBB);
785 }
786
787 return !Result.empty();
788 }
789
790 // If comparing a live-in value against a constant, see if we know the
791 // live-in value on any predecessors.
792 if (isa<Constant>(Val: CmpRHS) && !CmpType->isVectorTy()) {
793 Constant *CmpConst = cast<Constant>(Val: CmpRHS);
794
795 if (!isa<Instruction>(Val: CmpLHS) ||
796 cast<Instruction>(Val: CmpLHS)->getParent() != BB) {
797 for (BasicBlock *P : predecessors(BB)) {
798 // If the value is known by LazyValueInfo to be a constant in a
799 // predecessor, use that information to try to thread this block.
800 Constant *Res = LVI->getPredicateOnEdge(Pred, V: CmpLHS, C: CmpConst, FromBB: P, ToBB: BB,
801 CxtI: CxtI ? CxtI : Cmp);
802 if (Constant *KC = getKnownConstant(Val: Res, Preference: WantInteger))
803 Result.emplace_back(Args&: KC, Args&: P);
804 }
805
806 return !Result.empty();
807 }
808
809 // InstCombine can fold some forms of constant range checks into
810 // (icmp (add (x, C1)), C2). See if we have we have such a thing with
811 // x as a live-in.
812 {
813 using namespace PatternMatch;
814
815 Value *AddLHS;
816 ConstantInt *AddConst;
817 if (isa<ConstantInt>(Val: CmpConst) &&
818 match(V: CmpLHS, P: m_Add(L: m_Value(V&: AddLHS), R: m_ConstantInt(CI&: AddConst)))) {
819 if (!isa<Instruction>(Val: AddLHS) ||
820 cast<Instruction>(Val: AddLHS)->getParent() != BB) {
821 for (BasicBlock *P : predecessors(BB)) {
822 // If the value is known by LazyValueInfo to be a ConstantRange in
823 // a predecessor, use that information to try to thread this
824 // block.
825 ConstantRange CR = LVI->getConstantRangeOnEdge(
826 V: AddLHS, FromBB: P, ToBB: BB, CxtI: CxtI ? CxtI : cast<Instruction>(Val: CmpLHS));
827 // Propagate the range through the addition.
828 CR = CR.add(Other: AddConst->getValue());
829
830 // Get the range where the compare returns true.
831 ConstantRange CmpRange = ConstantRange::makeExactICmpRegion(
832 Pred, Other: cast<ConstantInt>(Val: CmpConst)->getValue());
833
834 Constant *ResC;
835 if (CmpRange.contains(CR))
836 ResC = ConstantInt::getTrue(Ty: CmpType);
837 else if (CmpRange.inverse().contains(CR))
838 ResC = ConstantInt::getFalse(Ty: CmpType);
839 else
840 continue;
841
842 Result.emplace_back(Args&: ResC, Args&: P);
843 }
844
845 return !Result.empty();
846 }
847 }
848 }
849
850 // Try to find a constant value for the LHS of a comparison,
851 // and evaluate it statically if we can.
852 PredValueInfoTy LHSVals;
853 computeValueKnownInPredecessorsImpl(V: I->getOperand(i: 0), BB, Result&: LHSVals,
854 Preference: WantInteger, RecursionSet, CxtI);
855
856 for (const auto &LHSVal : LHSVals) {
857 Constant *V = LHSVal.first;
858 Constant *Folded =
859 ConstantFoldCompareInstOperands(Predicate: Pred, LHS: V, RHS: CmpConst, DL);
860 if (Constant *KC = getKnownConstant(Val: Folded, Preference: WantInteger))
861 Result.emplace_back(Args&: KC, Args: LHSVal.second);
862 }
863
864 return !Result.empty();
865 }
866 }
867
868 if (SelectInst *SI = dyn_cast<SelectInst>(Val: I)) {
869 // Handle select instructions where at least one operand is a known constant
870 // and we can figure out the condition value for any predecessor block.
871 Constant *TrueVal = getKnownConstant(Val: SI->getTrueValue(), Preference);
872 Constant *FalseVal = getKnownConstant(Val: SI->getFalseValue(), Preference);
873 PredValueInfoTy Conds;
874 if ((TrueVal || FalseVal) &&
875 computeValueKnownInPredecessorsImpl(V: SI->getCondition(), BB, Result&: Conds,
876 Preference: WantInteger, RecursionSet, CxtI)) {
877 for (auto &C : Conds) {
878 Constant *Cond = C.first;
879
880 // Figure out what value to use for the condition.
881 bool KnownCond;
882 if (ConstantInt *CI = dyn_cast<ConstantInt>(Val: Cond)) {
883 // A known boolean.
884 KnownCond = CI->isOne();
885 } else {
886 assert(isa<UndefValue>(Cond) && "Unexpected condition value");
887 // Either operand will do, so be sure to pick the one that's a known
888 // constant.
889 // FIXME: Do this more cleverly if both values are known constants?
890 KnownCond = (TrueVal != nullptr);
891 }
892
893 // See if the select has a known constant value for this predecessor.
894 if (Constant *Val = KnownCond ? TrueVal : FalseVal)
895 Result.emplace_back(Args&: Val, Args&: C.second);
896 }
897
898 return !Result.empty();
899 }
900 }
901
902 // If all else fails, see if LVI can figure out a constant value for us.
903 assert(CxtI->getParent() == BB && "CxtI should be in BB");
904 Constant *CI = LVI->getConstant(V, CxtI);
905 if (Constant *KC = getKnownConstant(Val: CI, Preference)) {
906 for (BasicBlock *Pred : predecessors(BB))
907 Result.emplace_back(Args&: KC, Args&: Pred);
908 }
909
910 return !Result.empty();
911}
912
913/// GetBestDestForBranchOnUndef - If we determine that the specified block ends
914/// in an undefined jump, decide which block is best to revector to.
915///
916/// Since we can pick an arbitrary destination, we pick the successor with the
917/// fewest predecessors. This should reduce the in-degree of the others.
918static unsigned getBestDestForJumpOnUndef(BasicBlock *BB) {
919 Instruction *BBTerm = BB->getTerminator();
920 unsigned MinSucc = 0;
921 BasicBlock *TestBB = BBTerm->getSuccessor(Idx: MinSucc);
922 // Compute the successor with the minimum number of predecessors.
923 unsigned MinNumPreds = pred_size(BB: TestBB);
924 for (unsigned i = 1, e = BBTerm->getNumSuccessors(); i != e; ++i) {
925 TestBB = BBTerm->getSuccessor(Idx: i);
926 unsigned NumPreds = pred_size(BB: TestBB);
927 if (NumPreds < MinNumPreds) {
928 MinSucc = i;
929 MinNumPreds = NumPreds;
930 }
931 }
932
933 return MinSucc;
934}
935
936static bool hasAddressTakenAndUsed(BasicBlock *BB) {
937 if (!BB->hasAddressTaken()) return false;
938
939 // If the block has its address taken, it may be a tree of dead constants
940 // hanging off of it. These shouldn't keep the block alive.
941 BlockAddress *BA = BlockAddress::get(BB);
942 BA->removeDeadConstantUsers();
943 return !BA->use_empty();
944}
945
946/// processBlock - If there are any predecessors whose control can be threaded
947/// through to a successor, transform them now.
948bool JumpThreadingPass::processBlock(BasicBlock *BB) {
949 // If the block is trivially dead, just return and let the caller nuke it.
950 // This simplifies other transformations.
951 if (DTU->isBBPendingDeletion(DelBB: BB) ||
952 (pred_empty(BB) && BB != &BB->getParent()->getEntryBlock()))
953 return false;
954
955 // If this block has a single predecessor, and if that pred has a single
956 // successor, merge the blocks. This encourages recursive jump threading
957 // because now the condition in this block can be threaded through
958 // predecessors of our predecessor block.
959 if (maybeMergeBasicBlockIntoOnlyPred(BB))
960 return true;
961
962 if (tryToUnfoldSelectInCurrBB(BB))
963 return true;
964
965 // Look if we can propagate guards to predecessors.
966 if (HasGuards && processGuards(BB))
967 return true;
968
969 // What kind of constant we're looking for.
970 ConstantPreference Preference = WantInteger;
971
972 // Look to see if the terminator is a conditional branch, switch or indirect
973 // branch, if not we can't thread it.
974 Value *Condition;
975 Instruction *Terminator = BB->getTerminator();
976 if (CondBrInst *BI = dyn_cast<CondBrInst>(Val: Terminator)) {
977 Condition = BI->getCondition();
978 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Val: Terminator)) {
979 Condition = SI->getCondition();
980 } else if (IndirectBrInst *IB = dyn_cast<IndirectBrInst>(Val: Terminator)) {
981 // Can't thread indirect branch with no successors.
982 if (IB->getNumSuccessors() == 0) return false;
983 Condition = IB->getAddress()->stripPointerCasts();
984 Preference = WantBlockAddress;
985 } else {
986 return false; // Must be an invoke or callbr.
987 }
988
989 // Keep track if we constant folded the condition in this invocation.
990 bool ConstantFolded = false;
991
992 // Run constant folding to see if we can reduce the condition to a simple
993 // constant.
994 if (Instruction *I = dyn_cast<Instruction>(Val: Condition)) {
995 Value *SimpleVal =
996 ConstantFoldInstruction(I, DL: BB->getDataLayout(), TLI);
997 if (SimpleVal) {
998 I->replaceAllUsesWith(V: SimpleVal);
999 if (isInstructionTriviallyDead(I, TLI))
1000 I->eraseFromParent();
1001 Condition = SimpleVal;
1002 ConstantFolded = true;
1003 }
1004 }
1005
1006 // If the terminator is branching on an undef or freeze undef, we can pick any
1007 // of the successors to branch to. Let getBestDestForJumpOnUndef decide.
1008 auto *FI = dyn_cast<FreezeInst>(Val: Condition);
1009 if (isa<UndefValue>(Val: Condition) ||
1010 (FI && isa<UndefValue>(Val: FI->getOperand(i_nocapture: 0)) && FI->hasOneUse())) {
1011 unsigned BestSucc = getBestDestForJumpOnUndef(BB);
1012 std::vector<DominatorTree::UpdateType> Updates;
1013
1014 // Fold the branch/switch.
1015 Instruction *BBTerm = BB->getTerminator();
1016 Updates.reserve(n: BBTerm->getNumSuccessors());
1017 for (unsigned i = 0, e = BBTerm->getNumSuccessors(); i != e; ++i) {
1018 if (i == BestSucc) continue;
1019 BasicBlock *Succ = BBTerm->getSuccessor(Idx: i);
1020 Succ->removePredecessor(Pred: BB, KeepOneInputPHIs: true);
1021 Updates.push_back(x: {DominatorTree::Delete, BB, Succ});
1022 }
1023
1024 LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
1025 << "' folding undef terminator: " << *BBTerm << '\n');
1026 Instruction *NewBI = UncondBrInst::Create(Target: BBTerm->getSuccessor(Idx: BestSucc),
1027 InsertBefore: BBTerm->getIterator());
1028 NewBI->setDebugLoc(BBTerm->getDebugLoc());
1029 ++NumFolds;
1030 BBTerm->eraseFromParent();
1031 DTU->applyUpdatesPermissive(Updates);
1032 if (FI)
1033 FI->eraseFromParent();
1034 return true;
1035 }
1036
1037 // If the terminator of this block is branching on a constant, simplify the
1038 // terminator to an unconditional branch. This can occur due to threading in
1039 // other blocks.
1040 if (getKnownConstant(Val: Condition, Preference)) {
1041 LLVM_DEBUG(dbgs() << " In block '" << BB->getName()
1042 << "' folding terminator: " << *BB->getTerminator()
1043 << '\n');
1044 ++NumFolds;
1045 ConstantFoldTerminator(BB, DeleteDeadConditions: true, TLI: nullptr, DTU: DTU.get());
1046 if (auto *BPI = getBPI())
1047 BPI->eraseBlock(BB);
1048 return true;
1049 }
1050
1051 Instruction *CondInst = dyn_cast<Instruction>(Val: Condition);
1052
1053 // All the rest of our checks depend on the condition being an instruction.
1054 if (!CondInst) {
1055 // FIXME: Unify this with code below.
1056 if (processThreadableEdges(Cond: Condition, BB, Preference, CxtI: Terminator))
1057 return true;
1058 return ConstantFolded;
1059 }
1060
1061 // Some of the following optimization can safely work on the unfrozen cond.
1062 Value *CondWithoutFreeze = CondInst;
1063 if (auto *FI = dyn_cast<FreezeInst>(Val: CondInst))
1064 CondWithoutFreeze = FI->getOperand(i_nocapture: 0);
1065
1066 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Val: CondWithoutFreeze)) {
1067 // If we're branching on a conditional, LVI might be able to determine
1068 // it's value at the branch instruction. We only handle comparisons
1069 // against a constant at this time.
1070 if (Constant *CondConst = dyn_cast<Constant>(Val: CondCmp->getOperand(i_nocapture: 1))) {
1071 Constant *Res =
1072 LVI->getPredicateAt(Pred: CondCmp->getPredicate(), V: CondCmp->getOperand(i_nocapture: 0),
1073 C: CondConst, CxtI: BB->getTerminator(),
1074 /*UseBlockValue=*/false);
1075 if (Res) {
1076 // We can safely replace *some* uses of the CondInst if it has
1077 // exactly one value as returned by LVI. RAUW is incorrect in the
1078 // presence of guards and assumes, that have the `Cond` as the use. This
1079 // is because we use the guards/assume to reason about the `Cond` value
1080 // at the end of block, but RAUW unconditionally replaces all uses
1081 // including the guards/assumes themselves and the uses before the
1082 // guard/assume.
1083 if (replaceFoldableUses(Cond: CondCmp, ToVal: Res, KnownAtEndOfBB: BB))
1084 return true;
1085 }
1086
1087 // We did not manage to simplify this branch, try to see whether
1088 // CondCmp depends on a known phi-select pattern.
1089 if (tryToUnfoldSelect(CondCmp, BB))
1090 return true;
1091 }
1092 }
1093
1094 if (SwitchInst *SI = dyn_cast<SwitchInst>(Val: BB->getTerminator()))
1095 if (tryToUnfoldSelect(SI, BB))
1096 return true;
1097
1098 // Check for some cases that are worth simplifying. Right now we want to look
1099 // for loads that are used by a switch or by the condition for the branch. If
1100 // we see one, check to see if it's partially redundant. If so, insert a PHI
1101 // which can then be used to thread the values.
1102 Value *SimplifyValue = CondWithoutFreeze;
1103
1104 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Val: SimplifyValue))
1105 if (isa<Constant>(Val: CondCmp->getOperand(i_nocapture: 1)))
1106 SimplifyValue = CondCmp->getOperand(i_nocapture: 0);
1107
1108 // TODO: There are other places where load PRE would be profitable, such as
1109 // more complex comparisons.
1110 if (LoadInst *LoadI = dyn_cast<LoadInst>(Val: SimplifyValue))
1111 if (simplifyPartiallyRedundantLoad(LI: LoadI))
1112 return true;
1113
1114 // Before threading, try to propagate profile data backwards:
1115 if (PHINode *PN = dyn_cast<PHINode>(Val: CondInst))
1116 if (PN->getParent() == BB && isa<CondBrInst>(Val: BB->getTerminator()))
1117 updatePredecessorProfileMetadata(PN, BB);
1118
1119 // Handle a variety of cases where we are branching on something derived from
1120 // a PHI node in the current block. If we can prove that any predecessors
1121 // compute a predictable value based on a PHI node, thread those predecessors.
