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/GlobalsModRef.h" 29 #include "llvm/Analysis/TargetTransformInfo.h" 30 #include "llvm/IR/Attributes.h" 31 #include "llvm/IR/CFG.h" 32 #include "llvm/IR/Constants.h" 33 #include "llvm/IR/DataLayout.h" 34 #include "llvm/IR/Instructions.h" 35 #include "llvm/IR/IntrinsicInst.h" 36 #include "llvm/IR/Module.h" 37 #include "llvm/InitializePasses.h" 38 #include "llvm/Pass.h" 39 #include "llvm/Support/CommandLine.h" 40 #include "llvm/Transforms/Scalar.h" 41 #include "llvm/Transforms/Scalar/SimplifyCFG.h" 42 #include "llvm/Transforms/Utils/Local.h" 43 #include "llvm/Transforms/Utils/SimplifyCFGOptions.h" 44 #include <utility> 45 using namespace llvm; 46 47 #define DEBUG_TYPE "simplifycfg" 48 49 static cl::opt<unsigned> UserBonusInstThreshold( 50 "bonus-inst-threshold", cl::Hidden, cl::init(1), 51 cl::desc("Control the number of bonus instructions (default = 1)")); 52 53 static cl::opt<bool> UserKeepLoops( 54 "keep-loops", cl::Hidden, cl::init(true), 55 cl::desc("Preserve canonical loop structure (default = true)")); 56 57 static cl::opt<bool> UserSwitchToLookup( 58 "switch-to-lookup", cl::Hidden, cl::init(false), 59 cl::desc("Convert switches to lookup tables (default = false)")); 60 61 static cl::opt<bool> UserForwardSwitchCond( 62 "forward-switch-cond", cl::Hidden, cl::init(false), 63 cl::desc("Forward switch condition to phi ops (default = false)")); 64 65 static cl::opt<bool> UserHoistCommonInsts( 66 "hoist-common-insts", cl::Hidden, cl::init(false), 67 cl::desc("hoist common instructions (default = false)")); 68 69 static cl::opt<bool> UserSinkCommonInsts( 70 "sink-common-insts", cl::Hidden, cl::init(false), 71 cl::desc("Sink common instructions (default = false)")); 72 73 74 STATISTIC(NumSimpl, "Number of blocks simplified"); 75 76 /// If we have more than one empty (other than phi node) return blocks, 77 /// merge them together to promote recursive block merging. 78 static bool mergeEmptyReturnBlocks(Function &F) { 79 bool Changed = false; 80 81 BasicBlock *RetBlock = nullptr; 82 83 // Scan all the blocks in the function, looking for empty return blocks. 84 for (Function::iterator BBI = F.begin(), E = F.end(); BBI != E; ) { 85 BasicBlock &BB = *BBI++; 86 87 // Only look at return blocks. 88 ReturnInst *Ret = dyn_cast<ReturnInst>(BB.getTerminator()); 89 if (!Ret) continue; 90 91 // Only look at the block if it is empty or the only other thing in it is a 92 // single PHI node that is the operand to the return. 93 if (Ret != &BB.front()) { 94 // Check for something else in the block. 95 BasicBlock::iterator I(Ret); 96 --I; 97 // Skip over debug info. 98 while (isa<DbgInfoIntrinsic>(I) && I != BB.begin()) 99 --I; 100 if (!isa<DbgInfoIntrinsic>(I) && 101 (!isa<PHINode>(I) || I != BB.begin() || Ret->getNumOperands() == 0 || 102 Ret->getOperand(0) != &*I)) 103 continue; 104 } 105 106 // If this is the first returning block, remember it and keep going. 107 if (!RetBlock) { 108 RetBlock = &BB; 109 continue; 110 } 111 112 // Skip merging if this would result in a CallBr instruction with a 113 // duplicate destination. FIXME: See note in CodeGenPrepare.cpp. 114 bool SkipCallBr = false; 115 for (pred_iterator PI = pred_begin(&BB), E = pred_end(&BB); 116 PI != E && !SkipCallBr; ++PI) { 117 if (auto *CBI = dyn_cast<CallBrInst>((*PI)->getTerminator())) 118 for (unsigned i = 0, e = CBI->getNumSuccessors(); i != e; ++i) 119 if (RetBlock == CBI->getSuccessor(i)) { 120 SkipCallBr = true; 121 break; 122 } 123 } 124 if (SkipCallBr) 125 continue; 126 127 // Otherwise, we found a duplicate return block. Merge the two. 128 Changed = true; 129 130 // Case when there is no input to the return or when the returned values 131 // agree is trivial. Note that they can't agree if there are phis in the 132 // blocks. 