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/DebugInfoMetadata.h" 35 #include "llvm/IR/Dominators.h" 36 #include "llvm/IR/Instructions.h" 37 #include "llvm/IR/IntrinsicInst.h" 38 #include "llvm/IR/ValueHandle.h" 39 #include "llvm/InitializePasses.h" 40 #include "llvm/Pass.h" 41 #include "llvm/Support/CommandLine.h" 42 #include "llvm/Transforms/Scalar.h" 43 #include "llvm/Transforms/Scalar/SimplifyCFG.h" 44 #include "llvm/Transforms/Utils/Local.h" 45 #include "llvm/Transforms/Utils/SimplifyCFGOptions.h" 46 #include <utility> 47 using namespace llvm; 48 49 #define DEBUG_TYPE "simplifycfg" 50 51 static cl::opt<unsigned> UserBonusInstThreshold( 52 "bonus-inst-threshold", cl::Hidden, cl::init(1), 53 cl::desc("Control the number of bonus instructions (default = 1)")); 54 55 static cl::opt<bool> UserKeepLoops( 56 "keep-loops", cl::Hidden, cl::init(true), 57 cl::desc("Preserve canonical loop structure (default = true)")); 58 59 static cl::opt<bool> UserSwitchRangeToICmp( 60 "switch-range-to-icmp", cl::Hidden, cl::init(false), 61 cl::desc( 62 "Convert switches into an integer range comparison (default = false)")); 63 64 static cl::opt<bool> UserSwitchToLookup( 65 "switch-to-lookup", cl::Hidden, cl::init(false), 66 cl::desc("Convert switches to lookup tables (default = false)")); 67 68 static cl::opt<bool> UserForwardSwitchCond( 69 "forward-switch-cond", cl::Hidden, cl::init(false), 70 cl::desc("Forward switch condition to phi ops (default = false)")); 71 72 static cl::opt<bool> UserHoistCommonInsts( 73 "hoist-common-insts", cl::Hidden, cl::init(false), 74 cl::desc("hoist common instructions (default = false)")); 75 76 static cl::opt<bool> UserSinkCommonInsts( 77 "sink-common-insts", cl::Hidden, cl::init(false), 78 cl::desc("Sink common instructions (default = false)")); 79 80 static cl::opt<bool> UserSpeculateUnpredictables( 81 "speculate-unpredictables", cl::Hidden, cl::init(false), 82 cl::desc("Speculate unpredictable branches (default = false)")); 83 84 STATISTIC(NumSimpl, "Number of blocks simplified"); 85 86 static bool 87 performBlockTailMerging(Function &F, ArrayRef<BasicBlock *> BBs, 88 std::vector<DominatorTree::UpdateType> *Updates) { 89 SmallVector<PHINode *, 1> NewOps; 90 91 // We don't want to change IR just because we can. 92 // Only do that if there are at least two blocks we'll tail-merge. 93 if (BBs.size() < 2) 94 return false; 95 96 if (Updates) 97 Updates->reserve(Updates->size() + BBs.size()); 98 99 BasicBlock *CanonicalBB; 100 Instruction *CanonicalTerm; 101 { 102 auto *Term = BBs[0]->getTerminator(); 103 104 // Create a canonical block for this function terminator type now, 105 // placing it *before* the first block that will branch to it. 106 CanonicalBB = BasicBlock::Create( 107 F.getContext(), Twine("common.") + Term->getOpcodeName(), &F, BBs[0]); 108 // We'll also need a PHI node per each operand of the terminator. 109 NewOps.resize(Term->getNumOperands()); 110 for (auto I : zip(Term->operands(), NewOps)) { 111 std::get<1>(I) = PHINode::Create(std::get<0>(I)->getType(), 112 /*NumReservedValues=*/BBs.size(), 113 CanonicalBB->getName() + ".op"); 114 std::get<1>(I)->insertInto(CanonicalBB, CanonicalBB->end()); 115 } 116 // Make it so that this canonical block actually has the right 117 // terminator. 118 CanonicalTerm = Term->clone(); 119 CanonicalTerm->insertInto(CanonicalBB, CanonicalBB->end()); 120 // If the canonical terminator has operands, rewrite it to take PHI's. 121 for (auto I : zip(NewOps, CanonicalTerm->operands())) 122 std::get<1>(I) = std::get<0>(I); 123 } 124 125 // Now, go through each block (with the current terminator type) 126 // we've recorded, and rewrite it to branch to the new common block. 