1122 if (processThreadableEdges(Cond: CondInst, BB, Preference, CxtI: Terminator))
1123 return true;
1124
1125 // If this is an otherwise-unfoldable branch on a phi node or freeze(phi) in
1126 // the current block, see if we can simplify.
1127 PHINode *PN = dyn_cast<PHINode>(Val: CondWithoutFreeze);
1128 if (PN && PN->getParent() == BB && isa<CondBrInst>(Val: BB->getTerminator()))
1129 return processBranchOnPHI(PN);
1130
1131 // If this is an otherwise-unfoldable branch on a XOR, see if we can simplify.
1132 if (CondInst->getOpcode() == Instruction::Xor &&
1133 CondInst->getParent() == BB && isa<CondBrInst>(Val: BB->getTerminator()))
1134 return processBranchOnXOR(BO: cast<BinaryOperator>(Val: CondInst));
1135
1136 // Search for a stronger dominating condition that can be used to simplify a
1137 // conditional branch leaving BB.
1138 if (processImpliedCondition(BB))
1139 return true;
1140
1141 return false;
1142}
1143
1144bool JumpThreadingPass::processImpliedCondition(BasicBlock *BB) {
1145 auto *BI = dyn_cast<CondBrInst>(Val: BB->getTerminator());
1146 if (!BI)
1147 return false;
1148
1149 Value *Cond = BI->getCondition();
1150 // Assuming that predecessor's branch was taken, if pred's branch condition
1151 // (V) implies Cond, Cond can be either true, undef, or poison. In this case,
1152 // freeze(Cond) is either true or a nondeterministic value.
1153 // If freeze(Cond) has only one use, we can freely fold freeze(Cond) to true
1154 // without affecting other instructions.
1155 auto *FICond = dyn_cast<FreezeInst>(Val: Cond);
1156 if (FICond && FICond->hasOneUse())
1157 Cond = FICond->getOperand(i_nocapture: 0);
1158 else
1159 FICond = nullptr;
1160
1161 BasicBlock *CurrentBB = BB;
1162 BasicBlock *CurrentPred = BB->getSinglePredecessor();
1163 unsigned Iter = 0;
1164
1165 auto &DL = BB->getDataLayout();
1166
1167 while (CurrentPred && Iter++ < ImplicationSearchThreshold) {
1168 auto *PBI = dyn_cast<CondBrInst>(Val: CurrentPred->getTerminator());
1169 if (!PBI)
1170 return false;
1171 if (PBI->getSuccessor(i: 0) != CurrentBB && PBI->getSuccessor(i: 1) != CurrentBB)
1172 return false;
1173
1174 bool CondIsTrue = PBI->getSuccessor(i: 0) == CurrentBB;
1175 std::optional<bool> Implication =
1176 isImpliedCondition(LHS: PBI->getCondition(), RHS: Cond, DL, LHSIsTrue: CondIsTrue);
1177
1178 // If the branch condition of BB (which is Cond) and CurrentPred are
1179 // exactly the same freeze instruction, Cond can be folded into CondIsTrue.
1180 if (!Implication && FICond && isa<FreezeInst>(Val: PBI->getCondition())) {
1181 if (cast<FreezeInst>(Val: PBI->getCondition())->getOperand(i_nocapture: 0) ==
1182 FICond->getOperand(i_nocapture: 0))
1183 Implication = CondIsTrue;
1184 }
1185
1186 if (Implication) {
1187 BasicBlock *KeepSucc = BI->getSuccessor(i: *Implication ? 0 : 1);
1188 BasicBlock *RemoveSucc = BI->getSuccessor(i: *Implication ? 1 : 0);
1189 RemoveSucc->removePredecessor(Pred: BB);
1190 UncondBrInst *UncondBI =
1191 UncondBrInst::Create(Target: KeepSucc, InsertBefore: BI->getIterator());
1192 UncondBI->setDebugLoc(BI->getDebugLoc());
1193 ++NumFolds;
1194 BI->eraseFromParent();
1195 if (FICond)
1196 FICond->eraseFromParent();
1197
1198 DTU->applyUpdatesPermissive(Updates: {{DominatorTree::Delete, BB, RemoveSucc}});
1199 if (auto *BPI = getBPI())
1200 BPI->eraseBlock(BB);
1201 return true;
1202 }
1203 CurrentBB = CurrentPred;
1204 CurrentPred = CurrentBB->getSinglePredecessor();
1205 }
1206
1207 return false;
1208}
1209
1210/// Return true if Op is an instruction defined in the given block.
1211static bool isOpDefinedInBlock(Value *Op, BasicBlock *BB) {
1212 if (Instruction *OpInst = dyn_cast<Instruction>(Val: Op))
1213 if (OpInst->getParent() == BB)
1214 return true;
1215 return false;
1216}
1217
1218/// simplifyPartiallyRedundantLoad - If LoadI is an obviously partially
1219/// redundant load instruction, eliminate it by replacing it with a PHI node.
1220/// This is an important optimization that encourages jump threading, and needs
1221/// to be run interlaced with other jump threading tasks.
1222bool JumpThreadingPass::simplifyPartiallyRedundantLoad(LoadInst *LoadI) {
1223 // Don't hack volatile and ordered loads.
1224 if (!LoadI->isUnordered()) return false;
1225
1226 // If the load is defined in a block with exactly one predecessor, it can't be
1227 // partially redundant.
1228 BasicBlock *LoadBB = LoadI->getParent();
1229 if (LoadBB->getSinglePredecessor())
1230 return false;
1231
1232 // If the load is defined in an EH pad, it can't be partially redundant,
1233 // because the edges between the invoke and the EH pad cannot have other
1234 // instructions between them.
1235 if (LoadBB->isEHPad())
1236 return false;
1237
1238 Value *LoadedPtr = LoadI->getOperand(i_nocapture: 0);
1239
1240 // If the loaded operand is defined in the LoadBB and its not a phi,
1241 // it can't be available in predecessors.
1242 if (isOpDefinedInBlock(Op: LoadedPtr, BB: LoadBB) && !isa<PHINode>(Val: LoadedPtr))
1243 return false;
1244
1245 // Scan a few instructions up from the load, to see if it is obviously live at
1246 // the entry to its block.
1247 BasicBlock::iterator BBIt(LoadI);
1248 bool IsLoadCSE;
1249 BatchAAResults BatchAA(*AA);
1250 // The dominator tree is updated lazily and may not be valid at this point.
1251 BatchAA.disableDominatorTree();
1252 if (Value *AvailableVal = FindAvailableLoadedValue(
1253 Load: LoadI, ScanBB: LoadBB, ScanFrom&: BBIt, MaxInstsToScan: DefMaxInstsToScan, AA: &BatchAA, IsLoadCSE: &IsLoadCSE)) {
1254 // If the value of the load is locally available within the block, just use
1255 // it. This frequently occurs for reg2mem'd allocas.
1256
1257 if (IsLoadCSE) {
1258 LoadInst *NLoadI = cast<LoadInst>(Val: AvailableVal);
1259 combineMetadataForCSE(K: NLoadI, J: LoadI, DoesKMove: false);
1260 LVI->forgetValue(V: NLoadI);
1261 };
1262
1263 // If the returned value is the load itself, replace with poison. This can
1264 // only happen in dead loops.
1265 if (AvailableVal == LoadI)
1266 AvailableVal = PoisonValue::get(T: LoadI->getType());
1267 if (AvailableVal->getType() != LoadI->getType()) {
1268 AvailableVal = CastInst::CreateBitOrPointerCast(
1269 S: AvailableVal, Ty: LoadI->getType(), Name: "", InsertBefore: LoadI->getIterator());
1270 cast<Instruction>(Val: AvailableVal)->setDebugLoc(LoadI->getDebugLoc());
1271 }
1272 LoadI->replaceAllUsesWith(V: AvailableVal);
1273 LoadI->eraseFromParent();
1274 return true;
1275 }
1276
1277 // Otherwise, if we scanned the whole block and got to the top of the block,
1278 // we know the block is locally transparent to the load. If not, something
1279 // might clobber its value.
1280 if (BBIt != LoadBB->begin())
1281 return false;
1282
1283 // If all of the loads and stores that feed the value have the same AA tags,
1284 // then we can propagate them onto any newly inserted loads.
1285 AAMDNodes AATags = LoadI->getAAMetadata();
1286
1287 SmallPtrSet<BasicBlock*, 8> PredsScanned;
1288
1289 using AvailablePredsTy = SmallVector<std::pair<BasicBlock *, Value *>, 8>;
1290
1291 AvailablePredsTy AvailablePreds;
1292 BasicBlock *OneUnavailablePred = nullptr;
1293 SmallVector<LoadInst*, 8> CSELoads;
1294
1295 // If we got here, the loaded value is transparent through to the start of the
1296 // block. Check to see if it is available in any of the predecessor blocks.
1297 for (BasicBlock *PredBB : predecessors(BB: LoadBB)) {
1298 // If we already scanned this predecessor, skip it.
1299 if (!PredsScanned.insert(Ptr: PredBB).second)
1300 continue;
1301
1302 BBIt = PredBB->end();
1303 unsigned NumScanedInst = 0;
1304 Value *PredAvailable = nullptr;
1305 // NOTE: We don't CSE load that is volatile or anything stronger than
1306 // unordered, that should have been checked when we entered the function.
1307 assert(LoadI->isUnordered() &&
1308 "Attempting to CSE volatile or atomic loads");
1309 // If this is a load on a phi pointer, phi-translate it and search
1310 // for available load/store to the pointer in predecessors.
1311 Type *AccessTy = LoadI->getType();
1312 const auto &DL = LoadI->getDataLayout();
1313 MemoryLocation Loc(LoadedPtr->DoPHITranslation(CurBB: LoadBB, PredBB),
1314 LocationSize::precise(Value: DL.getTypeStoreSize(Ty: AccessTy)),
1315 AATags);
1316 PredAvailable = findAvailablePtrLoadStore(
1317 Loc, AccessTy, AtLeastAtomic: LoadI->isAtomic(), ScanBB: PredBB, ScanFrom&: BBIt, MaxInstsToScan: DefMaxInstsToScan,
1318 AA: &BatchAA, IsLoadCSE: &IsLoadCSE, NumScanedInst: &NumScanedInst);
1319
1320 // If PredBB has a single predecessor, continue scanning through the
1321 // single predecessor.
1322 BasicBlock *SinglePredBB = PredBB;
1323 while (!PredAvailable && SinglePredBB && BBIt == SinglePredBB->begin() &&
1324 NumScanedInst < DefMaxInstsToScan) {
1325 SinglePredBB = SinglePredBB->getSinglePredecessor();
1326 if (SinglePredBB) {
1327 BBIt = SinglePredBB->end();
1328 PredAvailable = findAvailablePtrLoadStore(
1329 Loc, AccessTy, AtLeastAtomic: LoadI->isAtomic(), ScanBB: SinglePredBB, ScanFrom&: BBIt,
1330 MaxInstsToScan: (DefMaxInstsToScan - NumScanedInst), AA: &BatchAA, IsLoadCSE: &IsLoadCSE,
1331 NumScanedInst: &NumScanedInst);
1332 }
1333 }
1334
1335 if (!PredAvailable) {
1336 OneUnavailablePred = PredBB;
1337 continue;
1338 }
1339
1340 if (IsLoadCSE)
1341 CSELoads.push_back(Elt: cast<LoadInst>(Val: PredAvailable));
1342
1343 // If so, this load is partially redundant. Remember this info so that we
1344 // can create a PHI node.
1345 AvailablePreds.emplace_back(Args&: PredBB, Args&: PredAvailable);
1346 }
1347
1348 // If the loaded value isn't available in any predecessor, it isn't partially
1349 // redundant.
1350 if (AvailablePreds.empty()) return false;
1351
1352 // Okay, the loaded value is available in at least one (and maybe all!)
1353 // predecessors. If the value is unavailable in more than one unique
1354 // predecessor, we want to insert a merge block for those common predecessors.
1355 // This ensures that we only have to insert one reload, thus not increasing
1356 // code size.
1357 BasicBlock *UnavailablePred = nullptr;
1358
1359 // If the value is unavailable in one of predecessors, we will end up
1360 // inserting a new instruction into them. It is only valid if all the
1361 // instructions before LoadI are guaranteed to pass execution to its
1362 // successor, or if LoadI is safe to speculate.
1363 // TODO: If this logic becomes more complex, and we will perform PRE insertion
1364 // farther than to a predecessor, we need to reuse the code from GVN's PRE.
1365 // It requires domination tree analysis, so for this simple case it is an
1366 // overkill.
1367 if (PredsScanned.size() != AvailablePreds.size() &&
1368 !isSafeToSpeculativelyExecute(I: LoadI))
1369 for (auto I = LoadBB->begin(); &*I != LoadI; ++I)
1370 if (!isGuaranteedToTransferExecutionToSuccessor(I: &*I))
1371 return false;
1372
1373 // If there is exactly one predecessor where the value is unavailable, the
1374 // already computed 'OneUnavailablePred' block is it. If it ends in an
1375 // unconditional branch, we know that it isn't a critical edge.
1376 if (PredsScanned.size() == AvailablePreds.size()+1 &&
1377 OneUnavailablePred->getTerminator()->getNumSuccessors() == 1) {
1378 UnavailablePred = OneUnavailablePred;
1379 } else if (PredsScanned.size() != AvailablePreds.size()) {
1380 // Otherwise, we had multiple unavailable predecessors or we had a critical
1381 // edge from the one.
1382 SmallVector<BasicBlock*, 8> PredsToSplit;
1383 SmallPtrSet<BasicBlock *, 8> AvailablePredSet(
1384 llvm::from_range, llvm::make_first_range(c&: AvailablePreds));
1385
1386 // Add all the unavailable predecessors to the PredsToSplit list.
1387 for (BasicBlock *P : predecessors(BB: LoadBB)) {
1388 // If the predecessor is an indirect goto, we can't split the edge.
1389 if (isa<IndirectBrInst>(Val: P->getTerminator()))
1390 return false;
1391
1392 if (!AvailablePredSet.count(Ptr: P))
1393 PredsToSplit.push_back(Elt: P);
1394 }
1395
1396 // Split them out to their own block.
1397 UnavailablePred = splitBlockPreds(BB: LoadBB, Preds: PredsToSplit, Suffix: "thread-pre-split");
1398 }
1399
1400 // If the value isn't available in all predecessors, then there will be
1401 // exactly one where it isn't available. Insert a load on that edge and add
1402 // it to the AvailablePreds list.
1403 if (UnavailablePred) {
1404 assert(UnavailablePred->getTerminator()->getNumSuccessors() == 1 &&
1405 "Can't handle critical edge here!");
1406 LoadInst *NewVal = new LoadInst(
1407 LoadI->getType(), LoadedPtr->DoPHITranslation(CurBB: LoadBB, PredBB: UnavailablePred),
1408 LoadI->getName() + ".pr", false, LoadI->getAlign(),
1409 LoadI->getOrdering(), LoadI->getSyncScopeID(),
1410 UnavailablePred->getTerminator()->getIterator());
1411 NewVal->setDebugLoc(LoadI->getDebugLoc());
1412 if (AATags)
1413 NewVal->setAAMetadata(AATags);
1414
1415 AvailablePreds.emplace_back(Args&: UnavailablePred, Args&: NewVal);
1416 }
1417
1418 // Now we know that each predecessor of this block has a value in
1419 // AvailablePreds, sort them for efficient access as we're walking the preds.
