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 // FIXME: switch to non-permissive DomTreeUpdater::applyUpdates(). 81 static bool mergeEmptyReturnBlocks(Function &F, DomTreeUpdater *DTU) { 82 bool Changed = false; 83 84 std::vector<DominatorTree::UpdateType> Updates; 85 SmallVector<BasicBlock *, 8> DeadBlocks; 86 87 BasicBlock *RetBlock = nullptr; 88 89 // Scan all the blocks in the function, looking for empty return blocks. 90 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) { 91 BasicBlock &BB = *BBI++; 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 Updates.push_back({DominatorTree::Delete, Predecessor, &BB}); 146 Updates.push_back({DominatorTree::Insert, Predecessor, RetBlock}); 147 } 148 } 149 BB.replaceAllUsesWith(RetBlock); 150 DeadBlocks.emplace_back(&BB); 151 continue; 152 } 153 154 // If the canonical return block has no PHI node, create one now. 155 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin()); 156 if (!RetBlockPHI) { 157 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0); 158 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock); 159 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), 160 std::distance(PB, PE), "merge", 161 &RetBlock->front()); 162 163 for (pred_iterator PI = PB; PI != PE; ++PI) 164 RetBlockPHI->addIncoming(InVal, *PI); 165 RetBlock->getTerminator()->setOperand(0, RetBlockPHI); 166 } 167 168 // Turn BB into a block that just unconditionally branches to the return 169 // block. This handles the case when the two return blocks have a common 170 // predecessor but that return different things. 171 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB); 172 BB.getTerminator()->eraseFromParent(); 173 BranchInst::Create(RetBlock, &BB); 174 if (DTU) 175 Updates.push_back({DominatorTree::Insert, &BB, RetBlock}); 176 } 177 178 if (DTU) { 179 DTU->applyUpdatesPermissive(Updates); 180 for (auto *BB : DeadBlocks) 181 DTU->deleteBB(BB); 182 } else { 183 for (auto *BB : DeadBlocks) 184 BB->eraseFromParent(); 185 } 186 187 return Changed; 188 } 189 190 /// Call SimplifyCFG on all the blocks in the function, 191 /// iterating until no more changes are made. 192 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, 193 DomTreeUpdater *DTU, 194 const SimplifyCFGOptions &Options) { 195 bool Changed = false; 196 bool LocalChange = true; 197 198 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges; 199 FindFunctionBackedges(F, Edges); 200 SmallPtrSet<BasicBlock *, 16> LoopHeaders; 201 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 202 LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second)); 203 204 while (LocalChange) { 205 LocalChange = false; 206 207 // Loop over all of the basic blocks and remove them if they are unneeded. 208 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { 209 if (simplifyCFG(&*BBIt++, TTI, DTU, Options, &LoopHeaders)) { 210 LocalChange = true; 211 ++NumSimpl; 212 } 213 } 214 Changed |= LocalChange; 215 } 216 return Changed; 217 } 218 219 static bool simplifyFunctionCFGImpl(Function &F, const TargetTransformInfo &TTI, 220 DominatorTree *DT, 221 const SimplifyCFGOptions &Options) { 222 DomTreeUpdater DTU(DT, DomTreeUpdater::UpdateStrategy::Eager); 223 224 bool EverChanged = removeUnreachableBlocks(F, DT ? &DTU : nullptr); 225 EverChanged |= mergeEmptyReturnBlocks(F, DT ? &DTU : nullptr); 226 EverChanged |= iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 227 228 // If neither pass changed anything, we're done. 229 if (!EverChanged) return false; 230 231 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens, 232 // removeUnreachableBlocks is needed to nuke them, which means we should 233 // iterate between the two optimizations. We structure the code like this to 234 // avoid rerunning iterativelySimplifyCFG if the second pass of 235 // removeUnreachableBlocks doesn't do anything. 236 if (!removeUnreachableBlocks(F, DT ? &DTU : nullptr)) 237 return true; 238 239 do { 240 EverChanged = iterativelySimplifyCFG(F, TTI, DT ? &DTU : nullptr, Options); 241 EverChanged |= removeUnreachableBlocks(F, DT ? &DTU : nullptr); 242 } while (EverChanged); 243 244 return true; 245 } 246 247 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, 248 DominatorTree *DT, 249 const SimplifyCFGOptions &Options) { 250 assert((!RequireAndPreserveDomTree || 251 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 252 "Original domtree is invalid?"); 253 254 bool Changed = simplifyFunctionCFGImpl(F, TTI, DT, Options); 255 256 assert((!RequireAndPreserveDomTree || 257 (DT && DT->verify(DominatorTree::VerificationLevel::Full))) && 258 "Failed to maintain validity of domtree!"); 259 260 return Changed; 261 } 262 263 // Command-line settings override compile-time settings. 