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