1420 array_pod_sort(Start: AvailablePreds.begin(), End: AvailablePreds.end());
1421
1422 // Create a PHI node at the start of the block for the PRE'd load value.
1423 PHINode *PN = PHINode::Create(Ty: LoadI->getType(), NumReservedValues: pred_size(BB: LoadBB), NameStr: "");
1424 PN->insertBefore(InsertPos: LoadBB->begin());
1425 PN->takeName(V: LoadI);
1426 PN->setDebugLoc(LoadI->getDebugLoc());
1427
1428 // Insert new entries into the PHI for each predecessor. A single block may
1429 // have multiple entries here.
1430 for (BasicBlock *P : predecessors(BB: LoadBB)) {
1431 AvailablePredsTy::iterator I =
1432 llvm::lower_bound(Range&: AvailablePreds, Value: std::make_pair(x&: P, y: (Value *)nullptr));
1433
1434 assert(I != AvailablePreds.end() && I->first == P &&
1435 "Didn't find entry for predecessor!");
1436
1437 // If we have an available predecessor but it requires casting, insert the
1438 // cast in the predecessor and use the cast. Note that we have to update the
1439 // AvailablePreds vector as we go so that all of the PHI entries for this
1440 // predecessor use the same bitcast.
1441 Value *&PredV = I->second;
1442 if (PredV->getType() != LoadI->getType()) {
1443 PredV = CastInst::CreateBitOrPointerCast(
1444 S: PredV, Ty: LoadI->getType(), Name: "", InsertBefore: P->getTerminator()->getIterator());
1445 // The new cast is producing the value used to replace the load
1446 // instruction, so uses the load's debug location. If P does not always
1447 // branch to the load BB however then the debug location must be dropped,
1448 // as it is hoisted past a conditional branch.
1449 DebugLoc DL = P->getTerminator()->getNumSuccessors() == 1
1450 ? LoadI->getDebugLoc()
1451 : DebugLoc::getDropped();
1452 cast<CastInst>(Val: PredV)->setDebugLoc(DL);
1453 }
1454
1455 PN->addIncoming(V: PredV, BB: I->first);
1456 }
1457
1458 for (LoadInst *PredLoadI : CSELoads) {
1459 combineMetadataForCSE(K: PredLoadI, J: LoadI, DoesKMove: true);
1460 LVI->forgetValue(V: PredLoadI);
1461 }
1462
1463 LoadI->replaceAllUsesWith(V: PN);
1464 LoadI->eraseFromParent();
1465
1466 return true;
1467}
1468
1469/// findMostPopularDest - The specified list contains multiple possible
1470/// threadable destinations. Pick the one that occurs the most frequently in
1471/// the list.
1472static BasicBlock *
1473findMostPopularDest(BasicBlock *BB,
1474 const SmallVectorImpl<std::pair<BasicBlock *,
1475 BasicBlock *>> &PredToDestList) {
1476 assert(!PredToDestList.empty());
1477
1478 // Determine popularity. If there are multiple possible destinations, we
1479 // explicitly choose to ignore 'undef' destinations. We prefer to thread
1480 // blocks with known and real destinations to threading undef. We'll handle
1481 // them later if interesting.
1482 MapVector<BasicBlock *, unsigned> DestPopularity;
1483
1484 // Populate DestPopularity with the successors in the order they appear in the
1485 // successor list. This way, we ensure determinism by iterating it in the
1486 // same order in llvm::max_element below. We map nullptr to 0 so that we can
1487 // return nullptr when PredToDestList contains nullptr only.
1488 DestPopularity[nullptr] = 0;
1489 for (auto *SuccBB : successors(BB))
1490 DestPopularity[SuccBB] = 0;
1491
1492 for (const auto &PredToDest : PredToDestList)
1493 if (PredToDest.second)
1494 DestPopularity[PredToDest.second]++;
1495
1496 // Find the most popular dest.
1497 auto MostPopular = llvm::max_element(Range&: DestPopularity, C: llvm::less_second());
1498
1499 // Okay, we have finally picked the most popular destination.
1500 return MostPopular->first;
1501}
1502
1503// Try to evaluate the value of V when the control flows from PredPredBB to
1504// BB->getSinglePredecessor() and then on to BB.
1505Constant *JumpThreadingPass::evaluateOnPredecessorEdge(BasicBlock *BB,
1506 BasicBlock *PredPredBB,
1507 Value *V,
1508 const DataLayout &DL) {
1509 SmallPtrSet<Value *, 8> Visited;
1510 return evaluateOnPredecessorEdge(BB, PredPredBB, cond: V, DL, Visited);
1511}
1512
1513Constant *JumpThreadingPass::evaluateOnPredecessorEdge(
1514 BasicBlock *BB, BasicBlock *PredPredBB, Value *V, const DataLayout &DL,
1515 SmallPtrSet<Value *, 8> &Visited) {
1516 if (!Visited.insert(Ptr: V).second)
1517 return nullptr;
1518 llvm::scope_exit _([&Visited, V]() { Visited.erase(Ptr: V); });
1519
1520 BasicBlock *PredBB = BB->getSinglePredecessor();
1521 assert(PredBB && "Expected a single predecessor");
1522
1523 if (Constant *Cst = dyn_cast<Constant>(Val: V)) {
1524 return Cst;
1525 }
1526
1527 // Consult LVI if V is not an instruction in BB or PredBB.
1528 Instruction *I = dyn_cast<Instruction>(Val: V);
1529 if (!I || (I->getParent() != BB && I->getParent() != PredBB)) {
1530 return LVI->getConstantOnEdge(V, FromBB: PredPredBB, ToBB: PredBB, CxtI: nullptr);
1531 }
1532
1533 // Look into a PHI argument.
1534 if (PHINode *PHI = dyn_cast<PHINode>(Val: V)) {
1535 if (PHI->getParent() == PredBB)
1536 return dyn_cast<Constant>(Val: PHI->getIncomingValueForBlock(BB: PredPredBB));
1537 return nullptr;
1538 }
1539
1540 // If we have a CmpInst, try to fold it for each incoming edge into PredBB.
1541 // Note that during the execution of the pass, phi nodes may become constant
1542 // and may be removed, which can lead to self-referencing instructions in
1543 // code that becomes unreachable. Consequently, we need to handle those
1544 // instructions in unreachable code and check before going into recursion.
1545 if (CmpInst *CondCmp = dyn_cast<CmpInst>(Val: V)) {
1546 if (CondCmp->getParent() == BB) {
1547 Constant *Op0 = evaluateOnPredecessorEdge(
1548 BB, PredPredBB, V: CondCmp->getOperand(i_nocapture: 0), DL, Visited);
1549 Constant *Op1 = evaluateOnPredecessorEdge(
1550 BB, PredPredBB, V: CondCmp->getOperand(i_nocapture: 1), DL, Visited);
1551 if (Op0 && Op1) {
1552 return ConstantFoldCompareInstOperands(Predicate: CondCmp->getPredicate(), LHS: Op0,
1553 RHS: Op1, DL);
1554 }
1555 }
1556 return nullptr;
1557 }
1558
1559 return nullptr;
1560}
1561
1562bool JumpThreadingPass::processThreadableEdges(Value *Cond, BasicBlock *BB,
1563 ConstantPreference Preference,
1564 Instruction *CxtI) {
1565 // If threading this would thread across a loop header, don't even try to
1566 // thread the edge.
1567 if (LoopHeaders.count(Ptr: BB))
1568 return false;
1569
1570 PredValueInfoTy PredValues;
1571 if (!computeValueKnownInPredecessors(V: Cond, BB, Result&: PredValues, Preference,
1572 CxtI)) {
1573 // We don't have known values in predecessors. See if we can thread through
1574 // BB and its sole predecessor.
1575 return maybethreadThroughTwoBasicBlocks(BB, Cond);
1576 }
1577
1578 assert(!PredValues.empty() &&
1579 "computeValueKnownInPredecessors returned true with no values");
1580
1581 LLVM_DEBUG(dbgs() << "IN BB: " << *BB;
1582 for (const auto &PredValue : PredValues) {
1583 dbgs() << " BB '" << BB->getName()
1584 << "': FOUND condition = " << *PredValue.first
1585 << " for pred '" << PredValue.second->getName() << "'.\n";
1586 });
1587
1588 // Decide what we want to thread through. Convert our list of known values to
1589 // a list of known destinations for each pred. This also discards duplicate
1590 // predecessors and keeps track of the undefined inputs (which are represented
1591 // as a null dest in the PredToDestList).
1592 SmallPtrSet<BasicBlock*, 16> SeenPreds;
1593 SmallVector<std::pair<BasicBlock*, BasicBlock*>, 16> PredToDestList;
1594
1595 BasicBlock *OnlyDest = nullptr;
1596 BasicBlock *MultipleDestSentinel = (BasicBlock*)(intptr_t)~0ULL;
1597 Constant *OnlyVal = nullptr;
1598 Constant *MultipleVal = (Constant *)(intptr_t)~0ULL;
1599
1600 for (const auto &PredValue : PredValues) {
1601 BasicBlock *Pred = PredValue.second;
1602 if (!SeenPreds.insert(Ptr: Pred).second)
1603 continue; // Duplicate predecessor entry.
1604
1605 Constant *Val = PredValue.first;
1606
1607 BasicBlock *DestBB;
1608 if (isa<UndefValue>(Val))
1609 DestBB = nullptr;
1610 else if (CondBrInst *BI = dyn_cast<CondBrInst>(Val: BB->getTerminator())) {
1611 assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
1612 DestBB = BI->getSuccessor(i: cast<ConstantInt>(Val)->isZero());
1613 } else if (SwitchInst *SI = dyn_cast<SwitchInst>(Val: BB->getTerminator())) {
1614 assert(isa<ConstantInt>(Val) && "Expecting a constant integer");
1615 DestBB = SI->findCaseValue(C: cast<ConstantInt>(Val))->getCaseSuccessor();
1616 } else {
1617 assert(isa<IndirectBrInst>(BB->getTerminator())
1618 && "Unexpected terminator");
1619 assert(isa<BlockAddress>(Val) && "Expecting a constant blockaddress");
1620 DestBB = cast<BlockAddress>(Val)->getBasicBlock();
1621 }
1622
1623 // If we have exactly one destination, remember it for efficiency below.
1624 if (PredToDestList.empty()) {
1625 OnlyDest = DestBB;
1626 OnlyVal = Val;
1627 } else {
1628 if (OnlyDest != DestBB)
1629 OnlyDest = MultipleDestSentinel;
1630 // It possible we have same destination, but different value, e.g. default
1631 // case in switchinst.
1632 if (Val != OnlyVal)
1633 OnlyVal = MultipleVal;
1634 }
1635
1636 // If the predecessor ends with an indirect goto, we can't change its
1637 // destination.
1638 if (isa<IndirectBrInst>(Val: Pred->getTerminator()))
1639 continue;
1640
1641 PredToDestList.emplace_back(Args&: Pred, Args&: DestBB);
1642 }
1643
1644 // If all edges were unthreadable, we fail.
1645 if (PredToDestList.empty())
1646 return false;
1647
1648 // If all the predecessors go to a single known successor, we want to fold,
1649 // not thread. By doing so, we do not need to duplicate the current block and
1650 // also miss potential opportunities in case we dont/cant duplicate.
1651 if (OnlyDest && OnlyDest != MultipleDestSentinel) {
1652 if (BB->hasNPredecessors(N: PredToDestList.size())) {
1653 bool SeenFirstBranchToOnlyDest = false;
1654 std::vector <DominatorTree::UpdateType> Updates;
1655 Updates.reserve(n: BB->getTerminator()->getNumSuccessors() - 1);
1656 for (BasicBlock *SuccBB : successors(BB)) {
1657 if (SuccBB == OnlyDest && !SeenFirstBranchToOnlyDest) {
1658 SeenFirstBranchToOnlyDest = true; // Don't modify the first branch.
1659 } else {
1660 SuccBB->removePredecessor(Pred: BB, KeepOneInputPHIs: true); // This is unreachable successor.
1661 Updates.push_back(x: {DominatorTree::Delete, BB, SuccBB});
1662 }
1663 }
1664
1665 // Finally update the terminator.
1666 Instruction *Term = BB->getTerminator();
1667 Instruction *NewBI = UncondBrInst::Create(Target: OnlyDest, InsertBefore: Term->getIterator());
1668 NewBI->setDebugLoc(Term->getDebugLoc());
1669 ++NumFolds;
1670 Term->eraseFromParent();
1671 DTU->applyUpdatesPermissive(Updates);
1672 if (auto *BPI = getBPI())
1673 BPI->eraseBlock(BB);
1674
1675 // If the condition is now dead due to the removal of the old terminator,
1676 // erase it.
1677 if (auto *CondInst = dyn_cast<Instruction>(Val: Cond)) {
1678 if (CondInst->use_empty() && !CondInst->mayHaveSideEffects())
1679 CondInst->eraseFromParent();
1680 // We can safely replace *some* uses of the CondInst if it has
1681 // exactly one value as returned by LVI. RAUW is incorrect in the
1682 // presence of guards and assumes, that have the `Cond` as the use. This
1683 // is because we use the guards/assume to reason about the `Cond` value
1684 // at the end of block, but RAUW unconditionally replaces all uses
1685 // including the guards/assumes themselves and the uses before the
1686 // guard/assume.
1687 else if (OnlyVal && OnlyVal != MultipleVal)
1688 replaceFoldableUses(Cond: CondInst, ToVal: OnlyVal, KnownAtEndOfBB: BB);
1689 }
1690 return true;
1691 }
1692 }
1693
1694 // Determine which is the most common successor. If we have many inputs and
1695 // this block is a switch, we want to start by threading the batch that goes
1696 // to the most popular destination first. If we only know about one
1697 // threadable destination (the common case) we can avoid this.
1698 BasicBlock *MostPopularDest = OnlyDest;
1699
1700 if (MostPopularDest == MultipleDestSentinel) {
1701 // Remove any loop headers from the Dest list, threadEdge conservatively
1702 // won't process them, but we might have other destination that are eligible
1703 // and we still want to process.
1704 erase_if(C&: PredToDestList,
1705 P: [&](const std::pair<BasicBlock *, BasicBlock *> &PredToDest) {
1706 return LoopHeaders.contains(Ptr: PredToDest.second);
1707 });
1708
1709 if (PredToDestList.empty())
1710 return false;
1711
1712 MostPopularDest = findMostPopularDest(BB, PredToDestList);
1713 }
1714
1715 // Now that we know what the most popular destination is, factor all
1716 // predecessors that will jump to it into a single predecessor.
1717 SmallVector<BasicBlock*, 16> PredsToFactor;
1718 for (const auto &PredToDest : PredToDestList)
1719 if (PredToDest.second == MostPopularDest) {
1720 BasicBlock *Pred = PredToDest.first;
1721
1722 // This predecessor may be a switch or something else that has multiple
1723 // edges to the block. Factor each of these edges by listing them
1724 // according to # occurrences in PredsToFactor.
1725 for (BasicBlock *Succ : successors(BB: Pred))
1726 if (Succ == BB)
1727 PredsToFactor.push_back(Elt: Pred);
1728 }
1729
1730 // If the threadable edges are branching on an undefined value, we get to pick
1731 // the destination that these predecessors should get to.
1732 if (!MostPopularDest)
1733 MostPopularDest = BB->getTerminator()->
1734 getSuccessor(Idx: getBestDestForJumpOnUndef(BB));
1735
1736 // Ok, try to thread it!