264 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) { 265 if (UserBonusInstThreshold.getNumOccurrences()) 266 Options.BonusInstThreshold = UserBonusInstThreshold; 267 if (UserForwardSwitchCond.getNumOccurrences()) 268 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond; 269 if (UserSwitchToLookup.getNumOccurrences()) 270 Options.ConvertSwitchToLookupTable = UserSwitchToLookup; 271 if (UserKeepLoops.getNumOccurrences()) 272 Options.NeedCanonicalLoop = UserKeepLoops; 273 if (UserHoistCommonInsts.getNumOccurrences()) 274 Options.HoistCommonInsts = UserHoistCommonInsts; 275 if (UserSinkCommonInsts.getNumOccurrences()) 276 Options.SinkCommonInsts = UserSinkCommonInsts; 277 } 278 279 SimplifyCFGPass::SimplifyCFGPass() : Options() { 280 applyCommandLineOverridesToOptions(Options); 281 } 282 283 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) 284 : Options(Opts) { 285 applyCommandLineOverridesToOptions(Options); 286 } 287 288 PreservedAnalyses SimplifyCFGPass::run(Function &F, 289 FunctionAnalysisManager &AM) { 290 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 291 Options.AC = &AM.getResult<AssumptionAnalysis>(F); 292 DominatorTree *DT = nullptr; 293 if (RequireAndPreserveDomTree) 294 DT = &AM.getResult<DominatorTreeAnalysis>(F); 295 if (F.hasFnAttribute(Attribute::OptForFuzzing)) { 296 Options.setSimplifyCondBranch(false).setFoldTwoEntryPHINode(false); 297 } else { 298 Options.setSimplifyCondBranch(true).setFoldTwoEntryPHINode(true); 299 } 300 if (!simplifyFunctionCFG(F, TTI, DT, Options)) 301 return PreservedAnalyses::all(); 302 PreservedAnalyses PA; 303 if (RequireAndPreserveDomTree) 304 PA.preserve<DominatorTreeAnalysis>(); 305 PA.preserve<GlobalsAA>(); 306 return PA; 307 } 308 309 namespace { 310 struct CFGSimplifyPass : public FunctionPass { 311 static char ID; 312 SimplifyCFGOptions Options; 313 std::function<bool(const Function &)> PredicateFtor; 314 315 CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(), 316 std::function<bool(const Function &)> Ftor = nullptr) 317 : FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) { 318 319 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry()); 320 321 // Check for command-line overrides of options for debug/customization. 322 applyCommandLineOverridesToOptions(Options); 323 } 324 325 bool runOnFunction(Function &F) override { 326 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F))) 327 return false; 328 329 Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 330 DominatorTree *DT = nullptr; 331 if (RequireAndPreserveDomTree) 332 DT = &getAnalysis<DominatorTreeWrapperPass>().getDomTree(); 333 if (F.hasFnAttribute(Attribute::OptForFuzzing)) { 334 Options.setSimplifyCondBranch(false) 335 .setFoldTwoEntryPHINode(false); 336 } else { 337 Options.setSimplifyCondBranch(true) 338 .setFoldTwoEntryPHINode(true); 339 } 340 341 auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); 342 return simplifyFunctionCFG(F, TTI, DT, Options); 343 } 344 void getAnalysisUsage(AnalysisUsage &AU) const override { 345 AU.addRequired<AssumptionCacheTracker>(); 346 if (RequireAndPreserveDomTree) 347 AU.addRequired<DominatorTreeWrapperPass>(); 348 AU.addRequired<TargetTransformInfoWrapperPass>(); 349 if (RequireAndPreserveDomTree) 350 AU.addPreserved<DominatorTreeWrapperPass>(); 351 AU.addPreserved<GlobalsAAWrapperPass>(); 352 } 353 }; 354 } 355 356 char CFGSimplifyPass::ID = 0; 357 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 358 false) 359 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 360 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 361 INITIALIZE_PASS_DEPENDENCY(DominatorTreeWrapperPass) 362 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 363 false) 364 365 // Public interface to the CFGSimplification pass 366 FunctionPass * 367 llvm::createCFGSimplificationPass(SimplifyCFGOptions Options, 368 std::function<bool(const Function &)> Ftor) { 369 return new CFGSimplifyPass(Options, std::move(Ftor)); 370 } 371