127 DILocation *CommonDebugLoc = nullptr; 128 for (BasicBlock *BB : BBs) { 129 auto *Term = BB->getTerminator(); 130 assert(Term->getOpcode() == CanonicalTerm->getOpcode() && 131 "All blocks to be tail-merged must be the same " 132 "(function-terminating) terminator type."); 133 134 // Aha, found a new non-canonical function terminator. If it has operands, 135 // forward them to the PHI nodes in the canonical block. 136 for (auto I : zip(Term->operands(), NewOps)) 137 std::get<1>(I)->addIncoming(std::get<0>(I), BB); 138 139 // Compute the debug location common to all the original terminators. 140 if (!CommonDebugLoc) 141 CommonDebugLoc = Term->getDebugLoc(); 142 else 143 CommonDebugLoc = 144 DILocation::getMergedLocation(CommonDebugLoc, Term->getDebugLoc()); 145 146 // And turn BB into a block that just unconditionally branches 147 // to the canonical block. 148 Instruction *BI = BranchInst::Create(CanonicalBB, BB); 149 BI->setDebugLoc(Term->getDebugLoc()); 150 Term->eraseFromParent(); 151 152 if (Updates) 153 Updates->push_back({DominatorTree::Insert, BB, CanonicalBB}); 154 } 155 156 CanonicalTerm->setDebugLoc(CommonDebugLoc); 157 158 return true; 159 } 160 161 static bool tailMergeBlocksWithSimilarFunctionTerminators(Function &F, 162 DomTreeUpdater *DTU) { 163 SmallMapVector<unsigned /*TerminatorOpcode*/, SmallVector<BasicBlock *, 2>, 4> 164 Structure; 165 166 // Scan all the blocks in the function, record the interesting-ones. 167 for (BasicBlock &BB : F) { 168 if (DTU && DTU->isBBPendingDeletion(&BB)) 169 continue; 170 171 // We are only interested in function-terminating blocks. 172 if (!succ_empty(&BB)) 173 continue; 174 175 auto *Term = BB.getTerminator(); 176 177 // Fow now only support `ret`/`resume` function terminators. 178 // FIXME: lift this restriction. 179 switch (Term->getOpcode()) { 180 case Instruction::Ret: 181 case Instruction::Resume: 182 break; 183 default: 184 continue; 185 } 186 187 // We can't tail-merge block that contains a musttail call. 188 if (BB.getTerminatingMustTailCall()) 189 continue; 190 191 // Calls to experimental_deoptimize must be followed by a return 192 // of the value computed by experimental_deoptimize. 193 // I.e., we can not change `ret` to `br` for this block. 194 if (auto *CI = 195 dyn_cast_or_null<CallInst>(Term->getPrevNonDebugInstruction())) { 196 if (Function *F = CI->getCalledFunction()) 197 if (Intrinsic::ID ID = F->getIntrinsicID()) 198 if (ID == Intrinsic::experimental_deoptimize) 199 continue; 200 } 201 202 // PHI nodes cannot have token type, so if the terminator has an operand 203 // with token type, we can not tail-merge this kind of function terminators. 204 if (any_of(Term->operands(), 205 [](Value *Op) { return Op->getType()->isTokenTy(); })) 206 continue; 207 208 // Canonical blocks are uniqued based on the terminator type (opcode). 209 Structure[Term->getOpcode()].emplace_back(&BB); 210 } 211 212 bool Changed = false; 213 214 std::vector<DominatorTree::UpdateType> Updates; 215 216 for (ArrayRef<BasicBlock *> BBs : make_second_range(Structure)) 217 Changed |= performBlockTailMerging(F, BBs, DTU ? &Updates : nullptr); 218 219 if (DTU) 220 DTU->applyUpdates(Updates); 221 222 return Changed; 223 } 224 225 /// Call SimplifyCFG on all the blocks in the function, 226 /// iterating until no more changes are made. 227 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, 228 DomTreeUpdater *DTU, 229 const SimplifyCFGOptions &Options) { 230 bool Changed = false; 231 bool LocalChange = true; 232 233 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges; 234 FindFunctionBackedges(F, Edges); 235 SmallPtrSet<BasicBlock *, 16> UniqueLoopHeaders; 236 for (const auto &Edge : Edges) 237 UniqueLoopHeaders.insert(const_cast<BasicBlock *>(Edge.second)); 238 239 SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(), 240 UniqueLoopHeaders.end()); 241 242 unsigned IterCnt = 0; 243 (void)IterCnt; 244 while (LocalChange) { 245 assert(IterCnt++ < 1000 && "Iterative simplification didn't converge!"); 246 LocalChange = false; 247 248 // Loop over all of the basic blocks and remove them if they are unneeded. 