1737 return tryThreadEdge(BB, PredBBs: PredsToFactor, SuccBB: MostPopularDest);
1738}
1739
1740/// processBranchOnPHI - We have an otherwise unthreadable conditional branch on
1741/// a PHI node (or freeze PHI) in the current block. See if there are any
1742/// simplifications we can do based on inputs to the phi node.
1743bool JumpThreadingPass::processBranchOnPHI(PHINode *PN) {
1744 BasicBlock *BB = PN->getParent();
1745
1746 // TODO: We could make use of this to do it once for blocks with common PHI
1747 // values.
1748 SmallVector<BasicBlock*, 1> PredBBs;
1749 PredBBs.resize(N: 1);
1750
1751 // If any of the predecessor blocks end in an unconditional branch, we can
1752 // *duplicate* the conditional branch into that block in order to further
1753 // encourage jump threading and to eliminate cases where we have branch on a
1754 // phi of an icmp (branch on icmp is much better).
1755 // This is still beneficial when a frozen phi is used as the branch condition
1756 // because it allows CodeGenPrepare to further canonicalize br(freeze(icmp))
1757 // to br(icmp(freeze ...)).
1758 for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
1759 BasicBlock *PredBB = PN->getIncomingBlock(i);
1760 if (isa<UncondBrInst>(Val: PredBB->getTerminator())) {
1761 PredBBs[0] = PredBB;
1762 // Try to duplicate BB into PredBB.
1763 if (duplicateCondBranchOnPHIIntoPred(BB, PredBBs))
1764 return true;
1765 }
1766 }
1767
1768 return false;
1769}
1770
1771/// processBranchOnXOR - We have an otherwise unthreadable conditional branch on
1772/// a xor instruction in the current block. See if there are any
1773/// simplifications we can do based on inputs to the xor.
1774bool JumpThreadingPass::processBranchOnXOR(BinaryOperator *BO) {
1775 BasicBlock *BB = BO->getParent();
1776
1777 // If either the LHS or RHS of the xor is a constant, don't do this
1778 // optimization.
1779 if (isa<ConstantInt>(Val: BO->getOperand(i_nocapture: 0)) ||
1780 isa<ConstantInt>(Val: BO->getOperand(i_nocapture: 1)))
1781 return false;
1782
1783 // If the first instruction in BB isn't a phi, we won't be able to infer
1784 // anything special about any particular predecessor.
1785 if (!isa<PHINode>(Val: BB->front()))
1786 return false;
1787
1788 // If this BB is a landing pad, we won't be able to split the edge into it.
1789 if (BB->isEHPad())
1790 return false;
1791
1792 // If we have a xor as the branch input to this block, and we know that the
1793 // LHS or RHS of the xor in any predecessor is true/false, then we can clone
1794 // the condition into the predecessor and fix that value to true, saving some
1795 // logical ops on that path and encouraging other paths to simplify.
1796 //
1797 // This copies something like this:
1798 //
1799 // BB:
1800 // %X = phi i1 [1], [%X']
1801 // %Y = icmp eq i32 %A, %B
1802 // %Z = xor i1 %X, %Y
1803 // br i1 %Z, ...
1804 //
1805 // Into:
1806 // BB':
1807 // %Y = icmp ne i32 %A, %B
1808 // br i1 %Y, ...
1809
1810 PredValueInfoTy XorOpValues;
1811 bool isLHS = true;
1812 if (!computeValueKnownInPredecessors(V: BO->getOperand(i_nocapture: 0), BB, Result&: XorOpValues,
1813 Preference: WantInteger, CxtI: BO)) {
1814 assert(XorOpValues.empty());
1815 if (!computeValueKnownInPredecessors(V: BO->getOperand(i_nocapture: 1), BB, Result&: XorOpValues,
1816 Preference: WantInteger, CxtI: BO))
1817 return false;
1818 isLHS = false;
1819 }
1820
1821 assert(!XorOpValues.empty() &&
1822 "computeValueKnownInPredecessors returned true with no values");
1823
1824 // Scan the information to see which is most popular: true or false. The
1825 // predecessors can be of the set true, false, or undef.
1826 unsigned NumTrue = 0, NumFalse = 0;
1827 for (const auto &XorOpValue : XorOpValues) {
1828 if (isa<UndefValue>(Val: XorOpValue.first))
1829 // Ignore undefs for the count.
1830 continue;
1831 if (cast<ConstantInt>(Val: XorOpValue.first)->isZero())
1832 ++NumFalse;
1833 else
1834 ++NumTrue;
1835 }
1836
1837 // Determine which value to split on, true, false, or undef if neither.
1838 ConstantInt *SplitVal = nullptr;
1839 if (NumTrue > NumFalse)
1840 SplitVal = ConstantInt::getTrue(Context&: BB->getContext());
1841 else if (NumTrue != 0 || NumFalse != 0)
1842 SplitVal = ConstantInt::getFalse(Context&: BB->getContext());
1843
1844 // Collect all of the blocks that this can be folded into so that we can
1845 // factor this once and clone it once.
1846 SmallVector<BasicBlock*, 8> BlocksToFoldInto;
1847 for (const auto &XorOpValue : XorOpValues) {
1848 if (XorOpValue.first != SplitVal && !isa<UndefValue>(Val: XorOpValue.first))
1849 continue;
1850
1851 BlocksToFoldInto.push_back(Elt: XorOpValue.second);
1852 }
1853
1854 // If we inferred a value for all of the predecessors, then duplication won't
1855 // help us. However, we can just replace the LHS or RHS with the constant.
1856 if (BlocksToFoldInto.size() ==
1857 cast<PHINode>(Val&: BB->front()).getNumIncomingValues()) {
1858 if (!SplitVal) {
1859 // If all preds provide undef, just nuke the xor, because it is undef too.
1860 BO->replaceAllUsesWith(V: UndefValue::get(T: BO->getType()));
1861 BO->eraseFromParent();
1862 } else if (SplitVal->isZero() && BO != BO->getOperand(i_nocapture: isLHS)) {
1863 // If all preds provide 0, replace the xor with the other input.
1864 BO->replaceAllUsesWith(V: BO->getOperand(i_nocapture: isLHS));
1865 BO->eraseFromParent();
1866 } else {
1867 // If all preds provide 1, set the computed value to 1.
1868 BO->setOperand(i_nocapture: !isLHS, Val_nocapture: SplitVal);
1869 }
1870
1871 return true;
1872 }
1873
1874 // If any of predecessors end with an indirect goto, we can't change its
1875 // destination.
1876 if (any_of(Range&: BlocksToFoldInto, P: [](BasicBlock *Pred) {
1877 return isa<IndirectBrInst>(Val: Pred->getTerminator());
1878 }))
1879 return false;
1880
1881 // Try to duplicate BB into PredBB.
1882 return duplicateCondBranchOnPHIIntoPred(BB, PredBBs: BlocksToFoldInto);
1883}
1884
1885/// addPHINodeEntriesForMappedBlock - We're adding 'NewPred' as a new
1886/// predecessor to the PHIBB block. If it has PHI nodes, add entries for
1887/// NewPred using the entries from OldPred (suitably mapped).
1888static void addPHINodeEntriesForMappedBlock(BasicBlock *PHIBB,
1889 BasicBlock *OldPred,
1890 BasicBlock *NewPred,
1891 ValueToValueMapTy &ValueMap) {
1892 for (PHINode &PN : PHIBB->phis()) {
1893 // Ok, we have a PHI node. Figure out what the incoming value was for the
1894 // DestBlock.
1895 Value *IV = PN.getIncomingValueForBlock(BB: OldPred);
1896
1897 // Remap the value if necessary.
1898 if (Instruction *Inst = dyn_cast<Instruction>(Val: IV)) {
1899 ValueToValueMapTy::iterator I = ValueMap.find(Val: Inst);
1900 if (I != ValueMap.end())
1901 IV = I->second;
1902 }
1903
1904 PN.addIncoming(V: IV, BB: NewPred);
1905 }
1906}
1907
1908/// Merge basic block BB into its sole predecessor if possible.
1909bool JumpThreadingPass::maybeMergeBasicBlockIntoOnlyPred(BasicBlock *BB) {
1910 BasicBlock *SinglePred = BB->getSinglePredecessor();
1911 if (!SinglePred)
1912 return false;
1913
1914 const Instruction *TI = SinglePred->getTerminator();
1915 if (TI->isSpecialTerminator() || TI->getNumSuccessors() != 1 ||
1916 SinglePred == BB || hasAddressTakenAndUsed(BB))
1917 return false;
1918
1919 // MergeBasicBlockIntoOnlyPred may delete SinglePred, we need to avoid
1920 // deleting a BB pointer from Unreachable.
1921 if (Unreachable.count(Ptr: SinglePred))
1922 return false;
1923
1924 // Don't merge if both the basic block and the predecessor contain loop or
1925 // entry convergent intrinsics, since there may only be one convergence token
1926 // per block.
1927 if (HasLoopOrEntryConvergenceToken(BB) &&
1928 HasLoopOrEntryConvergenceToken(BB: SinglePred))
1929 return false;
1930
1931 // If SinglePred was a loop header, BB becomes one.
1932 if (LoopHeaders.erase(Ptr: SinglePred))
1933 LoopHeaders.insert(Ptr: BB);
1934
1935 LVI->eraseBlock(BB: SinglePred);
1936 MergeBasicBlockIntoOnlyPred(BB, DTU: DTU.get());
1937
1938 // Now that BB is merged into SinglePred (i.e. SinglePred code followed by
1939 // BB code within one basic block `BB`), we need to invalidate the LVI
1940 // information associated with BB, because the LVI information need not be
1941 // true for all of BB after the merge. For example,
1942 // Before the merge, LVI info and code is as follows:
1943 // SinglePred: <LVI info1 for %p val>
1944 // %y = use of %p
1945 // call @exit() // need not transfer execution to successor.
1946 // assume(%p) // from this point on %p is true
1947 // br label %BB
1948 // BB: <LVI info2 for %p val, i.e. %p is true>
1949 // %x = use of %p
1950 // br label exit
1951 //
1952 // Note that this LVI info for blocks BB and SinglPred is correct for %p
1953 // (info2 and info1 respectively). After the merge and the deletion of the
1954 // LVI info1 for SinglePred. We have the following code:
1955 // BB: <LVI info2 for %p val>
1956 // %y = use of %p
1957 // call @exit()
1958 // assume(%p)
1959 // %x = use of %p <-- LVI info2 is correct from here onwards.
1960 // br label exit
1961 // LVI info2 for BB is incorrect at the beginning of BB.
1962
1963 // Invalidate LVI information for BB if the LVI is not provably true for
1964 // all of BB.
1965 if (!isGuaranteedToTransferExecutionToSuccessor(BB))
1966 LVI->eraseBlock(BB);
1967 return true;
1968}
1969
1970/// Update the SSA form. NewBB contains instructions that are copied from BB.
1971/// ValueMapping maps old values in BB to new ones in NewBB.
1972void JumpThreadingPass::updateSSA(BasicBlock *BB, BasicBlock *NewBB,
1973 ValueToValueMapTy &ValueMapping) {
1974 // If there were values defined in BB that are used outside the block, then we
1975 // now have to update all uses of the value to use either the original value,
1976 // the cloned value, or some PHI derived value. This can require arbitrary
1977 // PHI insertion, of which we are prepared to do, clean these up now.
1978 SSAUpdater SSAUpdate;
1979 SmallVector<Use *, 16> UsesToRename;
1980 SmallVector<DbgVariableRecord *, 4> DbgVariableRecords;
1981
1982 for (Instruction &I : *BB) {
1983 // Scan all uses of this instruction to see if it is used outside of its
1984 // block, and if so, record them in UsesToRename.
1985 for (Use &U : I.uses()) {
1986 Instruction *User = cast<Instruction>(Val: U.getUser());
1987 if (PHINode *UserPN = dyn_cast<PHINode>(Val: User)) {
1988 if (UserPN->getIncomingBlock(U) == BB)
1989 continue;
1990 } else if (User->getParent() == BB)
1991 continue;
1992
1993 UsesToRename.push_back(Elt: &U);
1994 }
1995
1996 // Find debug values outside of the block
1997 findDbgValues(V: &I, DbgVariableRecords);
1998 llvm::erase_if(C&: DbgVariableRecords, P: [&](const DbgVariableRecord *DbgVarRec) {
1999 return DbgVarRec->getParent() == BB;
2000 });
2001
2002 // If there are no uses outside the block, we're done with this instruction.
2003 if (UsesToRename.empty() && DbgVariableRecords.empty())
2004 continue;
2005 LLVM_DEBUG(dbgs() << "JT: Renaming non-local uses of: " << I << "\n");
2006
2007 // We found a use of I outside of BB. Rename all uses of I that are outside
2008 // its block to be uses of the appropriate PHI node etc. See ValuesInBlocks
2009 // with the two values we know.
2010 SSAUpdate.Initialize(Ty: I.getType(), Name: I.getName());
2011 SSAUpdate.AddAvailableValue(BB, V: &I);
2012 SSAUpdate.AddAvailableValue(BB: NewBB, V: ValueMapping[&I]);
2013
2014 while (!UsesToRename.empty())
2015 SSAUpdate.RewriteUse(U&: *UsesToRename.pop_back_val());
2016 if (!DbgVariableRecords.empty()) {
2017 SSAUpdate.UpdateDebugValues(I: &I, DbgValues&: DbgVariableRecords);
2018 DbgVariableRecords.clear();
2019 }
2020
2021 LLVM_DEBUG(dbgs() << "\n");
2022 }
2023}
2024
2025static void remapSourceAtoms(ValueToValueMapTy &VM, BasicBlock::iterator Begin,
2026 BasicBlock::iterator End) {
2027 if (VM.AtomMap.empty())
2028 return;
2029 for (auto It = Begin; It != End; ++It)
2030 RemapSourceAtom(I: &*It, VM);
2031}
2032
2033/// Clone instructions in range [BI, BE) to NewBB. For PHI nodes, we only clone
2034/// arguments that come from PredBB. Return the map from the variables in the
2035/// source basic block to the variables in the newly created basic block.
2036
2037void JumpThreadingPass::cloneInstructions(ValueToValueMapTy &ValueMapping,
2038 BasicBlock::iterator BI,
2039 BasicBlock::iterator BE,
2040 BasicBlock *NewBB,
2041 BasicBlock *PredBB) {
2042 // We are going to have to map operands from the source basic block to the new
2043 // copy of the block 'NewBB'. If there are PHI nodes in the source basic
2044 // block, evaluate them to account for entry from PredBB.
2045
2046 // Retargets dbg.value to any renamed variables.
2047 auto RetargetDbgVariableRecordIfPossible = [&](DbgVariableRecord *DVR) {
2048 SmallSet<std::pair<Value *, Value *>, 16> OperandsToRemap;
2049 for (auto *Op : DVR->location_ops()) {
2050 Instruction *OpInst = dyn_cast<Instruction>(Val: Op);
2051 if (!OpInst)
2052 continue;
2053
2054 auto I = ValueMapping.find(Val: OpInst);
2055 if (I != ValueMapping.end())
2056 OperandsToRemap.insert(V: {OpInst, I->second});
2057 }
2058
2059 for (auto &[OldOp, MappedOp] : OperandsToRemap)
2060 DVR->replaceVariableLocationOp(OldValue: OldOp, NewValue: MappedOp);
2061 };
2062
2063 BasicBlock *RangeBB = BI->getParent();
2064
2065 // Clone the phi nodes of the source basic block into NewBB. The resulting
2066 // phi nodes are trivial since NewBB only has one predecessor, but SSAUpdater
2067 // might need to rewrite the operand of the cloned phi.
