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/ValueHandle.h" 38 #include "llvm/InitializePasses.h" 39 #include "llvm/Pass.h" 40 #include "llvm/Support/CommandLine.h" 41 #include "llvm/Transforms/Scalar.h" 42 #include "llvm/Transforms/Scalar/SimplifyCFG.h" 43 #include "llvm/Transforms/Utils/Local.h" 44 #include "llvm/Transforms/Utils/SimplifyCFGOptions.h" 45 #include <utility> 46 using namespace llvm; 47 48 #define DEBUG_TYPE "simplifycfg" 49 50 static cl::opt<unsigned> UserBonusInstThreshold( 51 "bonus-inst-threshold", cl::Hidden, cl::init(1), 52 cl::desc("Control the number of bonus instructions (default = 1)")); 53 54 static cl::opt<bool> UserKeepLoops( 55 "keep-loops", cl::Hidden, cl::init(true), 56 cl::desc("Preserve canonical loop structure (default = true)")); 57 58 static cl::opt<bool> UserSwitchRangeToICmp( 59 "switch-range-to-icmp", cl::Hidden, cl::init(false), 60 cl::desc( 61 "Convert switches into an integer range comparison (default = false)")); 62 63 static cl::opt<bool> UserSwitchToLookup( 64 "switch-to-lookup", cl::Hidden, cl::init(false), 65 cl::desc("Convert switches to lookup tables (default = false)")); 66 67 static cl::opt<bool> UserForwardSwitchCond( 68 "forward-switch-cond", cl::Hidden, cl::init(false), 69 cl::desc("Forward switch condition to phi ops (default = false)")); 70 71 static cl::opt<bool> UserHoistCommonInsts( 72 "hoist-common-insts", cl::Hidden, cl::init(false), 73 cl::desc("hoist common instructions (default = false)")); 74 75 static cl::opt<bool> UserHoistLoadsStoresWithCondFaulting( 76 "hoist-loads-stores-with-cond-faulting", cl::Hidden, cl::init(false), 77 cl::desc("Hoist loads/stores if the target supports conditional faulting " 78 "(default = false)")); 79 80 static cl::opt<bool> UserSinkCommonInsts( 81 "sink-common-insts", cl::Hidden, cl::init(false), 82 cl::desc("Sink common instructions (default = false)")); 83 84 static cl::opt<bool> UserSpeculateUnpredictables( 85 "speculate-unpredictables", cl::Hidden, cl::init(false), 86 cl::desc("Speculate unpredictable branches (default = false)")); 87 88 STATISTIC(NumSimpl, "Number of blocks simplified"); 89 90 static bool 91 performBlockTailMerging(Function &F, ArrayRef<BasicBlock *> BBs, 92 std::vector<DominatorTree::UpdateType> *Updates) { 93 SmallVector<PHINode *, 1> NewOps; 94 95 // We don't want to change IR just because we can. 96 // Only do that if there are at least two blocks we'll tail-merge. 97 if (BBs.size() < 2) 98 return false; 99 100 if (Updates) 101 Updates->reserve(Updates->size() + BBs.size()); 102 103 BasicBlock *CanonicalBB; 104 Instruction *CanonicalTerm; 105 { 106 auto *Term = BBs[0]->getTerminator(); 107 108 // Create a canonical block for this function terminator type now, 109 // placing it *before* the first block that will branch to it. 110 CanonicalBB = BasicBlock::Create( 111 F.getContext(), Twine("common.") + Term->getOpcodeName(), &F, BBs[0]); 112 // We'll also need a PHI node per each operand of the terminator. 113 NewOps.resize(Term->getNumOperands()); 114 for (auto I : zip(Term->operands(), NewOps)) { 115 std::get<1>(I) = PHINode::Create(std::get<0>(I)->getType(), 116 /*NumReservedValues=*/BBs.size(), 117 CanonicalBB->getName() + ".op"); 118 std::get<1>(I)->insertInto(CanonicalBB, CanonicalBB->end()); 119 } 120 // Make it so that this canonical block actually has the right 121 // terminator. 122 CanonicalTerm = Term->clone(); 123 CanonicalTerm->insertInto(CanonicalBB, CanonicalBB->end()); 124 // If the canonical terminator has operands, rewrite it to take PHI's. 125 for (auto I : zip(NewOps, CanonicalTerm->operands())) 126 std::get<1>(I) = std::get<0>(I); 127 } 128 129 // Now, go through each block (with the current terminator type) 130 // we've recorded, and rewrite it to branch to the new common block. 131 DILocation *CommonDebugLoc = nullptr; 132 for (BasicBlock *BB : BBs) { 133 auto *Term = BB->getTerminator(); 134 assert(Term->getOpcode() == CanonicalTerm->getOpcode() && 135 "All blocks to be tail-merged must be the same " 136 "(function-terminating) terminator type."); 137 138 // Aha, found a new non-canonical function terminator. If it has operands, 139 // forward them to the PHI nodes in the canonical block. 140 for (auto I : zip(Term->operands(), NewOps)) 141 std::get<1>(I)->addIncoming(std::get<0>(I), BB); 142 143 // Compute the debug location common to all the original terminators. 