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