249 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { 250 BasicBlock &BB = *BBIt++; 251 if (DTU) { 252 assert( 253 !DTU->isBBPendingDeletion(&BB) && 254 "Should not end up trying to simplify blocks marked for removal."); 255 // Make sure that the advanced iterator does not point at the blocks 256 // that are marked for removal, skip over all such blocks. 257 while (BBIt != F.end() && DTU->isBBPendingDeletion(&*BBIt)) 258 ++BBIt; 259 } 260 if (simplifyCFG(&BB, TTI, DTU, Options, LoopHeaders)) { 261 LocalChange = true; 262 ++NumSimpl; 263 } 264 } 265 Changed |= LocalChange; 266 } 267 return Changed; 268 } 269 270 static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI, 271 DominatorTree *DT, 272 const SimplifyCFGOptions &Options) { 273 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 274 275 bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr); 276 EverChanged |= 277 tailMergeBlocksWithSimilarFunctionTerminators(F, DT ? &DTU : nullptr); 278 EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 279 280 // If neither pass changed anything, we're done. 281 if (!EverChanged) return false; 282 283 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens, 284 // removeUnreachableBlocks is needed to nuke them, which means we should 285 // iterate between the two optimizations. We structure the code like this to 286 // avoid rerunning iterativelySimplifyCFG if the second pass of 287 // removeUnreachableBlocks doesn't do anything. 288 if (!removeUnreachableBlocks(F, DT ? &DTU : nullptr)) 289 return true; 290 291 do { 292 EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 293 EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr); 294 } while (EverChanged); 295 296 return true; 297 } 298 299 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, 300 DominatorTree *DT, 301 const SimplifyCFGOptions &Options) { 302 assert((!RequireAndPreserveDomTree || 303 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 304 "Original domtree is invalid?"); 305 306 bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options); 307 308 assert((!RequireAndPreserveDomTree || 309 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 310 "Failed to maintain validity of domtree!"); 311 312 return Changed; 313 } 314 315 // Command-line settings override compile-time settings. 316 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) { 317 if (UserBonusInstThreshold.getNumOccurrences()) 318 Options.BonusInstThreshold = UserBonusInstThreshold; 319 if (UserForwardSwitchCond.getNumOccurrences()) 320 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond; 321 if (UserSwitchRangeToICmp.getNumOccurrences()) 322 Options.ConvertSwitchRangeToICmp = UserSwitchRangeToICmp; 323 if (UserSwitchToLookup.getNumOccurrences()) 324 Options.ConvertSwitchToLookupTable = UserSwitchToLookup; 325 if (UserKeepLoops.getNumOccurrences()) 326 Options.NeedCanonicalLoop = UserKeepLoops; 327 if (UserHoistCommonInsts.getNumOccurrences()) 328 Options.HoistCommonInsts = UserHoistCommonInsts; 329 if (UserSinkCommonInsts.getNumOccurrences()) 330 Options.SinkCommonInsts = UserSinkCommonInsts; 331 if (UserSpeculateUnpredictables.getNumOccurrences()) 332 Options.SpeculateUnpredictables = UserSpeculateUnpredictables; 333 } 334 335 SimplifyCFGPass::SimplifyCFGPass() { 336 applyCommandLineOverridesToOptions(Options); 337 } 338 339 