2068 for (; PHINode *PN = dyn_cast<PHINode>(Val&: BI); ++BI) {
2069 PHINode *NewPN = PHINode::Create(Ty: PN->getType(), NumReservedValues: 1, NameStr: PN->getName(), InsertBefore: NewBB);
2070 NewPN->addIncoming(V: PN->getIncomingValueForBlock(BB: PredBB), BB: PredBB);
2071 ValueMapping[PN] = NewPN;
2072 if (const DebugLoc &DL = PN->getDebugLoc())
2073 mapAtomInstance(DL, VMap&: ValueMapping);
2074 }
2075
2076 // Clone noalias scope declarations in the threaded block. When threading a
2077 // loop exit, we would otherwise end up with two idential scope declarations
2078 // visible at the same time.
2079 SmallVector<MDNode *> NoAliasScopes;
2080 DenseMap<MDNode *, MDNode *> ClonedScopes;
2081 LLVMContext &Context = PredBB->getContext();
2082 identifyNoAliasScopesToClone(Start: BI, End: BE, NoAliasDeclScopes&: NoAliasScopes);
2083 cloneNoAliasScopes(NoAliasDeclScopes: NoAliasScopes, ClonedScopes, Ext: "thread", Context);
2084
2085 auto CloneAndRemapDbgInfo = [&](Instruction *NewInst, Instruction *From) {
2086 auto DVRRange = NewInst->cloneDebugInfoFrom(From);
2087 for (DbgVariableRecord &DVR : filterDbgVars(R: DVRRange))
2088 RetargetDbgVariableRecordIfPossible(&DVR);
2089 };
2090
2091 // Clone the non-phi instructions of the source basic block into NewBB,
2092 // keeping track of the mapping and using it to remap operands in the cloned
2093 // instructions.
2094 for (; BI != BE; ++BI) {
2095 Instruction *New = BI->clone();
2096 New->setName(BI->getName());
2097 New->insertInto(ParentBB: NewBB, It: NewBB->end());
2098 ValueMapping[&*BI] = New;
2099 adaptNoAliasScopes(I: New, ClonedScopes, Context);
2100
2101 CloneAndRemapDbgInfo(New, &*BI);
2102 if (const DebugLoc &DL = New->getDebugLoc())
2103 mapAtomInstance(DL, VMap&: ValueMapping);
2104
2105 // Remap operands to patch up intra-block references.
2106 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
2107 if (Instruction *Inst = dyn_cast<Instruction>(Val: New->getOperand(i))) {
2108 ValueToValueMapTy::iterator I = ValueMapping.find(Val: Inst);
2109 if (I != ValueMapping.end())
2110 New->setOperand(i, Val: I->second);
2111 }
2112 }
2113
2114 // There may be DbgVariableRecords on the terminator, clone directly from
2115 // marker to marker as there isn't an instruction there.
2116 if (BE != RangeBB->end() && BE->hasDbgRecords()) {
2117 // Dump them at the end.
2118 DbgMarker *Marker = RangeBB->getMarker(It: BE);
2119 DbgMarker *EndMarker = NewBB->createMarker(It: NewBB->end());
2120 auto DVRRange = EndMarker->cloneDebugInfoFrom(From: Marker, FromHere: std::nullopt);
2121 for (DbgVariableRecord &DVR : filterDbgVars(R: DVRRange))
2122 RetargetDbgVariableRecordIfPossible(&DVR);
2123 }
2124}
2125
2126/// Attempt to thread through two successive basic blocks.
2127bool JumpThreadingPass::maybethreadThroughTwoBasicBlocks(BasicBlock *BB,
2128 Value *Cond) {
2129 // Consider:
2130 //
2131 // PredBB:
2132 // %var = phi i32* [ null, %bb1 ], [ @a, %bb2 ]
2133 // %tobool = icmp eq i32 %cond, 0
2134 // br i1 %tobool, label %BB, label ...
2135 //
2136 // BB:
2137 // %cmp = icmp eq i32* %var, null
2138 // br i1 %cmp, label ..., label ...
2139 //
2140 // We don't know the value of %var at BB even if we know which incoming edge
2141 // we take to BB. However, once we duplicate PredBB for each of its incoming
2142 // edges (say, PredBB1 and PredBB2), we know the value of %var in each copy of
2143 // PredBB. Then we can thread edges PredBB1->BB and PredBB2->BB through BB.
2144
2145 // Require that BB end with a Branch for simplicity.
2146 CondBrInst *CondBr = dyn_cast<CondBrInst>(Val: BB->getTerminator());
2147 if (!CondBr)
2148 return false;
2149
2150 // BB must have exactly one predecessor.
2151 BasicBlock *PredBB = BB->getSinglePredecessor();
2152 if (!PredBB)
2153 return false;
2154
2155 // Require that PredBB end with a conditional Branch. If PredBB ends with an
2156 // unconditional branch, we should be merging PredBB and BB instead. For
2157 // simplicity, we don't deal with a switch.
2158 CondBrInst *PredBBBranch = dyn_cast<CondBrInst>(Val: PredBB->getTerminator());
2159 if (!PredBBBranch)
2160 return false;
2161
2162 // If PredBB has exactly one incoming edge, we don't gain anything by copying
2163 // PredBB.
2164 if (PredBB->getSinglePredecessor())
2165 return false;
2166
2167 // Don't thread through PredBB if it contains a successor edge to itself, in
2168 // which case we would infinite loop. Suppose we are threading an edge from
2169 // PredPredBB through PredBB and BB to SuccBB with PredBB containing a
2170 // successor edge to itself. If we allowed jump threading in this case, we
2171 // could duplicate PredBB and BB as, say, PredBB.thread and BB.thread. Since
2172 // PredBB.thread has a successor edge to PredBB, we would immediately come up
2173 // with another jump threading opportunity from PredBB.thread through PredBB
2174 // and BB to SuccBB. This jump threading would repeatedly occur. That is, we
2175 // would keep peeling one iteration from PredBB.
2176 if (llvm::is_contained(Range: successors(BB: PredBB), Element: PredBB))
2177 return false;
2178
2179 // Don't thread across a loop header.
2180 if (LoopHeaders.count(Ptr: PredBB))
2181 return false;
2182
2183 // Avoid complication with duplicating EH pads.
2184 if (PredBB->isEHPad())
2185 return false;
2186
2187 // Find a predecessor that we can thread. For simplicity, we only consider a
2188 // successor edge out of BB to which we thread exactly one incoming edge into
2189 // PredBB.
2190 unsigned ZeroCount = 0;
2191 unsigned OneCount = 0;
2192 BasicBlock *ZeroPred = nullptr;
2193 BasicBlock *OnePred = nullptr;
2194 const DataLayout &DL = BB->getDataLayout();
2195 for (BasicBlock *P : predecessors(BB: PredBB)) {
2196 // If PredPred ends with IndirectBrInst, we can't handle it.
2197 if (isa<IndirectBrInst>(Val: P->getTerminator()))
2198 continue;
2199 if (ConstantInt *CI = dyn_cast_or_null<ConstantInt>(
2200 Val: evaluateOnPredecessorEdge(BB, PredPredBB: P, V: Cond, DL))) {
2201 if (CI->isZero()) {
2202 ZeroCount++;
2203 ZeroPred = P;
2204 } else if (CI->isOne()) {
2205 OneCount++;
2206 OnePred = P;
2207 }
2208 }
2209 }
2210
2211 // Disregard complicated cases where we have to thread multiple edges.
2212 BasicBlock *PredPredBB;
2213 if (ZeroCount == 1) {
2214 PredPredBB = ZeroPred;
2215 } else if (OneCount == 1) {
2216 PredPredBB = OnePred;
2217 } else {
2218 return false;
2219 }
2220
2221 BasicBlock *SuccBB = CondBr->getSuccessor(i: PredPredBB == ZeroPred);
2222
2223 // If threading to the same block as we come from, we would infinite loop.
2224 if (SuccBB == BB) {
2225 LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
2226 << "' - would thread to self!\n");
2227 return false;
2228 }
2229
2230 // If threading this would thread across a loop header, don't thread the edge.
2231 // See the comments above findLoopHeaders for justifications and caveats.
2232 if (LoopHeaders.count(Ptr: BB) || LoopHeaders.count(Ptr: SuccBB)) {
2233 LLVM_DEBUG({
2234 bool BBIsHeader = LoopHeaders.count(BB);
2235 bool SuccIsHeader = LoopHeaders.count(SuccBB);
2236 dbgs() << " Not threading across "
2237 << (BBIsHeader ? "loop header BB '" : "block BB '")
2238 << BB->getName() << "' to dest "
2239 << (SuccIsHeader ? "loop header BB '" : "block BB '")
2240 << SuccBB->getName()
2241 << "' - it might create an irreducible loop!\n";
2242 });
2243 return false;
2244 }
2245
2246 // Compute the cost of duplicating BB and PredBB.
2247 unsigned BBCost = getJumpThreadDuplicationCost(
2248 TTI, BB, StopAt: BB->getTerminator(), Threshold: BBDupThreshold);
2249 unsigned PredBBCost = getJumpThreadDuplicationCost(
2250 TTI, BB: PredBB, StopAt: PredBB->getTerminator(), Threshold: BBDupThreshold);
2251
2252 // Give up if costs are too high. We need to check BBCost and PredBBCost
2253 // individually before checking their sum because getJumpThreadDuplicationCost
2254 // return (unsigned)~0 for those basic blocks that cannot be duplicated.
2255 if (BBCost > BBDupThreshold || PredBBCost > BBDupThreshold ||
2256 BBCost + PredBBCost > BBDupThreshold) {
2257 LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
2258 << "' - Cost is too high: " << PredBBCost
2259 << " for PredBB, " << BBCost << "for BB\n");
2260 return false;
2261 }
2262
2263 // Now we are ready to duplicate PredBB.
2264 threadThroughTwoBasicBlocks(PredPredBB, PredBB, BB, SuccBB);
2265 return true;
2266}
2267
2268void JumpThreadingPass::threadThroughTwoBasicBlocks(BasicBlock *PredPredBB,
2269 BasicBlock *PredBB,
2270 BasicBlock *BB,
2271 BasicBlock *SuccBB) {
2272 LLVM_DEBUG(dbgs() << " Threading through '" << PredBB->getName() << "' and '"
2273 << BB->getName() << "'\n");
2274
2275 // Build BPI/BFI before any changes are made to IR.
2276 bool HasProfile = doesBlockHaveProfileData(BB);
2277 auto *BFI = getOrCreateBFI(Force: HasProfile);
2278 auto *BPI = getOrCreateBPI(Force: BFI != nullptr);
2279
2280 CondBrInst *CondBr = cast<CondBrInst>(Val: BB->getTerminator());
2281 CondBrInst *PredBBBranch = cast<CondBrInst>(Val: PredBB->getTerminator());
2282
2283 BasicBlock *NewBB =
2284 BasicBlock::Create(Context&: PredBB->getContext(), Name: PredBB->getName() + ".thread",
2285 Parent: PredBB->getParent(), InsertBefore: PredBB);
2286 NewBB->moveAfter(MovePos: PredBB);
2287
2288 // Set the block frequency of NewBB.
2289 if (BFI) {
2290 assert(BPI && "It's expected BPI to exist along with BFI");
2291 auto NewBBFreq = BFI->getBlockFreq(BB: PredPredBB) *
2292 BPI->getEdgeProbability(Src: PredPredBB, Dst: PredBB);
2293 BFI->setBlockFreq(BB: NewBB, Freq: NewBBFreq);
2294 }
2295
2296 // We are going to have to map operands from the original BB block to the new
2297 // copy of the block 'NewBB'. If there are PHI nodes in PredBB, evaluate them
2298 // to account for entry from PredPredBB.
2299 ValueToValueMapTy ValueMapping;
2300 cloneInstructions(ValueMapping, BI: PredBB->begin(), BE: PredBB->end(), NewBB,
2301 PredBB: PredPredBB);
2302
2303 // Copy the edge probabilities from PredBB to NewBB.
2304 if (BPI)
2305 BPI->copyEdgeProbabilities(Src: PredBB, Dst: NewBB);
2306
2307 // Update the terminator of PredPredBB to jump to NewBB instead of PredBB.
2308 // This eliminates predecessors from PredPredBB, which requires us to simplify
2309 // any PHI nodes in PredBB.
2310 Instruction *PredPredTerm = PredPredBB->getTerminator();
2311 for (unsigned i = 0, e = PredPredTerm->getNumSuccessors(); i != e; ++i)
2312 if (PredPredTerm->getSuccessor(Idx: i) == PredBB) {
2313 PredBB->removePredecessor(Pred: PredPredBB, KeepOneInputPHIs: true);
2314 PredPredTerm->setSuccessor(Idx: i, BB: NewBB);
2315 }
2316
2317 addPHINodeEntriesForMappedBlock(PHIBB: PredBBBranch->getSuccessor(i: 0), OldPred: PredBB, NewPred: NewBB,
2318 ValueMap&: ValueMapping);
2319 addPHINodeEntriesForMappedBlock(PHIBB: PredBBBranch->getSuccessor(i: 1), OldPred: PredBB, NewPred: NewBB,
2320 ValueMap&: ValueMapping);
2321
2322 DTU->applyUpdatesPermissive(
2323 Updates: {{DominatorTree::Insert, NewBB, CondBr->getSuccessor(i: 0)},
2324 {DominatorTree::Insert, NewBB, CondBr->getSuccessor(i: 1)},
2325 {DominatorTree::Insert, PredPredBB, NewBB},
2326 {DominatorTree::Delete, PredPredBB, PredBB}});
2327
2328 // Remap source location atoms beacuse we're duplicating control flow.
2329 remapSourceAtoms(VM&: ValueMapping, Begin: NewBB->begin(), End: NewBB->end());
2330
2331 updateSSA(BB: PredBB, NewBB, ValueMapping);
2332
2333 // Clean up things like PHI nodes with single operands, dead instructions,
2334 // etc.
2335 SimplifyInstructionsInBlock(BB: NewBB, TLI);
2336 SimplifyInstructionsInBlock(BB: PredBB, TLI);
2337
2338 SmallVector<BasicBlock *, 1> PredsToFactor;
2339 PredsToFactor.push_back(Elt: NewBB);
2340 threadEdge(BB, PredBBs: PredsToFactor, SuccBB);
2341}
2342
2343/// tryThreadEdge - Thread an edge if it's safe and profitable to do so.
2344bool JumpThreadingPass::tryThreadEdge(
2345 BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs,
2346 BasicBlock *SuccBB) {
2347 // If threading to the same block as we come from, we would infinite loop.
2348 if (SuccBB == BB) {
2349 LLVM_DEBUG(dbgs() << " Not threading across BB '" << BB->getName()
2350 << "' - would thread to self!\n");
2351 return false;
2352 }
2353
2354 // If threading this would thread across a loop header, don't thread the edge.
2355 // See the comments above findLoopHeaders for justifications and caveats.