133 if (Ret->getNumOperands() == 0 || 134 Ret->getOperand(0) == 135 cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0)) { 136 BB.replaceAllUsesWith(RetBlock); 137 BB.eraseFromParent(); 138 continue; 139 } 140 141 // If the canonical return block has no PHI node, create one now. 142 PHINode *RetBlockPHI = dyn_cast<PHINode>(RetBlock->begin()); 143 if (!RetBlockPHI) { 144 Value *InVal = cast<ReturnInst>(RetBlock->getTerminator())->getOperand(0); 145 pred_iterator PB = pred_begin(RetBlock), PE = pred_end(RetBlock); 146 RetBlockPHI = PHINode::Create(Ret->getOperand(0)->getType(), 147 std::distance(PB, PE), "merge", 148 &RetBlock->front()); 149 150 for (pred_iterator PI = PB; PI != PE; ++PI) 151 RetBlockPHI->addIncoming(InVal, *PI); 152 RetBlock->getTerminator()->setOperand(0, RetBlockPHI); 153 } 154 155 // Turn BB into a block that just unconditionally branches to the return 156 // block. This handles the case when the two return blocks have a common 157 // predecessor but that return different things. 158 RetBlockPHI->addIncoming(Ret->getOperand(0), &BB); 159 BB.getTerminator()->eraseFromParent(); 160 BranchInst::Create(RetBlock, &BB); 161 } 162 163 return Changed; 164 } 165 166 /// Call SimplifyCFG on all the blocks in the function, 167 /// iterating until no more changes are made. 168 static bool iterativelySimplifyCFG(Function &F, const TargetTransformInfo &TTI, 169 const SimplifyCFGOptions &Options) { 170 bool Changed = false; 171 bool LocalChange = true; 172 173 SmallVector<std::pair<const BasicBlock *, const BasicBlock *>, 32> Edges; 174 FindFunctionBackedges(F, Edges); 175 SmallPtrSet<BasicBlock *, 16> LoopHeaders; 176 for (unsigned i = 0, e = Edges.size(); i != e; ++i) 177 LoopHeaders.insert(const_cast<BasicBlock *>(Edges[i].second)); 178 179 while (LocalChange) { 180 LocalChange = false; 181 182 // Loop over all of the basic blocks and remove them if they are unneeded. 183 for (Function::iterator BBIt = F.begin(); BBIt != F.end(); ) { 184 if (simplifyCFG(&*BBIt++, TTI, Options, &LoopHeaders)) { 185 LocalChange = true; 186 ++NumSimpl; 187 } 188 } 189 Changed |= LocalChange; 190 } 191 return Changed; 192 } 193 194 static bool simplifyFunctionCFG(Function &F, const TargetTransformInfo &TTI, 195 const SimplifyCFGOptions &Options) { 196 bool EverChanged = removeUnreachableBlocks(F); 197 EverChanged |= mergeEmptyReturnBlocks(F); 198 EverChanged |= iterativelySimplifyCFG(F, TTI, Options); 199 200 // If neither pass changed anything, we're done. 201 if (!EverChanged) return false; 202 203 // iterativelySimplifyCFG can (rarely) make some loops dead. If this happens, 204 // removeUnreachableBlocks is needed to nuke them, which means we should 205 // iterate between the two optimizations. We structure the code like this to 206 // avoid rerunning iterativelySimplifyCFG if the second pass of 207 // removeUnreachableBlocks doesn't do anything. 