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/SmallPtrSet.h" 24 #include "llvm/ADT/SmallVector.h" 25 #include "llvm/ADT/Statistic.h" 26 #include "llvm/Analysis/AssumptionCache.h" 27 #include "llvm/Analysis/CFG.h" 28 #include "llvm/Analysis/DomTreeUpdater.h" 29 #include "llvm/Analysis/GlobalsModRef.h" 30 #include "llvm/Analysis/TargetTransformInfo.h" 31 #include "llvm/IR/Attributes.h" 32 #include "llvm/IR/CFG.h" 33 #include "llvm/IR/Constants.h" 34 #include "llvm/IR/DataLayout.h" 35 #include "llvm/IR/Dominators.h" 36 #include "llvm/IR/Instructions.h" 37 #include "llvm/IR/IntrinsicInst.h" 38 #include "llvm/IR/Module.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> UserSwitchToLookup( 60 "switch-to-lookup", cl::Hidden, cl::init(false), 61 cl::desc("Convert switches to lookup tables (default = false)")); 62 63 static cl::opt<bool> UserForwardSwitchCond( 64 "forward-switch-cond", cl::Hidden, cl::init(false), 65 cl::desc("Forward switch condition to phi ops (default = false)")); 66 67 static cl::opt<bool> UserHoistCommonInsts( 68 "hoist-common-insts", cl::Hidden, cl::init(false), 69 cl::desc("hoist common instructions (default = false)")); 70 71 static cl::opt<bool> UserSinkCommonInsts( 72 "sink-common-insts", cl::Hidden, cl::init(false), 73 cl::desc("Sink common instructions (default = false)")); 74 75 76 STATISTIC(NumSimpl, "Number of blocks simplified"); 77 78 /// If we have more than one empty (other than phi node) return blocks, 79 /// merge them together to promote recursive block merging. 80 static bool mergeEmptyReturnBlocks(Function &F, DomTreeUpdater *DTU) { 81 bool Changed = false; 82 83 std::vector<DominatorTree::UpdateType> Updates; 84 SmallVector<BasicBlock *, 8> DeadBlocks; 85 86 BasicBlock *RetBlock = nullptr; 87 88 // Scan all the blocks in the function, looking for empty return blocks. 89 for (BasicBlock &BB : make_early_inc_range(F)) { 90 if (DTU && DTU->isBBPendingDeletion(&BB)) 91 continue; 92 93 // Only look at return blocks. 94 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator()); 95 if (!Ret) continue; 96 97 // Only look at the block if it is empty or the only other thing in it is a 98 // single PHI node that is the operand to the return. 99 if (Ret != &BB.front()) { 100 // Check for something else in the block. 101 BasicBlock::iterator I(Ret); 102 --I; 103 // Skip over debug info. 104 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin()) 105 --I; 106 if (!isa<DbgInfoIntrinsic>(I) && 107 (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 || 108 Ret->getOperand(0) != &*I)) 109 continue; 110 } 111 112 // If this is the first returning block, remember it and keep going. 113 if (!RetBlock) { 114 RetBlock = &BB; 115 continue; 116 } 117 118 // Skip merging if this would result in a CallBr instruction with a 119 // duplicate destination. FIXME: See note in CodeGenPrepare.cpp. 120 bool SkipCallBr = false; 121 for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB); 122 PI != E && !SkipCallBr; ++PI) { 123 if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator())) 124 for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i) 125 if (RetBlock == CBI->getSuccessor(i)) { 126 SkipCallBr = true; 127 break; 128 } 129 } 130 if (SkipCallBr) 131 continue; 132 133 // Otherwise, we found a duplicate return block. Merge the two. 134 Changed = true; 135 136 // Case when there is no input to the return or when the returned values 137 // agree is trivial. Note that they can't agree if there are phis in the 138 // blocks. 139 if (Ret->getNumOperands() == 0 || 140 Ret->getOperand(0) == 141 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) { 142 // All predecessors of BB should now branch to RetBlock instead. 143 if (DTU) { 144 for (auto *Predecessor : predecessors(&BB)) { 145 // But, iff Predecessor already branches to RetBlock, 146 // don't (re-)add DomTree edge, because it already exists. 