2356 if (LoopHeaders.count(Ptr: BB) || LoopHeaders.count(Ptr: SuccBB)) {
2357 LLVM_DEBUG({
2358 bool BBIsHeader = LoopHeaders.count(BB);
2359 bool SuccIsHeader = LoopHeaders.count(SuccBB);
2360 dbgs() << " Not threading across "
2361 << (BBIsHeader ? "loop header BB '" : "block BB '") << BB->getName()
2362 << "' to dest " << (SuccIsHeader ? "loop header BB '" : "block BB '")
2363 << SuccBB->getName() << "' - it might create an irreducible loop!\n";
2364 });
2365 return false;
2366 }
2367
2368 unsigned JumpThreadCost = getJumpThreadDuplicationCost(
2369 TTI, BB, StopAt: BB->getTerminator(), Threshold: BBDupThreshold);
2370 if (JumpThreadCost > BBDupThreshold) {
2371 LLVM_DEBUG(dbgs() << " Not threading BB '" << BB->getName()
2372 << "' - Cost is too high: " << JumpThreadCost << "\n");
2373 return false;
2374 }
2375
2376 threadEdge(BB, PredBBs, SuccBB);
2377 return true;
2378}
2379
2380/// threadEdge - We have decided that it is safe and profitable to factor the
2381/// blocks in PredBBs to one predecessor, then thread an edge from it to SuccBB
2382/// across BB. Transform the IR to reflect this change.
2383void JumpThreadingPass::threadEdge(BasicBlock *BB,
2384 const SmallVectorImpl<BasicBlock *> &PredBBs,
2385 BasicBlock *SuccBB) {
2386 assert(SuccBB != BB && "Don't create an infinite loop");
2387
2388 assert(!LoopHeaders.count(BB) && !LoopHeaders.count(SuccBB) &&
2389 "Don't thread across loop headers");
2390
2391 // Build BPI/BFI before any changes are made to IR.
2392 bool HasProfile = doesBlockHaveProfileData(BB);
2393 auto *BFI = getOrCreateBFI(Force: HasProfile);
2394 auto *BPI = getOrCreateBPI(Force: BFI != nullptr);
2395
2396 // And finally, do it! Start by factoring the predecessors if needed.
2397 BasicBlock *PredBB;
2398 if (PredBBs.size() == 1)
2399 PredBB = PredBBs[0];
2400 else {
2401 LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
2402 << " common predecessors.\n");
2403 PredBB = splitBlockPreds(BB, Preds: PredBBs, Suffix: ".thr_comm");
2404 }
2405
2406 // And finally, do it!
2407 LLVM_DEBUG(dbgs() << " Threading edge from '" << PredBB->getName()
2408 << "' to '" << SuccBB->getName()
2409 << ", across block:\n " << *BB << "\n");
2410
2411 LVI->threadEdge(PredBB, OldSucc: BB, NewSucc: SuccBB);
2412
2413 BasicBlock *NewBB = BasicBlock::Create(Context&: BB->getContext(),
2414 Name: BB->getName()+".thread",
2415 Parent: BB->getParent(), InsertBefore: BB);
2416 NewBB->moveAfter(MovePos: PredBB);
2417
2418 // Set the block frequency of NewBB.
2419 if (BFI) {
2420 assert(BPI && "It's expected BPI to exist along with BFI");
2421 auto NewBBFreq =
2422 BFI->getBlockFreq(BB: PredBB) * BPI->getEdgeProbability(Src: PredBB, Dst: BB);
2423 BFI->setBlockFreq(BB: NewBB, Freq: NewBBFreq);
2424 }
2425
2426 // Copy all the instructions from BB to NewBB except the terminator.
2427 ValueToValueMapTy ValueMapping;
2428 cloneInstructions(ValueMapping, BI: BB->begin(), BE: std::prev(x: BB->end()), NewBB,
2429 PredBB);
2430
2431 // We didn't copy the terminator from BB over to NewBB, because there is now
2432 // an unconditional jump to SuccBB. Insert the unconditional jump.
2433 UncondBrInst *NewBI = UncondBrInst::Create(Target: SuccBB, InsertBefore: NewBB);
2434 NewBI->setDebugLoc(BB->getTerminator()->getDebugLoc());
2435
2436 // Check to see if SuccBB has PHI nodes. If so, we need to add entries to the
2437 // PHI nodes for NewBB now.
2438 addPHINodeEntriesForMappedBlock(PHIBB: SuccBB, OldPred: BB, NewPred: NewBB, ValueMap&: ValueMapping);
2439
2440 // Update the terminator of PredBB to jump to NewBB instead of BB. This
2441 // eliminates predecessors from BB, which requires us to simplify any PHI
2442 // nodes in BB.
2443 Instruction *PredTerm = PredBB->getTerminator();
2444 for (unsigned i = 0, e = PredTerm->getNumSuccessors(); i != e; ++i)
2445 if (PredTerm->getSuccessor(Idx: i) == BB) {
2446 BB->removePredecessor(Pred: PredBB, KeepOneInputPHIs: true);
2447 PredTerm->setSuccessor(Idx: i, BB: NewBB);
2448 }
2449
2450 // Enqueue required DT updates.
2451 DTU->applyUpdatesPermissive(Updates: {{DominatorTree::Insert, NewBB, SuccBB},
2452 {DominatorTree::Insert, PredBB, NewBB},
2453 {DominatorTree::Delete, PredBB, BB}});
2454
2455 remapSourceAtoms(VM&: ValueMapping, Begin: NewBB->begin(), End: NewBB->end());
2456 updateSSA(BB, NewBB, ValueMapping);
2457
2458 // At this point, the IR is fully up to date and consistent. Do a quick scan
2459 // over the new instructions and zap any that are constants or dead. This
2460 // frequently happens because of phi translation.
2461 SimplifyInstructionsInBlock(BB: NewBB, TLI);
2462
2463 // Update the edge weight from BB to SuccBB, which should be less than before.
2464 updateBlockFreqAndEdgeWeight(PredBB, BB, NewBB, SuccBB, BFI, BPI, HasProfile);
2465
2466 // Threaded an edge!
2467 ++NumThreads;
2468}
2469
2470/// Create a new basic block that will be the predecessor of BB and successor of
2471/// all blocks in Preds. When profile data is available, update the frequency of
2472/// this new block.
2473BasicBlock *JumpThreadingPass::splitBlockPreds(BasicBlock *BB,
2474 ArrayRef<BasicBlock *> Preds,
2475 const char *Suffix) {
2476 SmallVector<BasicBlock *, 2> NewBBs;
2477
2478 // Collect the frequencies of all predecessors of BB, which will be used to
2479 // update the edge weight of the result of splitting predecessors.
2480 DenseMap<BasicBlock *, BlockFrequency> FreqMap;
2481 auto *BFI = getBFI();
2482 if (BFI) {
2483 auto *BPI = getOrCreateBPI(Force: true);
2484 for (auto *Pred : Preds)
2485 FreqMap.insert(KV: std::make_pair(
2486 x&: Pred, y: BFI->getBlockFreq(BB: Pred) * BPI->getEdgeProbability(Src: Pred, Dst: BB)));
2487 }
2488
2489 // In the case when BB is a LandingPad block we create 2 new predecessors
2490 // instead of just one.
2491 if (BB->isLandingPad()) {
2492 std::string NewName = std::string(Suffix) + ".split-lp";
2493 SplitLandingPadPredecessors(OrigBB: BB, Preds, Suffix, Suffix2: NewName.c_str(), NewBBs);
2494 } else {
2495 NewBBs.push_back(Elt: SplitBlockPredecessors(BB, Preds, Suffix));
2496 }
2497
2498 std::vector<DominatorTree::UpdateType> Updates;
2499 Updates.reserve(n: (2 * Preds.size()) + NewBBs.size());
2500 for (auto *NewBB : NewBBs) {
2501 BlockFrequency NewBBFreq(0);
2502 Updates.push_back(x: {DominatorTree::Insert, NewBB, BB});
2503 for (auto *Pred : predecessors(BB: NewBB)) {
2504 Updates.push_back(x: {DominatorTree::Delete, Pred, BB});
2505 Updates.push_back(x: {DominatorTree::Insert, Pred, NewBB});
2506 if (BFI) // Update frequencies between Pred -> NewBB.
2507 NewBBFreq += FreqMap.lookup(Val: Pred);
2508 }
2509 if (BFI) // Apply the summed frequency to NewBB.
2510 BFI->setBlockFreq(BB: NewBB, Freq: NewBBFreq);
2511 }
2512
2513 DTU->applyUpdatesPermissive(Updates);
2514 return NewBBs[0];
2515}
2516
2517bool JumpThreadingPass::doesBlockHaveProfileData(BasicBlock *BB) {
2518 const Instruction *TI = BB->getTerminator();
2519 if (!TI || TI->getNumSuccessors() < 2)
2520 return false;
2521
2522 return hasValidBranchWeightMD(I: *TI);
2523}
2524
2525/// Update the block frequency of BB and branch weight and the metadata on the
2526/// edge BB->SuccBB. This is done by scaling the weight of BB->SuccBB by 1 -
2527/// Freq(PredBB->BB) / Freq(BB->SuccBB).
2528void JumpThreadingPass::updateBlockFreqAndEdgeWeight(BasicBlock *PredBB,
2529 BasicBlock *BB,
2530 BasicBlock *NewBB,
2531 BasicBlock *SuccBB,
2532 BlockFrequencyInfo *BFI,
2533 BranchProbabilityInfo *BPI,
2534 bool HasProfile) {
2535 assert(((BFI && BPI) || (!BFI && !BFI)) &&
2536 "Both BFI & BPI should either be set or unset");
2537
2538 if (!BFI) {
2539 assert(!HasProfile &&
2540 "It's expected to have BFI/BPI when profile info exists");
2541 return;
2542 }
2543
2544 // As the edge from PredBB to BB is deleted, we have to update the block
2545 // frequency of BB.
2546 auto BBOrigFreq = BFI->getBlockFreq(BB);
2547 auto NewBBFreq = BFI->getBlockFreq(BB: NewBB);
2548 auto BBNewFreq = BBOrigFreq - NewBBFreq;
2549 BFI->setBlockFreq(BB, Freq: BBNewFreq);
2550
2551 // Collect updated outgoing edges' frequencies from BB and use them to update
2552 // edge probabilities.
2553 SmallVector<uint64_t, 4> BBSuccFreq;
2554 for (auto It : enumerate(First: successors(BB))) {
2555 auto BB2SuccBBFreq = BBOrigFreq * BPI->getEdgeProbability(Src: BB, IndexInSuccessors: It.index());
2556 auto SuccFreq =
2557 (It.value() == SuccBB) ? BB2SuccBBFreq - NewBBFreq : BB2SuccBBFreq;
2558 BBSuccFreq.push_back(Elt: SuccFreq.getFrequency());
2559 }
2560
2561 uint64_t MaxBBSuccFreq = *llvm::max_element(Range&: BBSuccFreq);
2562
2563 SmallVector<BranchProbability, 4> BBSuccProbs;
2564 if (MaxBBSuccFreq == 0)
2565 BBSuccProbs.assign(NumElts: BBSuccFreq.size(),
2566 Elt: {1, static_cast<unsigned>(BBSuccFreq.size())});
2567 else {
2568 for (uint64_t Freq : BBSuccFreq)
2569 BBSuccProbs.push_back(
2570 Elt: BranchProbability::getBranchProbability(Numerator: Freq, Denominator: MaxBBSuccFreq));
2571 // Normalize edge probabilities so that they sum up to one.
2572 BranchProbability::normalizeProbabilities(Begin: BBSuccProbs.begin(),
2573 End: BBSuccProbs.end());
2574 }
2575
2576 // Update edge probabilities in BPI.
2577 BPI->setEdgeProbability(Src: BB, Probs: BBSuccProbs);
2578
2579 // Update the profile metadata as well.
2580 //
2581 // Don't do this if the profile of the transformed blocks was statically
2582 // estimated. (This could occur despite the function having an entry
2583 // frequency in completely cold parts of the CFG.)
2584 //
2585 // In this case we don't want to suggest to subsequent passes that the
2586 // calculated weights are fully consistent. Consider this graph:
2587 //
2588 // check_1
2589 // 50% / |
2590 // eq_1 | 50%
2591 // \ |
2592 // check_2
2593 // 50% / |
2594 // eq_2 | 50%
2595 // \ |
2596 // check_3
2597 // 50% / |
2598 // eq_3 | 50%
2599 // \ |
2600 //
2601 // Assuming the blocks check_* all compare the same value against 1, 2 and 3,
2602 // the overall probabilities are inconsistent; the total probability that the
2603 // value is either 1, 2 or 3 is 150%.
2604 //
2605 // As a consequence if we thread eq_1 -> check_2 to check_3, check_2->check_3
2606 // becomes 0%. This is even worse if the edge whose probability becomes 0% is
2607 // the loop exit edge. Then based solely on static estimation we would assume
2608 // the loop was extremely hot.
2609 //
2610 // FIXME this locally as well so that BPI and BFI are consistent as well. We
2611 // shouldn't make edges extremely likely or unlikely based solely on static
2612 // estimation.
2613 if (BBSuccProbs.size() >= 2 && HasProfile) {
2614 SmallVector<uint32_t, 4> Weights;
2615 for (auto Prob : BBSuccProbs)
2616 Weights.push_back(Elt: Prob.getNumerator());
2617
2618 auto TI = BB->getTerminator();
2619 setBranchWeights(I&: *TI, Weights, IsExpected: hasBranchWeightOrigin(I: *TI));
2620 }
2621}
2622
2623/// duplicateCondBranchOnPHIIntoPred - PredBB contains an unconditional branch
2624/// to BB which contains an i1 PHI node and a conditional branch on that PHI.
2625/// If we can duplicate the contents of BB up into PredBB do so now, this
2626/// improves the odds that the branch will be on an analyzable instruction like
2627/// a compare.
2628bool JumpThreadingPass::duplicateCondBranchOnPHIIntoPred(
2629 BasicBlock *BB, const SmallVectorImpl<BasicBlock *> &PredBBs) {
2630 assert(!PredBBs.empty() && "Can't handle an empty set");
2631
2632 // If BB is a loop header, then duplicating this block outside the loop would
2633 // cause us to transform this into an irreducible loop, don't do this.
2634 // See the comments above findLoopHeaders for justifications and caveats.
2635 if (LoopHeaders.count(Ptr: BB)) {
2636 LLVM_DEBUG(dbgs() << " Not duplicating loop header '" << BB->getName()
2637 << "' into predecessor block '" << PredBBs[0]->getName()
2638 << "' - it might create an irreducible loop!\n");
2639 return false;
2640 }
2641
2642 unsigned DuplicationCost = getJumpThreadDuplicationCost(
2643 TTI, BB, StopAt: BB->getTerminator(), Threshold: BBDupThreshold);
2644 if (DuplicationCost > BBDupThreshold) {
2645 LLVM_DEBUG(dbgs() << " Not duplicating BB '" << BB->getName()
2646 << "' - Cost is too high: " << DuplicationCost << "\n");
2647 return false;
2648 }
2649
2650 // And finally, do it! Start by factoring the predecessors if needed.
2651 std::vector<DominatorTree::UpdateType> Updates;
2652 BasicBlock *PredBB;
2653 if (PredBBs.size() == 1)
2654 PredBB = PredBBs[0];
2655 else {
2656 LLVM_DEBUG(dbgs() << " Factoring out " << PredBBs.size()
2657 << " common predecessors.\n");
2658 PredBB = splitBlockPreds(BB, Preds: PredBBs, Suffix: ".thr_comm");
2659 }
2660 Updates.push_back(x: {DominatorTree::Delete, PredBB, BB});
2661
2662 // Okay, we decided to do this! Clone all the instructions in BB onto the end
2663 // of PredBB.