208 if (!removeUnreachableBlocks(F)) 209 return true; 210 211 do { 212 EverChanged = iterativelySimplifyCFG(F, TTI, Options); 213 EverChanged |= removeUnreachableBlocks(F); 214 } while (EverChanged); 215 216 return true; 217 } 218 219 // Command-line settings override compile-time settings. 220 static void applyCommandLineOverridesToOptions(SimplifyCFGOptions &Options) { 221 if (UserBonusInstThreshold.getNumOccurrences()) 222 Options.BonusInstThreshold = UserBonusInstThreshold; 223 if (UserForwardSwitchCond.getNumOccurrences()) 224 Options.ForwardSwitchCondToPhi = UserForwardSwitchCond; 225 if (UserSwitchToLookup.getNumOccurrences()) 226 Options.ConvertSwitchToLookupTable = UserSwitchToLookup; 227 if (UserKeepLoops.getNumOccurrences()) 228 Options.NeedCanonicalLoop = UserKeepLoops; 229 if (UserHoistCommonInsts.getNumOccurrences()) 230 Options.HoistCommonInsts = UserHoistCommonInsts; 231 if (UserSinkCommonInsts.getNumOccurrences()) 232 Options.SinkCommonInsts = UserSinkCommonInsts; 233 } 234 235 SimplifyCFGPass::SimplifyCFGPass(const SimplifyCFGOptions &Opts) 236 : Options(Opts) { 237 applyCommandLineOverridesToOptions(Options); 238 } 239 240 PreservedAnalyses SimplifyCFGPass::run(Function &F, 241 FunctionAnalysisManager &AM) { 242 auto &TTI = AM.getResult<TargetIRAnalysis>(F); 243 Options.AC = &AM.getResult<AssumptionAnalysis>(F); 244 if (!simplifyFunctionCFG(F, TTI, Options)) 245 return PreservedAnalyses::all(); 246 PreservedAnalyses PA; 247 PA.preserve<GlobalsAA>(); 248 return PA; 249 } 250 251 namespace { 252 struct CFGSimplifyPass : public FunctionPass { 253 static char ID; 254 SimplifyCFGOptions Options; 255 std::function<bool(const Function &)> PredicateFtor; 256 257 CFGSimplifyPass(SimplifyCFGOptions Options_ = SimplifyCFGOptions(), 258 std::function<bool(const Function &)> Ftor = nullptr) 259 : FunctionPass(ID), Options(Options_), PredicateFtor(std::move(Ftor)) { 260 261 initializeCFGSimplifyPassPass(*PassRegistry::getPassRegistry()); 262 263 // Check for command-line overrides of options for debug/customization. 264 applyCommandLineOverridesToOptions(Options); 265 } 266 267 bool runOnFunction(Function &F) override { 268 if (skipFunction(F) || (PredicateFtor && !PredicateFtor(F))) 269 return false; 270 271 Options.AC = &getAnalysis<AssumptionCacheTracker>().getAssumptionCache(F); 272 if (F.hasFnAttribute(Attribute::OptForFuzzing)) { 273 Options.setSimplifyCondBranch(false) 274 .setFoldTwoEntryPHINode(false); 275 } else { 276 Options.setSimplifyCondBranch(true) 277 .setFoldTwoEntryPHINode(true); 278 } 279 280 auto &TTI = getAnalysis<TargetTransformInfoWrapperPass>().getTTI(F); 281 return simplifyFunctionCFG(F, TTI, Options); 282 } 283 void getAnalysisUsage(AnalysisUsage &AU) const override { 284 AU.addRequired<AssumptionCacheTracker>(); 285 AU.addRequired<TargetTransformInfoWrapperPass>(); 286 AU.addPreserved<GlobalsAAWrapperPass>(); 287 } 288 }; 289 } 290 291 char CFGSimplifyPass::ID = 0; 292 INITIALIZE_PASS_BEGIN(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 293 false) 294 INITIALIZE_PASS_DEPENDENCY(TargetTransformInfoWrapperPass) 295 INITIALIZE_PASS_DEPENDENCY(AssumptionCacheTracker) 296 INITIALIZE_PASS_END(CFGSimplifyPass, "simplifycfg", "Simplify the CFG", false, 297 false) 298 299 // Public interface to the CFGSimplification pass 300 FunctionPass * 301 llvm::createCFGSimplificationPass(SimplifyCFGOptions Options, 302 std::function<bool(const Function &)> Ftor) { 303 return new CFGSimplifyPass(Options, std::move(Ftor)); 304 } 305