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