1//===- SimplifyCFGPass.cpp - CFG Simplification Pass ----------------------===//
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 dead code elimination and basic block merging, along
10// with a collection of other peephole control flow optimizations. For example:
11//
12// * Removes basic blocks with no predecessors.
13// * Merges a basic block into its predecessor if there is only one and the
14// predecessor only has one successor.
15// * Eliminates PHI nodes for basic blocks with a single predecessor.
16// * Eliminates a basic block that only contains an unconditional branch.
17// * Changes invoke instructions to nounwind functions to be calls.
18// * Change things like "if (x) if (y)" into "if (x&y)".
19// * etc..
20//
21//===----------------------------------------------------------------------===//
22
23#include "llvm/ADT/MapVector.h"
24#include "llvm/ADT/SmallPtrSet.h"
25#include "llvm/ADT/SmallVector.h"
26#include "llvm/ADT/Statistic.h"
27#include "llvm/Analysis/AssumptionCache.h"
28#include "llvm/Analysis/CFG.h"
29#include "llvm/Analysis/DomTreeUpdater.h"
30#include "llvm/Analysis/GlobalsModRef.h"
31#include "llvm/Analysis/TargetTransformInfo.h"
32#include "llvm/IR/Attributes.h"
33#include "llvm/IR/CFG.h"
34#include "llvm/IR/Dominators.h"
35#include "llvm/IR/Instructions.h"
36#include "llvm/IR/ValueHandle.h"
37#include "llvm/InitializePasses.h"
38#include "llvm/Pass.h"
39#include "llvm/Support/CommandLine.h"
40#include "llvm/Transforms/Scalar.h"
41#include "llvm/Transforms/Scalar/SimplifyCFG.h"
42#include "llvm/Transforms/Utils/Local.h"
43#include "llvm/Transforms/Utils/SimplifyCFGOptions.h"
44#include <utility>
45using namespace llvm;
46
47#define DEBUG_TYPE "simplifycfg"
48
49static cl::opt<unsigned> UserBonusInstThreshold(
50 "bonus-inst-threshold", cl::Hidden, cl::init(Val: 1),
51 cl::desc("Control the number of bonus instructions (default = 1)"));
52
53static cl::opt<bool> UserKeepLoops(
54 "keep-loops", cl::Hidden, cl::init(Val: true),
55 cl::desc("Preserve canonical loop structure (default = true)"));
56
57static cl::opt<bool> UserSwitchRangeToICmp(
58 "switch-range-to-icmp", cl::Hidden, cl::init(Val: false),
59 cl::desc(
60 "Convert switches into an integer range comparison (default = false)"));
61
62static cl::opt<bool> UserSwitchToLookup(
63 "switch-to-lookup", cl::Hidden, cl::init(Val: false),
64 cl::desc("Convert switches to lookup tables (default = false)"));
65
66static cl::opt<bool> UserForwardSwitchCond(
67 "forward-switch-cond", cl::Hidden, cl::init(Val: false),
68 cl::desc("Forward switch condition to phi ops (default = false)"));
69
70static cl::opt<bool> UserHoistCommonInsts(
71 "hoist-common-insts", cl::Hidden, cl::init(Val: false),
72 cl::desc("hoist common instructions (default = false)"));
73
74static cl::opt<bool> UserHoistLoadsStoresWithCondFaulting(
75 "hoist-loads-stores-with-cond-faulting", cl::Hidden, cl::init(Val: false),
76 cl::desc("Hoist loads/stores if the target supports conditional faulting "
77 "(default = false)"));
78
79static cl::opt<bool> UserSinkCommonInsts(
80 "sink-common-insts", cl::Hidden, cl::init(Val: false),
81 cl::desc("Sink common instructions (default = false)"));
82
83static cl::opt<bool> UserSpeculateUnpredictables(
84 "speculate-unpredictables", cl::Hidden, cl::init(Val: false),
85 cl::desc("Speculate unpredictable branches (default = false)"));
86
87STATISTIC(NumSimpl, "Number of blocks simplified");
88
89static bool
90performBlockTailMerging(Function &F, ArrayRef<BasicBlock *> BBs,
91 std::vector<DominatorTree::UpdateType> *Updates) {
92 SmallVector<PHINode *, 1> NewOps;
93
94 // We don't want to change IR just because we can.
95 // Only do that if there are at least two blocks we'll tail-merge.
96 if (BBs.size() < 2)
97 return false;
98
99 if (Updates)
100 Updates->reserve(n: Updates->size() + BBs.size());
101
102 BasicBlock *CanonicalBB;
103 Instruction *CanonicalTerm;
104 {
105 auto *Term = BBs[0]->getTerminator();
106
107 // Create a canonical block for this function terminator type now,
108 // placing it *before* the first block that will branch to it.