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) 340 : Options(Opts) { 341 applyCommandLineOverridesToOptions(Options); 342 } 343 344 void SimplifyCFGPass::printPipeline( 345 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) { 346 static_cast<PassInfoMixin<SimplifyCFGPass> *>(this)->printPipeline( 347 OS, MapClassName2PassName); 348 OS << '<'; 349 OS << "bonus-inst-threshold=" << Options.BonusInstThreshold << ';'; 350 OS << (Options.ForwardSwitchCondToPhi ? "" : "no-") << "forward-switch-cond;"; 351 OS << (Options.ConvertSwitchRangeToICmp ? "" : "no-") 352 << "switch-range-to-icmp;"; 353 OS << (Options.ConvertSwitchToLookupTable ? "" : "no-") 354 << "switch-to-lookup;"; 355 OS << (Options.NeedCanonicalLoop ? "" : "no-") << "keep-loops;"; 356 OS << (Options.HoistCommonInsts ? "" : "no-") << "hoist-common-insts;"; 357 OS << (Options.SinkCommonInsts ? "" : "no-") << "sink-common-insts;"; 358 OS << (Options.SpeculateBlocks ? "" : "no-") << "speculate-blocks;"; 359 OS << (Options.SimplifyCondBranch ? "" : "no-") << "simplify-cond-branch;"; 360 OS << (Options.SpeculateUnpredictables ? "" : "no-") 361 << "speculate-unpredictables"; 362 OS << '>'; 363 } 364 365 PreservedAnalyses SimplifyCFGPass::run(Function &F, 366 FunctionAnalysisManager &AM) { 367 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 368 Options.AC = &AM.getResult<AssumptionAnalysis>(F); 369 DominatorTree *DT = nullptr; 370 if (RequireAndPreserveDomTree) 371 DT = &AM.getResult<DominatorTreeAnalysis>(F); 372 if (!simplifyFunctionCFG(F, TTI, DT, Options)) 373 return PreservedAnalyses::all(); 374 PreservedAnalyses PA; 375 if (RequireAndPreserveDomTree) 376 PA.preserve<DominatorTreeAnalysis>(); 377 return PA; 378 } 379 380 namespace { 381 struct CFGSimplifyPass : public FunctionPass { 382 static char ID; 383 SimplifyCFGOptions Options; 384 std::function<bool(const Function &)> PredicateFtor; 385 386 CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(), 387 std::function<bool(const Function &)> Ftor = nullptr) 388 : FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) { 389 390 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry()); 391 392 // Check for command-line overrides of options for debug/customization. 393 applyCommandLineOverridesToOptions(Options); 394 } 395 396 bool runOnFunction(Function &F) override { 397 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F))) 398 return false; 399 400 Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 401 DominatorTree *DT = nullptr; 402 if (RequireAndPreserveDomTree) 403 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 404 405 auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); 406 return simplifyFunctionCFG(F, TTI, DT, Options); 407 } 408 void getAnalysisUsage(AnalysisUsage &AU) const override { 409 AU.addRequired<AssumptionCacheTracker>(); 410 if (RequireAndPreserveDomTree) 411 AU.addRequired<DominatorTreeWrapperPass>(); 412 AU.addRequired<TargetTransformInfoWrapperPass>(); 413 if (RequireAndPreserveDomTree) 414 AU.addPreserved<DominatorTreeWrapperPass>(); 415 AU.addPreserved<GlobalsAAWrapperPass>(); 416 } 417 }; 418 } 419 420 char CFGSimplifyPass::ID = 0; 421 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 422 false) 423 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 424 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 425 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 426 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 427 false) 428 429 // Public interface to the CFGSimplification pass 430 FunctionPass * 431 llvm::createCFGSimplificationPass(SimplifyCFGOptions Options, 432 std::function<bool(const Function &)> Ftor) { 433 return new CFGSimplifyPass(Options, std::move(Ftor)); 434 } 435