144 if (!CommonDebugLoc) 145 CommonDebugLoc = Term->getDebugLoc(); 146 else 147 CommonDebugLoc = 148 DILocation::getMergedLocation(CommonDebugLoc, Term->getDebugLoc()); 149 150 // And turn BB into a block that just unconditionally branches 151 // to the canonical block. 152 Instruction *BI = BranchInst::Create(CanonicalBB, BB); 153 BI->setDebugLoc(Term->getDebugLoc()); 154 Term->eraseFromParent(); 155 156 if (Updates) 157 Updates->push_back({DominatorTree::Insert, BB, CanonicalBB}); 158 } 159 160 CanonicalTerm->setDebugLoc(CommonDebugLoc); 161 162 return true; 163 } 164 165 static bool tailMergeBlocksWithSimilarFunctionTerminators(Function &F, 166 DomTreeUpdater *DTU) { 167 SmallMapVector<unsigned /*TerminatorOpcode*/, SmallVector<BasicBlock *, 2>, 4> 168 Structure; 169 170 // Scan all the blocks in the function, record the interesting-ones. 171 for (BasicBlock &BB : F) { 172 if (DTU && DTU->isBBPendingDeletion(&BB)) 173 continue; 174 175 // We are only interested in function-terminating blocks. 176 if (!succ_empty(&BB)) 177 continue; 178 179 auto *Term = BB.getTerminator(); 180 181 // Fow now only support `ret`/`resume` function terminators. 182 // FIXME: lift this restriction. 183 switch (Term->getOpcode()) { 184 case Instruction::Ret: 185 case Instruction::Resume: 186 break; 187 default: 188 continue; 189 } 190 191 // We can't tail-merge block that contains a musttail call. 192 if (BB.getTerminatingMustTailCall()) 193 continue; 194 195 // Calls to experimental_deoptimize must be followed by a return 196 // of the value computed by experimental_deoptimize. 197 // I.e., we can not change `ret` to `br` for this block. 198 if (auto *CI = 199 dyn_cast_or_null<CallInst>(Term->getPrevNonDebugInstruction())) { 200 if (Function *F = CI->getCalledFunction()) 201 if (Intrinsic::ID ID = F->getIntrinsicID()) 202 if (ID == Intrinsic::experimental_deoptimize) 203 continue; 204 } 205 206 // PHI nodes cannot have token type, so if the terminator has an operand 207 // with token type, we can not tail-merge this kind of function terminators. 208 if (any_of(Term->operands(), 209 [](Value *Op) { return Op->getType()->isTokenTy(); })) 210 continue; 211 212 // Canonical blocks are uniqued based on the terminator type (opcode). 213 Structure[Term->getOpcode()].emplace_back(&BB); 214 } 215 216 bool Changed = false; 217 218 std::vector<DominatorTree::UpdateType> Updates; 219 220 for (ArrayRef<BasicBlock *> BBs : make_second_range(Structure)) 221 Changed |= performBlockTailMerging(F, BBs, DTU ? &Updates : nullptr); 222 223 if (DTU) 224 DTU->applyUpdates(Updates); 225 226 return Changed; 227 } 228 229 /// Call SimplifyCFG on all the blocks in the function, 230 /// iterating until no more changes are made. 231 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, 232 DomTreeUpdater *DTU, 233 const SimplifyCFGOptions &Options) { 234 bool Changed = false; 235 bool LocalChange = true; 236 237 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges; 238 FindFunctionBackedges(F, Edges); 239 SmallPtrSet<BasicBlock *, 16> UniqueLoopHeaders; 240 for (const auto &Edge : Edges) 241 UniqueLoopHeaders.insert(const_cast<BasicBlock *>(Edge.second)); 242 243 SmallVector<WeakVH, 16> LoopHeaders(UniqueLoopHeaders.begin(), 244 UniqueLoopHeaders.end()); 245 246 unsigned IterCnt = 0; 247 (void)IterCnt; 248 while (LocalChange) { 249 assert(IterCnt++ < 1000 && "Iterative simplification didn't converge!"); 250 LocalChange = false; 251 252 // Loop over all of the basic blocks and remove them if they are unneeded. 253 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { 254 BasicBlock &BB = *BBIt++; 255 if (DTU) { 256 assert( 257 !DTU->isBBPendingDeletion(&BB) && 258 "Should not end up trying to simplify blocks marked for removal."); 259 // Make sure that the advanced iterator does not point at the blocks 260 // that are marked for removal, skip over all such blocks. 