109 CanonicalBB = BasicBlock::Create(
110 Context&: F.getContext(), Name: Twine("common.") + Term->getOpcodeName(), Parent: &F, InsertBefore: BBs[0]);
111 // We'll also need a PHI node per each operand of the terminator.
112 NewOps.resize(N: Term->getNumOperands());
113 for (auto I : zip(t: Term->operands(), u&: NewOps)) {
114 std::get<1>(t&: I) = PHINode::Create(Ty: std::get<0>(t&: I)->getType(),
115 /*NumReservedValues=*/BBs.size(),
116 NameStr: CanonicalBB->getName() + ".op");
117 std::get<1>(t&: I)->insertInto(ParentBB: CanonicalBB, It: CanonicalBB->end());
118 }
119 // Make it so that this canonical block actually has the right
120 // terminator.
121 CanonicalTerm = Term->clone();
122 CanonicalTerm->insertInto(ParentBB: CanonicalBB, It: CanonicalBB->end());
123 // If the canonical terminator has operands, rewrite it to take PHI's.
124 for (auto I : zip(t&: NewOps, u: CanonicalTerm->operands()))
125 std::get<1>(t&: I) = std::get<0>(t&: I);
126 }
127
128 // Now, go through each block (with the current terminator type)
129 // we've recorded, and rewrite it to branch to the new common block.
130 DebugLoc CommonDebugLoc;
131 for (BasicBlock *BB : BBs) {
132 auto *Term = BB->getTerminator();
133 assert(Term->getOpcode() == CanonicalTerm->getOpcode() &&
134 "All blocks to be tail-merged must be the same "
135 "(function-terminating) terminator type.");
136
137 // Aha, found a new non-canonical function terminator. If it has operands,
138 // forward them to the PHI nodes in the canonical block.
139 for (auto I : zip(t: Term->operands(), u&: NewOps))
140 std::get<1>(t&: I)->addIncoming(V: std::get<0>(t&: I), BB);
141
142 // Compute the debug location common to all the original terminators.
143 if (!CommonDebugLoc)
144 CommonDebugLoc = Term->getDebugLoc();
145 else
146 CommonDebugLoc =
147 DebugLoc::getMergedLocation(LocA: CommonDebugLoc, LocB: Term->getDebugLoc());
148
149 // And turn BB into a block that just unconditionally branches
150 // to the canonical block.
151 Instruction *BI = BranchInst::Create(IfTrue: CanonicalBB, InsertBefore: BB);
152 BI->setDebugLoc(Term->getDebugLoc());
153 Term->eraseFromParent();
154
155 if (Updates)
156 Updates->push_back(x: {DominatorTree::Insert, BB, CanonicalBB});
157 }
158
159 CanonicalTerm->setDebugLoc(CommonDebugLoc);
160
161 return true;
162}
163
164static bool tailMergeBlocksWithSimilarFunctionTerminators(Function &F,
165 DomTreeUpdater *DTU) {
166 SmallMapVector<unsigned /*TerminatorOpcode*/, SmallVector<BasicBlock *, 2>, 4>
167 Structure;
168
169 // Scan all the blocks in the function, record the interesting-ones.
170 for (BasicBlock &BB : F) {
171 if (DTU && DTU->isBBPendingDeletion(DelBB: &BB))
172 continue;
173
174 // We are only interested in function-terminating blocks.
175 if (!succ_empty(BB: &BB))
176 continue;
177
178 auto *Term = BB.getTerminator();
179
180 // Fow now only support `ret`/`resume` function terminators.
181 // FIXME: lift this restriction.
182 switch (Term->getOpcode()) {
183 case Instruction::Ret:
184 case Instruction::Resume:
185 break;
186 default:
187 continue;
188 }
189
190 // We can't tail-merge block that contains a musttail call.
191 if (BB.getTerminatingMustTailCall())
192 continue;
193
194 // Calls to experimental_deoptimize must be followed by a return
195 // of the value computed by experimental_deoptimize.
196 // I.e., we can not change `ret` to `br` for this block.
197 if (auto *CI = dyn_cast_or_null<CallInst>(Val: Term->getPrevNode())) {
198 if (Function *F = CI->getCalledFunction())
199 if (Intrinsic::ID ID = F->getIntrinsicID())
200 if (ID == Intrinsic::experimental_deoptimize)
201 continue;
202 }
203
204 // PHI nodes cannot have token type, so if the terminator has an operand
205 // with token type, we can not tail-merge this kind of function terminators.
206 if (any_of(Range: Term->operands(),
207 P: [](Value *Op) { return Op->getType()->isTokenTy(); }))
208 continue;
209
210 // Canonical blocks are uniqued based on the terminator type (opcode).