147 if (!is_contained(successors(Predecessor), RetBlock)) 148 Updates.push_back({DominatorTree::Insert, Predecessor, RetBlock}); 149 Updates.push_back({DominatorTree::Delete, Predecessor, &BB}); 150 } 151 } 152 BB.replaceAllUsesWith(RetBlock); 153 DeadBlocks.emplace_back(&BB); 154 continue; 155 } 156 157 // If the canonical return block has no PHI node, create one now. 158 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin()); 159 if (!RetBlockPHI) { 160 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0); 161 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock); 162 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), 163 std::distance(PB, PE), "merge", 164 &RetBlock->front()); 165 166 for (pred_iterator PI = PB; PI != PE; ++PI) 167 RetBlockPHI->addIncoming(InVal, *PI); 168 RetBlock->getTerminator()->setOperand(0, RetBlockPHI); 169 } 170 171 // Turn BB into a block that just unconditionally branches to the return 172 // block. This handles the case when the two return blocks have a common 173 // predecessor but that return different things. 174 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB); 175 BB.getTerminator()->eraseFromParent(); 176 BranchInst::Create(RetBlock, &BB); 177 if (DTU) 178 Updates.push_back({DominatorTree::Insert, &BB, RetBlock}); 179 } 180 181 if (DTU) { 182 DTU->applyUpdates(Updates); 183 for (auto *BB : DeadBlocks) 184 DTU->deleteBB(BB); 185 } else { 186 for (auto *BB : DeadBlocks) 187 BB->eraseFromParent(); 188 } 189 190 return Changed; 191 } 192 193 /// Call SimplifyCFG on all the blocks in the function, 194 /// iterating until no more changes are made. 195 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, 196 DomTreeUpdater *DTU, 197 const SimplifyCFGOptions &Options) { 198 bool Changed = false; 199 bool LocalChange = true; 200 201 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges; 202 FindFunctionBackedges(F, Edges); 203 SmallPtrSet<BasicBlock *, 16> LoopHeaders; 204 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 205 LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second)); 206 207 while (LocalChange) { 208 LocalChange = false; 209 210 // Loop over all of the basic blocks and remove them if they are unneeded. 211 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { 212 if (simplifyCFG(&*BBIt++, TTI, DTU, Options, &LoopHeaders)) { 213 LocalChange = true; 214 ++NumSimpl; 215 } 216 } 217 Changed |= LocalChange; 218 } 219 return Changed; 220 } 221 222 static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI, 223 DominatorTree *DT, 224 const SimplifyCFGOptions &Options) { 225 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 226 227 bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr); 228 EverChanged |= mergeEmptyReturnBlocks(F, DT ? &DTU : nullptr); 229 EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 230 231 // If neither pass changed anything, we're done. 232 if (!EverChanged) return false; 233 234 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens, 235 // removeUnreachableBlocks is needed to nuke them, which means we should 236 // iterate between the two optimizations. We structure the code like this to 237 // avoid rerunning iterativelySimplifyCFG if the second pass of 238 // removeUnreachableBlocks doesn't do anything. 239 if (!removeUnreachableBlocks(F, DT ? &DTU : nullptr)) 240 return true; 241 242 do { 243 EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 244 EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr); 245 } while (EverChanged); 246 247 return true; 248 } 249 250 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, 251 DominatorTree *DT, 252 const SimplifyCFGOptions &Options) { 253 assert((!RequireAndPreserveDomTree || 254 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 255 "Original domtree is invalid?"); 256 257 bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options); 258 259 assert((!RequireAndPreserveDomTree || 260 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 261 "Failed to maintain validity of domtree!"); 262 263 return Changed; 264 } 265 266 // Command-line settings override compile-time settings. 