2664 LLVM_DEBUG(dbgs() << " Duplicating block '" << BB->getName()
2665 << "' into end of '" << PredBB->getName()
2666 << "' to eliminate branch on phi. Cost: "
2667 << DuplicationCost << " block is:" << *BB << "\n");
2668
2669 // Unless PredBB ends with an unconditional branch, split the edge so that we
2670 // can just clone the bits from BB into the end of the new PredBB.
2671 UncondBrInst *OldPredBranch = dyn_cast<UncondBrInst>(Val: PredBB->getTerminator());
2672
2673 if (!OldPredBranch) {
2674 BasicBlock *OldPredBB = PredBB;
2675 PredBB = SplitEdge(From: OldPredBB, To: BB);
2676 Updates.push_back(x: {DominatorTree::Insert, OldPredBB, PredBB});
2677 Updates.push_back(x: {DominatorTree::Insert, PredBB, BB});
2678 Updates.push_back(x: {DominatorTree::Delete, OldPredBB, BB});
2679 OldPredBranch = cast<UncondBrInst>(Val: PredBB->getTerminator());
2680 }
2681
2682 // We are going to have to map operands from the original BB block into the
2683 // PredBB block. Evaluate PHI nodes in BB.
2684 ValueToValueMapTy ValueMapping;
2685
2686 // Remember the position before the inserted instructions.
2687 auto RItBeforeInsertPt = std::next(x: OldPredBranch->getReverseIterator());
2688
2689 BasicBlock::iterator BI = BB->begin();
2690 for (; PHINode *PN = dyn_cast<PHINode>(Val&: BI); ++BI)
2691 ValueMapping[PN] = PN->getIncomingValueForBlock(BB: PredBB);
2692 // Clone the non-phi instructions of BB into PredBB, keeping track of the
2693 // mapping and using it to remap operands in the cloned instructions.
2694 for (; BI != BB->end(); ++BI) {
2695 Instruction *New = BI->clone();
2696 New->insertInto(ParentBB: PredBB, It: OldPredBranch->getIterator());
2697
2698 // Remap operands to patch up intra-block references.
2699 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
2700 if (Instruction *Inst = dyn_cast<Instruction>(Val: New->getOperand(i))) {
2701 ValueToValueMapTy::iterator I = ValueMapping.find(Val: Inst);
2702 if (I != ValueMapping.end())
2703 New->setOperand(i, Val: I->second);
2704 }
2705
2706 // Remap debug variable operands.
2707 remapDebugVariable(Mapping&: ValueMapping, Inst: New);
2708 if (const DebugLoc &DL = New->getDebugLoc())
2709 mapAtomInstance(DL, VMap&: ValueMapping);
2710
2711 // If this instruction can be simplified after the operands are updated,
2712 // just use the simplified value instead. This frequently happens due to
2713 // phi translation.
2714 if (Value *IV = simplifyInstruction(
2715 I: New,
2716 Q: {BB->getDataLayout(), TLI, nullptr, nullptr, New})) {
2717 ValueMapping[&*BI] = IV;
2718 if (!New->mayHaveSideEffects()) {
2719 New->eraseFromParent();
2720 New = nullptr;
2721 // Clone debug-info on the elided instruction to the destination
2722 // position.
2723 OldPredBranch->cloneDebugInfoFrom(From: &*BI, FromHere: std::nullopt, InsertAtHead: true);
2724 }
2725 } else {
2726 ValueMapping[&*BI] = New;
2727 }
2728 if (New) {
2729 // Otherwise, insert the new instruction into the block.
2730 New->setName(BI->getName());
2731 // Clone across any debug-info attached to the old instruction.
2732 New->cloneDebugInfoFrom(From: &*BI);
2733 // Update Dominance from simplified New instruction operands.
2734 for (unsigned i = 0, e = New->getNumOperands(); i != e; ++i)
2735 if (BasicBlock *SuccBB = dyn_cast<BasicBlock>(Val: New->getOperand(i)))
2736 Updates.push_back(x: {DominatorTree::Insert, PredBB, SuccBB});
2737 }
2738 }
2739
2740 // Check to see if the targets of the branch had PHI nodes. If so, we need to
2741 // add entries to the PHI nodes for branch from PredBB now.
2742 CondBrInst *BBBranch = cast<CondBrInst>(Val: BB->getTerminator());
2743 addPHINodeEntriesForMappedBlock(PHIBB: BBBranch->getSuccessor(i: 0), OldPred: BB, NewPred: PredBB,
2744 ValueMap&: ValueMapping);
2745 addPHINodeEntriesForMappedBlock(PHIBB: BBBranch->getSuccessor(i: 1), OldPred: BB, NewPred: PredBB,
2746 ValueMap&: ValueMapping);
2747
2748 // KeyInstructions: Remap the cloned instructions' atoms only.
2749 remapSourceAtoms(VM&: ValueMapping, Begin: std::prev(x: RItBeforeInsertPt)->getIterator(),
2750 End: OldPredBranch->getIterator());
2751
2752 updateSSA(BB, NewBB: PredBB, ValueMapping);
2753
2754 // PredBB no longer jumps to BB, remove entries in the PHI node for the edge
2755 // that we nuked.
2756 BB->removePredecessor(Pred: PredBB, KeepOneInputPHIs: true);
2757
2758 // Remove the unconditional branch at the end of the PredBB block.
2759 OldPredBranch->eraseFromParent();
2760 if (auto *BPI = getBPI())
2761 BPI->copyEdgeProbabilities(Src: BB, Dst: PredBB);
2762 DTU->applyUpdatesPermissive(Updates);
2763
2764 ++NumDupes;
2765 return true;
2766}
2767
2768// Pred is a predecessor of BB with an unconditional branch to BB. SI is
2769// a Select instruction in Pred. BB has other predecessors and SI is used in
2770// a PHI node in BB. SI has no other use.
2771// A new basic block, NewBB, is created and SI is converted to compare and
2772// conditional branch. SI is erased from parent.
2773void JumpThreadingPass::unfoldSelectInstr(BasicBlock *Pred, BasicBlock *BB,
2774 SelectInst *SI, PHINode *SIUse,
2775 unsigned Idx) {
2776 // Expand the select.
2777 //
2778 // Pred --
2779 // | v
2780 // | NewBB
2781 // | |
2782 // |-----
2783 // v
2784 // BB
2785 UncondBrInst *PredTerm = cast<UncondBrInst>(Val: Pred->getTerminator());
2786 BasicBlock *NewBB = BasicBlock::Create(Context&: BB->getContext(), Name: "select.unfold",
2787 Parent: BB->getParent(), InsertBefore: BB);
2788 // Move the unconditional branch to NewBB.
2789 PredTerm->removeFromParent();
2790 PredTerm->insertInto(ParentBB: NewBB, It: NewBB->end());
2791 // Create a conditional branch and update PHI nodes.
2792 auto *BI = CondBrInst::Create(Cond: SI->getCondition(), IfTrue: NewBB, IfFalse: BB, InsertBefore: Pred);
2793 BI->applyMergedLocation(LocA: PredTerm->getDebugLoc(), LocB: SI->getDebugLoc());
2794 BI->copyMetadata(SrcInst: *SI, WL: {LLVMContext::MD_prof});
2795 SIUse->setIncomingValue(i: Idx, V: SI->getFalseValue());
2796 SIUse->addIncoming(V: SI->getTrueValue(), BB: NewBB);
2797
2798 uint64_t TrueWeight = 1;
2799 uint64_t FalseWeight = 1;
2800 // Copy probabilities from 'SI' to created conditional branch in 'Pred'.
2801 if (extractBranchWeights(I: *SI, TrueVal&: TrueWeight, FalseVal&: FalseWeight) &&
2802 (TrueWeight + FalseWeight) != 0) {
2803 SmallVector<BranchProbability, 2> BP;
2804 BP.emplace_back(Args: BranchProbability::getBranchProbability(
2805 Numerator: TrueWeight, Denominator: TrueWeight + FalseWeight));
2806 BP.emplace_back(Args: BranchProbability::getBranchProbability(
2807 Numerator: FalseWeight, Denominator: TrueWeight + FalseWeight));
2808 // Update BPI if exists.
2809 if (auto *BPI = getBPI())
2810 BPI->setEdgeProbability(Src: Pred, Probs: BP);
2811 }
2812 // Set the block frequency of NewBB.
2813 if (auto *BFI = getBFI()) {
2814 if ((TrueWeight + FalseWeight) == 0) {
2815 TrueWeight = 1;
2816 FalseWeight = 1;
2817 }
2818 BranchProbability PredToNewBBProb = BranchProbability::getBranchProbability(
2819 Numerator: TrueWeight, Denominator: TrueWeight + FalseWeight);
2820 auto NewBBFreq = BFI->getBlockFreq(BB: Pred) * PredToNewBBProb;
2821 BFI->setBlockFreq(BB: NewBB, Freq: NewBBFreq);
2822 }
2823
2824 // The select is now dead.
2825 SI->eraseFromParent();
2826 DTU->applyUpdatesPermissive(Updates: {{DominatorTree::Insert, NewBB, BB},
2827 {DominatorTree::Insert, Pred, NewBB}});
2828
2829 // Update any other PHI nodes in BB.
2830 for (BasicBlock::iterator BI = BB->begin();
2831 PHINode *Phi = dyn_cast<PHINode>(Val&: BI); ++BI)
2832 if (Phi != SIUse)
2833 Phi->addIncoming(V: Phi->getIncomingValueForBlock(BB: Pred), BB: NewBB);
2834}
2835
2836bool JumpThreadingPass::tryToUnfoldSelect(SwitchInst *SI, BasicBlock *BB) {
2837 PHINode *CondPHI = dyn_cast<PHINode>(Val: SI->getCondition());
2838
2839 if (!CondPHI || CondPHI->getParent() != BB)
2840 return false;
2841
2842 for (unsigned I = 0, E = CondPHI->getNumIncomingValues(); I != E; ++I) {
2843 BasicBlock *Pred = CondPHI->getIncomingBlock(i: I);
2844 SelectInst *PredSI = dyn_cast<SelectInst>(Val: CondPHI->getIncomingValue(i: I));
2845
2846 // The second and third condition can be potentially relaxed. Currently
2847 // the conditions help to simplify the code and allow us to reuse existing
2848 // code, developed for tryToUnfoldSelect(CmpInst *, BasicBlock *)
2849 if (!PredSI || PredSI->getParent() != Pred || !PredSI->hasOneUse())
2850 continue;
2851
2852 UncondBrInst *PredTerm = dyn_cast<UncondBrInst>(Val: Pred->getTerminator());
2853 if (!PredTerm)
2854 continue;
2855
2856 unfoldSelectInstr(Pred, BB, SI: PredSI, SIUse: CondPHI, Idx: I);
2857 return true;
2858 }
2859 return false;
2860}
2861
2862/// tryToUnfoldSelect - Look for blocks of the form
2863/// bb1:
2864/// %a = select
2865/// br bb2
2866///
2867/// bb2:
2868/// %p = phi [%a, %bb1] ...
2869/// %c = icmp %p
2870/// br i1 %c
2871///
2872/// And expand the select into a branch structure if one of its arms allows %c
2873/// to be folded. This later enables threading from bb1 over bb2.
2874bool JumpThreadingPass::tryToUnfoldSelect(CmpInst *CondCmp, BasicBlock *BB) {
2875 CondBrInst *CondBr = dyn_cast<CondBrInst>(Val: BB->getTerminator());
2876 PHINode *CondLHS = dyn_cast<PHINode>(Val: CondCmp->getOperand(i_nocapture: 0));
2877 Constant *CondRHS = cast<Constant>(Val: CondCmp->getOperand(i_nocapture: 1));
2878
2879 if (!CondBr || !CondLHS || CondLHS->getParent() != BB)
2880 return false;
2881
2882 for (unsigned I = 0, E = CondLHS->getNumIncomingValues(); I != E; ++I) {
2883 BasicBlock *Pred = CondLHS->getIncomingBlock(i: I);
2884 SelectInst *SI = dyn_cast<SelectInst>(Val: CondLHS->getIncomingValue(i: I));
2885
2886 // Look if one of the incoming values is a select in the corresponding
2887 // predecessor.
2888 if (!SI || SI->getParent() != Pred || !SI->hasOneUse())
2889 continue;
2890
2891 UncondBrInst *PredTerm = dyn_cast<UncondBrInst>(Val: Pred->getTerminator());
2892 if (!PredTerm)
2893 continue;
2894
2895 // Now check if one of the select values would allow us to constant fold the
2896 // terminator in BB. We don't do the transform if both sides fold, those
2897 // cases will be threaded in any case.
2898 Constant *LHSRes =
2899 LVI->getPredicateOnEdge(Pred: CondCmp->getPredicate(), V: SI->getOperand(i_nocapture: 1),
2900 C: CondRHS, FromBB: Pred, ToBB: BB, CxtI: CondCmp);
2901 Constant *RHSRes =
2902 LVI->getPredicateOnEdge(Pred: CondCmp->getPredicate(), V: SI->getOperand(i_nocapture: 2),
2903 C: CondRHS, FromBB: Pred, ToBB: BB, CxtI: CondCmp);
2904 if ((LHSRes || RHSRes) && LHSRes != RHSRes) {
2905 unfoldSelectInstr(Pred, BB, SI, SIUse: CondLHS, Idx: I);
2906 return true;
2907 }
2908 }
2909 return false;
2910}
2911
2912/// tryToUnfoldSelectInCurrBB - Look for PHI/Select or PHI/CMP/Select in the
2913/// same BB in the form
2914/// bb:
2915/// %p = phi [false, %bb1], [true, %bb2], [false, %bb3], [true, %bb4], ...
2916/// %s = select %p, trueval, falseval
2917///
2918/// or
2919///
2920/// bb:
2921/// %p = phi [0, %bb1], [1, %bb2], [0, %bb3], [1, %bb4], ...
2922/// %c = cmp %p, 0
2923/// %s = select %c, trueval, falseval
2924///
2925/// And expand the select into a branch structure. This later enables
2926/// jump-threading over bb in this pass.
2927///
2928/// Using the similar approach of SimplifyCFG::FoldCondBranchOnPHI(), unfold
2929/// select if the associated PHI has at least one constant. If the unfolded
2930/// select is not jump-threaded, it will be folded again in the later
2931/// optimizations.
2932bool JumpThreadingPass::tryToUnfoldSelectInCurrBB(BasicBlock *BB) {
2933 // This transform would reduce the quality of msan diagnostics.
2934 // Disable this transform under MemorySanitizer.
2935 if (BB->getParent()->hasFnAttribute(Kind: Attribute::SanitizeMemory))
2936 return false;
2937
2938 // If threading this would thread across a loop header, don't thread the edge.
2939 // See the comments above findLoopHeaders for justifications and caveats.
2940 if (LoopHeaders.count(Ptr: BB))
2941 return false;
2942
2943 for (BasicBlock::iterator BI = BB->begin();
2944 PHINode *PN = dyn_cast<PHINode>(Val&: BI); ++BI) {
2945 // Look for a Phi having at least one constant incoming value.
2946 if (llvm::all_of(Range: PN->incoming_values(),
2947 P: [](Value *V) { return !isa<ConstantInt>(Val: V); }))
2948 continue;
2949
2950 auto isUnfoldCandidate = [BB](SelectInst *SI, Value *V) {
2951 using namespace PatternMatch;
2952
2953 // Check if SI is in BB and use V as condition.