211 Structure[Term->getOpcode()].emplace_back(Args: &BB);
212 }
213
214 bool Changed = false;
215
216 std::vector<DominatorTree::UpdateType> Updates;
217
218 for (ArrayRef<BasicBlock *> BBs : make_second_range(c&: Structure))
219 Changed |= performBlockTailMerging(F, BBs, Updates: DTU ? &Updates : nullptr);
220
221 if (DTU)
222 DTU->applyUpdates(Updates);
223
224 return Changed;
225}
226
227/// Call SimplifyCFG on all the blocks in the function,
228/// iterating until no more changes are made.
229static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI,
230 DomTreeUpdater *DTU,
231 const SimplifyCFGOptions &Options) {
232 bool Changed = false;
233 bool LocalChange = true;
234
235 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges;
236 FindFunctionBackedges(F, Result&: Edges);
237 SmallPtrSet<BasicBlock *, 16> UniqueLoopHeaders;
238 for (const auto &Edge : Edges)
239 UniqueLoopHeaders.insert(Ptr: const_cast<BasicBlock *>(Edge.second));
240
241 SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(),
242 UniqueLoopHeaders.end());
243
244 unsigned IterCnt = 0;
245 (void)IterCnt;
246 while (LocalChange) {
247 assert(IterCnt++ < 2000 && "Iterative simplification didn't converge!");
248 LocalChange = false;
249
250 // Loop over all of the basic blocks and remove them if they are unneeded.
251 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) {
252 BasicBlock &BB = *BBIt++;
253 if (DTU) {
254 assert(
255 !DTU->isBBPendingDeletion(&BB) &&
256 "Should not end up trying to simplify blocks marked for removal.");
257 // Make sure that the advanced iterator does not point at the blocks
258 // that are marked for removal, skip over all such blocks.
259 while (BBIt != F.end() && DTU->isBBPendingDeletion(DelBB: &*BBIt))
260 ++BBIt;
261 }
262 if (simplifyCFG(BB: &BB, TTI, DTU, Options, LoopHeaders)) {
263 LocalChange = true;
264 ++NumSimpl;
265 }
266 }
267 Changed |= LocalChange;
268 }
269 return Changed;
270}
271
272static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI,
273 DominatorTree *DT,
274 const SimplifyCFGOptions &Options) {
275 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager);
276
277 bool EverChanged = removeUnreachableBlocks(F, DTU: DT ? &DTU : nullptr);
278 EverChanged |=
279 tailMergeBlocksWithSimilarFunctionTerminators(F, DTU: DT ? &DTU : nullptr);
280 EverChanged |= iterativelySimplifyCFG(F, TTI, DTU: DT ? &DTU : nullptr, Options);
281
282 // If neither pass changed anything, we're done.
283 if (!EverChanged) return false;
284
285 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens,
286 // removeUnreachableBlocks is needed to nuke them, which means we should
287 // iterate between the two optimizations. We structure the code like this to
288 // avoid rerunning iterativelySimplifyCFG if the second pass of
289 // removeUnreachableBlocks doesn't do anything.
290 if (!removeUnreachableBlocks(F, DTU: DT ? &DTU : nullptr))
291 return true;
292
293 do {
294 EverChanged = iterativelySimplifyCFG(F, TTI, DTU: DT ? &DTU : nullptr, Options);
295 EverChanged |= removeUnreachableBlocks(F, DTU: DT ? &DTU : nullptr);
296 } while (EverChanged);
297
298 return true;
299}
300
301static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI,
302 DominatorTree *DT,
303 const SimplifyCFGOptions &Options) {
304 assert((!RequireAndPreserveDomTree ||
305 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
306 "Original domtree is invalid?");
307
308 bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options);
309
310 assert((!RequireAndPreserveDomTree ||
311 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) &&
312 "Failed to maintain validity of domtree!");
313
314 return Changed;
315}
316
317// Command-line settings override compile-time settings.