267 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) { 268 if (UserBonusInstThreshold.getNumOccurrences()) 269 Options.BonusInstThreshold = UserBonusInstThreshold; 270 if (UserForwardSwitchCond.getNumOccurrences()) 271 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond; 272 if (UserSwitchToLookup.getNumOccurrences()) 273 Options.ConvertSwitchToLookupTable = UserSwitchToLookup; 274 if (UserKeepLoops.getNumOccurrences()) 275 Options.NeedCanonicalLoop = UserKeepLoops; 276 if (UserHoistCommonInsts.getNumOccurrences()) 277 Options.HoistCommonInsts = UserHoistCommonInsts; 278 if (UserSinkCommonInsts.getNumOccurrences()) 279 Options.SinkCommonInsts = UserSinkCommonInsts; 280 } 281 282 SimplifyCFGPass::SimplifyCFGPass() : Options() { 283 applyCommandLineOverridesToOptions(Options); 284 } 285 286 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) 287 : Options(Opts) { 288 applyCommandLineOverridesToOptions(Options); 289 } 290 291 PreservedAnalyses SimplifyCFGPass::run(Function &F, 292 FunctionAnalysisManager &AM) { 293 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 294 Options.AC = &AM.getResult<AssumptionAnalysis>(F); 295 DominatorTree *DT = nullptr; 296 if (RequireAndPreserveDomTree) 297 DT = &AM.getResult<DominatorTreeAnalysis>(F); 298 if (F.hasFnAttribute(Attribute::OptForFuzzing)) { 299 Options.setSimplifyCondBranch(false).setFoldTwoEntryPHINode(false); 300 } else { 301 Options.setSimplifyCondBranch(true).setFoldTwoEntryPHINode(true); 302 } 303 if (!simplifyFunctionCFG(F, TTI, DT, Options)) 304 return PreservedAnalyses::all(); 305 PreservedAnalyses PA; 306 if (RequireAndPreserveDomTree) 307 PA.preserve<DominatorTreeAnalysis>(); 308 PA.preserve<GlobalsAA>(); 309 return PA; 310 } 311 312 namespace { 313 struct CFGSimplifyPass : public FunctionPass { 314 static char ID; 315 SimplifyCFGOptions Options; 316 std::function<bool(const Function &)> PredicateFtor; 317 318 CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(), 319 std::function<bool(const Function &)> Ftor = nullptr) 320 : FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) { 321 322 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry()); 323 324 // Check for command-line overrides of options for debug/customization. 325 applyCommandLineOverridesToOptions(Options); 326 } 327 328 bool runOnFunction(Function &F) override { 329 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F))) 330 return false; 331 332 Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 333 DominatorTree *DT = nullptr; 334 if (RequireAndPreserveDomTree) 335 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 336 if (F.hasFnAttribute(Attribute::OptForFuzzing)) { 337 Options.setSimplifyCondBranch(false) 338 .setFoldTwoEntryPHINode(false); 339 } else { 340 Options.setSimplifyCondBranch(true) 341 .setFoldTwoEntryPHINode(true); 342 } 343 344 auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); 345 return simplifyFunctionCFG(F, TTI, DT, Options); 346 } 347 void getAnalysisUsage(AnalysisUsage &AU) const override { 348 AU.addRequired<AssumptionCacheTracker>(); 349 if (RequireAndPreserveDomTree) 350 AU.addRequired<DominatorTreeWrapperPass>(); 351 AU.addRequired<TargetTransformInfoWrapperPass>(); 352 if (RequireAndPreserveDomTree) 353 AU.addPreserved<DominatorTreeWrapperPass>(); 354 AU.addPreserved<GlobalsAAWrapperPass>(); 355 } 356 }; 357 } 358 359 char CFGSimplifyPass::ID = 0; 360 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 361 false) 362 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 363 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 364 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 365 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 366 false) 367 368 // Public interface to the CFGSimplification pass 369 FunctionPass * 370 llvm::createCFGSimplificationPass(SimplifyCFGOptions Options, 371 std::function<bool(const Function &)> Ftor) { 372 return new CFGSimplifyPass(Options, std::move(Ftor)); 373 } 374