2954 if (SI->getParent() != BB)
2955 return false;
2956 Value *Cond = SI->getCondition();
2957 bool IsAndOr = match(V: SI, P: m_CombineOr(L: m_LogicalAnd(), R: m_LogicalOr()));
2958 return Cond && Cond == V && Cond->getType()->isIntegerTy(Bitwidth: 1) && !IsAndOr;
2959 };
2960
2961 SelectInst *SI = nullptr;
2962 for (Use &U : PN->uses()) {
2963 if (ICmpInst *Cmp = dyn_cast<ICmpInst>(Val: U.getUser())) {
2964 // Look for a ICmp in BB that compares PN with a constant and is the
2965 // condition of a Select.
2966 if (Cmp->getParent() == BB && Cmp->hasOneUse() &&
2967 isa<ConstantInt>(Val: Cmp->getOperand(i_nocapture: 1 - U.getOperandNo())))
2968 if (SelectInst *SelectI = dyn_cast<SelectInst>(Val: Cmp->user_back()))
2969 if (isUnfoldCandidate(SelectI, Cmp->use_begin()->get())) {
2970 SI = SelectI;
2971 break;
2972 }
2973 } else if (SelectInst *SelectI = dyn_cast<SelectInst>(Val: U.getUser())) {
2974 // Look for a Select in BB that uses PN as condition.
2975 if (isUnfoldCandidate(SelectI, U.get())) {
2976 SI = SelectI;
2977 break;
2978 }
2979 }
2980 }
2981
2982 if (!SI)
2983 continue;
2984 // Expand the select.
2985 Value *Cond = SI->getCondition();
2986 if (!isGuaranteedNotToBeUndefOrPoison(V: Cond, AC: nullptr, CtxI: SI)) {
2987 Cond = new FreezeInst(Cond, "cond.fr", SI->getIterator());
2988 cast<FreezeInst>(Val: Cond)->setDebugLoc(DebugLoc::getTemporary());
2989 }
2990 MDNode *BranchWeights = getBranchWeightMDNode(I: *SI);
2991 Instruction *Term =
2992 SplitBlockAndInsertIfThen(Cond, SplitBefore: SI, Unreachable: false, BranchWeights);
2993 BasicBlock *SplitBB = SI->getParent();
2994 BasicBlock *NewBB = Term->getParent();
2995 PHINode *NewPN = PHINode::Create(Ty: SI->getType(), NumReservedValues: 2, NameStr: "", InsertBefore: SI->getIterator());
2996 NewPN->addIncoming(V: SI->getTrueValue(), BB: Term->getParent());
2997 NewPN->addIncoming(V: SI->getFalseValue(), BB);
2998 NewPN->setDebugLoc(SI->getDebugLoc());
2999 SI->replaceAllUsesWith(V: NewPN);
3000
3001 auto *BPI = getBPI();
3002 auto *BFI = getBFI();
3003 if (!ProfcheckDisableMetadataFixes && BranchWeights) {
3004 SmallVector<uint32_t, 2> BW;
3005 [[maybe_unused]] bool Extracted = extractBranchWeights(ProfileData: BranchWeights, Weights&: BW);
3006 assert(Extracted);
3007 uint64_t Denominator =
3008 sum_of(Range: llvm::map_range(C&: BW, F: StaticCastTo<uint64_t>));
3009 assert(Denominator > 0 &&
3010 "At least one of the branch probabilities should be non-zero");
3011 BranchProbability TrueProb =
3012 BranchProbability::getBranchProbability(Numerator: BW[0], Denominator);
3013 BranchProbability FalseProb =
3014 BranchProbability::getBranchProbability(Numerator: BW[1], Denominator);
3015 SmallVector<BranchProbability, 2> BP = {TrueProb, FalseProb};
3016
3017 if (BPI)
3018 BPI->setEdgeProbability(Src: BB, Probs: BP);
3019
3020 if (BFI) {
3021 auto BBOrigFreq = BFI->getBlockFreq(BB);
3022 auto NewBBFreq = BBOrigFreq * TrueProb;
3023 BFI->setBlockFreq(BB: NewBB, Freq: NewBBFreq);
3024 BFI->setBlockFreq(BB: SplitBB, Freq: BBOrigFreq);
3025 }
3026 }
3027 SI->eraseFromParent();
3028 // NewBB and SplitBB are newly created blocks which require insertion.
3029 std::vector<DominatorTree::UpdateType> Updates;
3030 Updates.reserve(n: (2 * SplitBB->getTerminator()->getNumSuccessors()) + 3);
3031 Updates.push_back(x: {DominatorTree::Insert, BB, SplitBB});
3032 Updates.push_back(x: {DominatorTree::Insert, BB, NewBB});
3033 Updates.push_back(x: {DominatorTree::Insert, NewBB, SplitBB});
3034 // BB's successors were moved to SplitBB, update DTU accordingly.
3035 for (auto *Succ : successors(BB: SplitBB)) {
3036 Updates.push_back(x: {DominatorTree::Delete, BB, Succ});
3037 Updates.push_back(x: {DominatorTree::Insert, SplitBB, Succ});
3038 }
3039 DTU->applyUpdatesPermissive(Updates);
3040 return true;
3041 }
3042 return false;
3043}
3044
3045/// Try to propagate a guard from the current BB into one of its predecessors
3046/// in case if another branch of execution implies that the condition of this
3047/// guard is always true. Currently we only process the simplest case that
3048/// looks like:
3049///
3050/// Start:
3051/// %cond = ...
3052/// br i1 %cond, label %T1, label %F1
3053/// T1:
3054/// br label %Merge
3055/// F1:
3056/// br label %Merge
3057/// Merge:
3058/// %condGuard = ...
3059/// call void(i1, ...) @llvm.experimental.guard( i1 %condGuard )[ "deopt"() ]
3060///
3061/// And cond either implies condGuard or !condGuard. In this case all the
3062/// instructions before the guard can be duplicated in both branches, and the
3063/// guard is then threaded to one of them.
3064bool JumpThreadingPass::processGuards(BasicBlock *BB) {
3065 using namespace PatternMatch;
3066
3067 // We only want to deal with two predecessors.
3068 BasicBlock *Pred1, *Pred2;
3069 auto PI = pred_begin(BB), PE = pred_end(BB);
3070 if (PI == PE)
3071 return false;
3072 Pred1 = *PI++;
3073 if (PI == PE)
3074 return false;
3075 Pred2 = *PI++;
3076 if (PI != PE)
3077 return false;
3078 if (Pred1 == Pred2)
3079 return false;
3080
3081 // Try to thread one of the guards of the block.
3082 // TODO: Look up deeper than to immediate predecessor?
3083 auto *Parent = Pred1->getSinglePredecessor();
3084 if (!Parent || Parent != Pred2->getSinglePredecessor())
3085 return false;
3086
3087 if (auto *BI = dyn_cast<CondBrInst>(Val: Parent->getTerminator()))
3088 for (auto &I : *BB)
3089 if (isGuard(U: &I) && threadGuard(BB, Guard: cast<IntrinsicInst>(Val: &I), BI))
3090 return true;
3091
3092 return false;
3093}
3094
3095/// Try to propagate the guard from BB which is the lower block of a diamond
3096/// to one of its branches, in case if diamond's condition implies guard's
3097/// condition.
3098bool JumpThreadingPass::threadGuard(BasicBlock *BB, IntrinsicInst *Guard,
3099 CondBrInst *BI) {
3100 Value *GuardCond = Guard->getArgOperand(i: 0);
3101 Value *BranchCond = BI->getCondition();
3102 BasicBlock *TrueDest = BI->getSuccessor(i: 0);
3103 BasicBlock *FalseDest = BI->getSuccessor(i: 1);
3104
3105 auto &DL = BB->getDataLayout();
3106 bool TrueDestIsSafe = false;
3107 bool FalseDestIsSafe = false;
3108
3109 // True dest is safe if BranchCond => GuardCond.
3110 auto Impl = isImpliedCondition(LHS: BranchCond, RHS: GuardCond, DL);
3111 if (Impl && *Impl)
3112 TrueDestIsSafe = true;
3113 else {
3114 // False dest is safe if !BranchCond => GuardCond.
3115 Impl = isImpliedCondition(LHS: BranchCond, RHS: GuardCond, DL, /* LHSIsTrue */ false);
3116 if (Impl && *Impl)
3117 FalseDestIsSafe = true;
3118 }
3119
3120 if (!TrueDestIsSafe && !FalseDestIsSafe)
3121 return false;
3122
3123 BasicBlock *PredUnguardedBlock = TrueDestIsSafe ? TrueDest : FalseDest;
3124 BasicBlock *PredGuardedBlock = FalseDestIsSafe ? TrueDest : FalseDest;
3125
3126 ValueToValueMapTy UnguardedMapping, GuardedMapping;
3127 Instruction *AfterGuard = Guard->getNextNode();
3128 unsigned Cost =
3129 getJumpThreadDuplicationCost(TTI, BB, StopAt: AfterGuard, Threshold: BBDupThreshold);
3130 if (Cost > BBDupThreshold)
3131 return false;
3132 // Duplicate all instructions before the guard and the guard itself to the
3133 // branch where implication is not proved.
3134 BasicBlock *GuardedBlock = DuplicateInstructionsInSplitBetween(
3135 BB, PredBB: PredGuardedBlock, StopAt: AfterGuard, ValueMapping&: GuardedMapping, DTU&: *DTU);
3136 assert(GuardedBlock && "Could not create the guarded block?");
3137 // Duplicate all instructions before the guard in the unguarded branch.
3138 // Since we have successfully duplicated the guarded block and this block
3139 // has fewer instructions, we expect it to succeed.
3140 BasicBlock *UnguardedBlock = DuplicateInstructionsInSplitBetween(
3141 BB, PredBB: PredUnguardedBlock, StopAt: Guard, ValueMapping&: UnguardedMapping, DTU&: *DTU);
3142 assert(UnguardedBlock && "Could not create the unguarded block?");
3143 LLVM_DEBUG(dbgs() << "Moved guard " << *Guard << " to block "
3144 << GuardedBlock->getName() << "\n");
3145 // Some instructions before the guard may still have uses. For them, we need
3146 // to create Phi nodes merging their copies in both guarded and unguarded
3147 // branches. Those instructions that have no uses can be just removed.
3148 SmallVector<Instruction *, 4> ToRemove;
3149 for (auto BI = BB->begin(); &*BI != AfterGuard; ++BI)
3150 if (!isa<PHINode>(Val: &*BI))
3151 ToRemove.push_back(Elt: &*BI);
3152
3153 BasicBlock::iterator InsertionPoint = BB->getFirstInsertionPt();
3154 assert(InsertionPoint != BB->end() && "Empty block?");
3155 // Substitute with Phis & remove.
3156 for (auto *Inst : reverse(C&: ToRemove)) {
3157 if (!Inst->use_empty()) {
3158 PHINode *NewPN = PHINode::Create(Ty: Inst->getType(), NumReservedValues: 2);
3159 NewPN->addIncoming(V: UnguardedMapping[Inst], BB: UnguardedBlock);
3160 NewPN->addIncoming(V: GuardedMapping[Inst], BB: GuardedBlock);
3161 NewPN->setDebugLoc(Inst->getDebugLoc());
3162 NewPN->insertBefore(InsertPos: InsertionPoint);
3163 Inst->replaceAllUsesWith(V: NewPN);
3164 }
3165 Inst->dropDbgRecords();
3166 Inst->eraseFromParent();
3167 }
3168 return true;
3169}
3170
3171PreservedAnalyses JumpThreadingPass::getPreservedAnalysis() const {
3172 PreservedAnalyses PA;
3173 PA.preserve<LazyValueAnalysis>();
3174 PA.preserve<DominatorTreeAnalysis>();
3175
3176 // TODO: We would like to preserve BPI/BFI. Enable once all paths update them.
3177 // TODO: Would be nice to verify BPI/BFI consistency as well.
3178 return PA;
3179}
3180
3181template <typename AnalysisT>
3182typename AnalysisT::Result *JumpThreadingPass::runExternalAnalysis() {
3183 assert(FAM && "Can't run external analysis without FunctionAnalysisManager");
3184
3185 // If there were no changes since last call to 'runExternalAnalysis' then all
3186 // analysis is either up to date or explicitly invalidated. Just go ahead and
3187 // run the "external" analysis.
3188 if (!ChangedSinceLastAnalysisUpdate) {
3189 assert(!DTU->hasPendingUpdates() &&
3190 "Lost update of 'ChangedSinceLastAnalysisUpdate'?");
3191 // Run the "external" analysis.
3192 return &FAM->getResult<AnalysisT>(*F);
3193 }
3194 ChangedSinceLastAnalysisUpdate = false;
3195
3196 auto PA = getPreservedAnalysis();
3197 // TODO: This shouldn't be needed once 'getPreservedAnalysis' reports BPI/BFI
3198 // as preserved.
3199 PA.preserve<BranchProbabilityAnalysis>();
3200 PA.preserve<BlockFrequencyAnalysis>();
3201 // Report everything except explicitly preserved as invalid.
3202 FAM->invalidate(IR&: *F, PA);
3203 // Update DT/PDT.
3204 DTU->flush();
3205 // Make sure DT/PDT are valid before running "external" analysis.
3206 assert(DTU->getDomTree().verify(DominatorTree::VerificationLevel::Fast));
3207 assert((!DTU->hasPostDomTree() ||
3208 DTU->getPostDomTree().verify(
3209 PostDominatorTree::VerificationLevel::Fast)));
3210 // Run the "external" analysis.
3211 auto *Result = &FAM->getResult<AnalysisT>(*F);
3212 // Update analysis JumpThreading depends on and not explicitly preserved.
3213 TTI = &FAM->getResult<TargetIRAnalysis>(IR&: *F);
3214 TLI = &FAM->getResult<TargetLibraryAnalysis>(IR&: *F);
3215 AA = &FAM->getResult<AAManager>(IR&: *F);
3216
3217 return Result;
3218}
3219
3220BranchProbabilityInfo *JumpThreadingPass::getBPI() {
3221 if (!BPI) {
3222 assert(FAM && "Can't create BPI without FunctionAnalysisManager");
3223 BPI = FAM->getCachedResult<BranchProbabilityAnalysis>(IR&: *F);
3224 }
3225 return BPI;
3226}
3227
3228BlockFrequencyInfo *JumpThreadingPass::getBFI() {
3229 if (!BFI) {
3230 assert(FAM && "Can't create BFI without FunctionAnalysisManager");
3231 BFI = FAM->getCachedResult<BlockFrequencyAnalysis>(IR&: *F);
3232 }
3233 return BFI;
3234}
3235
3236// Important note on validity of BPI/BFI. JumpThreading tries to preserve
3237// BPI/BFI as it goes. Thus if cached instance exists it will be updated.
3238// Otherwise, new instance of BPI/BFI is created (up to date by definition).
3239BranchProbabilityInfo *JumpThreadingPass::getOrCreateBPI(bool Force) {
3240 auto *Res = getBPI();
3241 if (Res)
3242 return Res;
3243
3244 if (Force)
3245 BPI = runExternalAnalysis<BranchProbabilityAnalysis>();
3246
3247 return BPI;
3248}
3249
3250BlockFrequencyInfo *JumpThreadingPass::getOrCreateBFI(bool Force) {
3251 auto *Res = getBFI();
3252 if (Res)
3253 return Res;
3254
3255 if (Force)
3256 BFI = runExternalAnalysis<BlockFrequencyAnalysis>();
3257
3258 return BFI;
3259}
3260