261 while (BBIt != F.end() && DTU->isBBPendingDeletion(&*BBIt)) 262 ++BBIt; 263 } 264 if (simplifyCFG(&BB, TTI, DTU, Options, LoopHeaders)) { 265 LocalChange = true; 266 ++NumSimpl; 267 } 268 } 269 Changed |= LocalChange; 270 } 271 return Changed; 272 } 273 274 static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI, 275 DominatorTree *DT, 276 const SimplifyCFGOptions &Options) { 277 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 278 279 bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr); 280 EverChanged |= 281 tailMergeBlocksWithSimilarFunctionTerminators(F, DT ? &DTU : nullptr); 282 EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 283 284 // If neither pass changed anything, we're done. 285 if (!EverChanged) return false; 286 287 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens, 288 // removeUnreachableBlocks is needed to nuke them, which means we should 289 // iterate between the two optimizations. We structure the code like this to 290 // avoid rerunning iterativelySimplifyCFG if the second pass of 291 // removeUnreachableBlocks doesn't do anything. 292 if (!removeUnreachableBlocks(F, DT ? &DTU : nullptr)) 293 return true; 294 295 do { 296 EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 297 EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr); 298 } while (EverChanged); 299 300 return true; 301 } 302 303 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, 304 DominatorTree *DT, 305 const SimplifyCFGOptions &Options) { 306 assert((!RequireAndPreserveDomTree || 307 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 308 "Original domtree is invalid?"); 309 310 bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options); 311 312 assert((!RequireAndPreserveDomTree || 313 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 314 "Failed to maintain validity of domtree!"); 315 316 return Changed; 317 } 318 319 // Command-line settings override compile-time settings. 320 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) { 321 if (UserBonusInstThreshold.getNumOccurrences()) 322 Options.BonusInstThreshold = UserBonusInstThreshold; 323 if (UserForwardSwitchCond.getNumOccurrences()) 324 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond; 325 if (UserSwitchRangeToICmp.getNumOccurrences()) 326 Options.ConvertSwitchRangeToICmp = UserSwitchRangeToICmp; 327 if (UserSwitchToLookup.getNumOccurrences()) 328 Options.ConvertSwitchToLookupTable = UserSwitchToLookup; 329 if (UserKeepLoops.getNumOccurrences()) 330 Options.NeedCanonicalLoop = UserKeepLoops; 331 if (UserHoistCommonInsts.getNumOccurrences()) 332 Options.HoistCommonInsts = UserHoistCommonInsts; 333 if (UserHoistLoadsStoresWithCondFaulting.getNumOccurrences()) 334 Options.HoistLoadsStoresWithCondFaulting = 335 UserHoistLoadsStoresWithCondFaulting; 336 if (UserSinkCommonInsts.getNumOccurrences()) 337 Options.SinkCommonInsts = UserSinkCommonInsts; 338 if (UserSpeculateUnpredictables.getNumOccurrences()) 339 Options.SpeculateUnpredictables = UserSpeculateUnpredictables; 340 } 341 342 SimplifyCFGPass::SimplifyCFGPass() { 343 applyCommandLineOverridesToOptions(Options); 344 } 345 346 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) 347 : Options(Opts) { 348 applyCommandLineOverridesToOptions(Options); 349 } 350 351 void SimplifyCFGPass::printPipeline( 352 raw_ostream &OS, function_ref<StringRef(StringRef)> MapClassName2PassName) { 353 static_cast<PassInfoMixin<SimplifyCFGPass> *>(this)->printPipeline( 354 OS, MapClassName2PassName); 355 OS << '<'; 356 OS << "bonus-inst-threshold=" << Options.BonusInstThreshold << ';'; 357 OS << (Options.ForwardSwitchCondToPhi ? "" : "no-") << "forward-switch-cond;"; 358 OS << (Options.ConvertSwitchRangeToICmp ? "" : "no-") 359 << "switch-range-to-icmp;"; 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 } 373 374 PreservedAnalyses SimplifyCFGPass::run(Function &F, 375 FunctionAnalysisManager &AM) { 376 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 377 Options.AC = &AM.getResult<AssumptionAnalysis>(F); 378 DominatorTree *DT = nullptr; 379 if (RequireAndPreserveDomTree) 380 DT = &AM.getResult<DominatorTreeAnalysis>(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 } 388 389 namespace { 390 struct 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 } 428 429 char CFGSimplifyPass::ID = 0; 430 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 431 false) 432 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 433 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 434 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 435 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 436 false) 437 438 // Public interface to the CFGSimplification pass 439 FunctionPass * 440 llvm::createCFGSimplificationPass(SimplifyCFGOptions Options, 441 std::function<bool(const Function &)> Ftor) { 442 return new CFGSimplifyPass(Options, std::move(Ftor)); 443 } 444