318static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) {
319 if (UserBonusInstThreshold.getNumOccurrences())
320 Options.BonusInstThreshold = UserBonusInstThreshold;
321 if (UserForwardSwitchCond.getNumOccurrences())
322 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond;
323 if (UserSwitchRangeToICmp.getNumOccurrences())
324 Options.ConvertSwitchRangeToICmp = UserSwitchRangeToICmp;
325 if (UserSwitchToLookup.getNumOccurrences())
326 Options.ConvertSwitchToLookupTable = UserSwitchToLookup;
327 if (UserKeepLoops.getNumOccurrences())
328 Options.NeedCanonicalLoop = UserKeepLoops;
329 if (UserHoistCommonInsts.getNumOccurrences())
330 Options.HoistCommonInsts = UserHoistCommonInsts;
331 if (UserHoistLoadsStoresWithCondFaulting.getNumOccurrences())
332 Options.HoistLoadsStoresWithCondFaulting =
333 UserHoistLoadsStoresWithCondFaulting;
334 if (UserSinkCommonInsts.getNumOccurrences())
335 Options.SinkCommonInsts = UserSinkCommonInsts;
336 if (UserSpeculateUnpredictables.getNumOccurrences())
337 Options.SpeculateUnpredictables = UserSpeculateUnpredictables;
338}
339
340SimplifyCFGPass::SimplifyCFGPass() {
341 applyCommandLineOverridesToOptions(Options);
342}
343
344SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts)
345 : Options(Opts) {
346 applyCommandLineOverridesToOptions(Options);
347}
348
349void SimplifyCFGPass::printPipeline(
350 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) {
351 static_cast<PassInfoMixin<SimplifyCFGPass> *>(this)->printPipeline(
352 OS, MapClassName2PassName);
353 OS << '<';
354 OS << "bonus-inst-threshold=" << Options.BonusInstThreshold << ';';
355 OS << (Options.ForwardSwitchCondToPhi ? "" : "no-") << "forward-switch-cond;";
356 OS << (Options.ConvertSwitchRangeToICmp ? "" : "no-")
357 << "switch-range-to-icmp;";
358 OS << (Options.ConvertSwitchToArithmetic ? "" : "no-")
359 << "switch-to-arithmetic;";
360 OS << (Options.ConvertSwitchToLookupTable ? "" : "no-")
361 << "switch-to-lookup;";
362 OS << (Options.NeedCanonicalLoop ? "" : "no-") << "keep-loops;";
363 OS << (Options.HoistCommonInsts ? "" : "no-") << "hoist-common-insts;";
364 OS << (Options.HoistLoadsStoresWithCondFaulting ? "" : "no-")
365 << "hoist-loads-stores-with-cond-faulting;";
366 OS << (Options.SinkCommonInsts ? "" : "no-") << "sink-common-insts;";
367 OS << (Options.SpeculateBlocks ? "" : "no-") << "speculate-blocks;";
368 OS << (Options.SimplifyCondBranch ? "" : "no-") << "simplify-cond-branch;";
369 OS << (Options.SpeculateUnpredictables ? "" : "no-")
370 << "speculate-unpredictables";
371 OS << '>';
372}
373
374PreservedAnalyses SimplifyCFGPass::run(Function &F,
375 FunctionAnalysisManager &AM) {
376 auto &TTI = AM.getResult<TargetIRAnalysis>(IR&: F);
377 Options.AC = &AM.getResult<AssumptionAnalysis>(IR&: F);
378 DominatorTree *DT = nullptr;
379 if (RequireAndPreserveDomTree)
380 DT = &AM.getResult<DominatorTreeAnalysis>(IR&: F);
381 if (!simplifyFunctionCFG(F, TTI, DT, Options))
382 return PreservedAnalyses::all();
383 PreservedAnalyses PA;
384 if (RequireAndPreserveDomTree)
385 PA.preserve<DominatorTreeAnalysis>();
386 return PA;
387}
388
389namespace {
390struct CFGSimplifyPass : public FunctionPass {
391 static char ID;
392 SimplifyCFGOptions Options;
393 std::function<bool(const Function &)> PredicateFtor;
394
395 CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(),
396 std::function<bool(const Function &)> Ftor = nullptr)
397 : FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) {
398
399 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry());
400
401 // Check for command-line overrides of options for debug/customization.
402 applyCommandLineOverridesToOptions(Options);
403 }
404
405 bool runOnFunction(Function &F) override {
406 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F)))
407 return false;
408
409 Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F);
410 DominatorTree *DT = nullptr;
411 if (RequireAndPreserveDomTree)
412 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree();
413
414 auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F);
415 return simplifyFunctionCFG(F, TTI, DT, Options);
416 }
417 void getAnalysisUsage(AnalysisUsage &AU) const override {
418 AU.addRequired<AssumptionCacheTracker>();
419 if (RequireAndPreserveDomTree)
420 AU.addRequired<DominatorTreeWrapperPass>();
421 AU.addRequired<TargetTransformInfoWrapperPass>();
422 if (RequireAndPreserveDomTree)
423 AU.addPreserved<DominatorTreeWrapperPass>();
424 AU.addPreserved<GlobalsAAWrapperPass>();
425 }
426};
427}
428
429char CFGSimplifyPass::ID = 0;
430INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
431 false)
432INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass)
433INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker)
434INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass)
435INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false,
436 false)
437
438// Public interface to the CFGSimplification pass
439FunctionPass *
440llvm::createCFGSimplificationPass(SimplifyCFGOptions Options,
441 std::function<bool(const Function &)> Ftor) {
442 return new CFGSimplifyPass(Options, std::move(Ftor));
443}
444