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