1 //===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===// 2 // 3 // The LLVM Compiler Infrastructure 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 // 10 // This file implements the Correlated Value Propagation pass. 11 // 12 //===----------------------------------------------------------------------===// 13 14 #include "llvm/Transforms/Scalar.h" 15 #include "llvm/ADT/Statistic.h" 16 #include "llvm/Analysis/InstructionSimplify.h" 17 #include "llvm/Analysis/LazyValueInfo.h" 18 #include "llvm/IR/CFG.h" 19 #include "llvm/IR/Constants.h" 20 #include "llvm/IR/Function.h" 21 #include "llvm/IR/Instructions.h" 22 #include "llvm/Pass.h" 23 #include "llvm/Support/Debug.h" 24 #include "llvm/Support/raw_ostream.h" 25 #include "llvm/Transforms/Utils/Local.h" 26 using namespace llvm; 27 28 #define DEBUG_TYPE "correlated-value-propagation" 29 30 STATISTIC(NumPhis, "Number of phis propagated"); 31 STATISTIC(NumSelects, "Number of selects propagated"); 32 STATISTIC(NumMemAccess, "Number of memory access targets propagated"); 33 STATISTIC(NumCmps, "Number of comparisons propagated"); 34 STATISTIC(NumDeadCases, "Number of switch cases removed"); 35 36 namespace { 37 class CorrelatedValuePropagation : public FunctionPass { 38 LazyValueInfo *LVI; 39 40 bool processSelect(SelectInst *SI); 41 bool processPHI(PHINode *P); 42 bool processMemAccess(Instruction *I); 43 bool processCmp(CmpInst *C); 44 bool processSwitch(SwitchInst *SI); 45 46 public: 47 static char ID; 48 CorrelatedValuePropagation(): FunctionPass(ID) { 49 initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry()); 50 } 51 52 bool runOnFunction(Function &F) override; 53 54 void getAnalysisUsage(AnalysisUsage &AU) const override { 55 AU.addRequired<LazyValueInfo>(); 56 } 57 }; 58 } 59 60 char CorrelatedValuePropagation::ID = 0; 61 INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation", 62 "Value Propagation", false, false) 63 INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) 64 INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation", 65 "Value Propagation", false, false) 66 67 // Public interface to the Value Propagation pass 68 Pass *llvm::createCorrelatedValuePropagationPass() { 69 return new CorrelatedValuePropagation(); 70 } 71 72 bool CorrelatedValuePropagation::processSelect(SelectInst *S) { 73 if (S->getType()->isVectorTy()) return false; 74 if (isa<Constant>(S->getOperand(0))) return false; 75 76 Constant *C = LVI->getConstant(S->getOperand(0), S->getParent()); 77 if (!C) return false; 78 79 ConstantInt *CI = dyn_cast<ConstantInt>(C); 80 if (!CI) return false; 81 82 Value *ReplaceWith = S->getOperand(1); 83 Value *Other = S->getOperand(2); 84 if (!CI->isOne()) std::swap(ReplaceWith, Other); 85 if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType()); 86 87 S->replaceAllUsesWith(ReplaceWith); 88 S->eraseFromParent(); 89 90 ++NumSelects; 91 92 return true; 93 } 94 95 bool CorrelatedValuePropagation::processPHI(PHINode *P) { 96 bool Changed = false; 97 98 BasicBlock *BB = P->getParent(); 99 for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) { 100 Value *Incoming = P->getIncomingValue(i); 101 if (isa<Constant>(Incoming)) continue; 102 103 Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB); 104 105 // Look if the incoming value is a select with a constant but LVI tells us 106 // that the incoming value can never be that constant. In that case replace 107 // the incoming value with the other value of the select. This often allows 108 // us to remove the select later. 109 if (!V) { 110 SelectInst *SI = dyn_cast<SelectInst>(Incoming); 111 if (!SI) continue; 112 113 Constant *C = dyn_cast<Constant>(SI->getFalseValue()); 114 if (!C) continue; 115 116 if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, 117 P->getIncomingBlock(i), BB) != 118 LazyValueInfo::False) 119 continue; 120 121 DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n'); 122 V = SI->getTrueValue(); 123 } 124 125 P->setIncomingValue(i, V); 126 Changed = true; 127 } 128 129 if (Value *V = SimplifyInstruction(P)) { 130 P->replaceAllUsesWith(V); 131 P->eraseFromParent(); 132 Changed = true; 133 } 134 135 if (Changed) 136 ++NumPhis; 137 138 return Changed; 139 } 140 141 bool CorrelatedValuePropagation::processMemAccess(Instruction *I) { 142 Value *Pointer = nullptr; 143 if (LoadInst *L = dyn_cast<LoadInst>(I)) 144 Pointer = L->getPointerOperand(); 145 else 146 Pointer = cast<StoreInst>(I)->getPointerOperand(); 147 148 if (isa<Constant>(Pointer)) return false; 149 150 Constant *C = LVI->getConstant(Pointer, I->getParent()); 151 if (!C) return false; 152 153 ++NumMemAccess; 154 I->replaceUsesOfWith(Pointer, C); 155 return true; 156 } 157 158 /// processCmp - If the value of this comparison could be determined locally, 159 /// constant propagation would already have figured it out. Instead, walk 160 /// the predecessors and statically evaluate the comparison based on information 161 /// available on that edge. If a given static evaluation is true on ALL 162 /// incoming edges, then it's true universally and we can simplify the compare. 163 bool CorrelatedValuePropagation::processCmp(CmpInst *C) { 164 Value *Op0 = C->getOperand(0); 165 if (isa<Instruction>(Op0) && 166 cast<Instruction>(Op0)->getParent() == C->getParent()) 167 return false; 168 169 Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); 170 if (!Op1) return false; 171 172 pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent()); 173 if (PI == PE) return false; 174 175 LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(), 176 C->getOperand(0), Op1, *PI, C->getParent()); 177 if (Result == LazyValueInfo::Unknown) return false; 178 179 ++PI; 180 while (PI != PE) { 181 LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(), 182 C->getOperand(0), Op1, *PI, C->getParent()); 183 if (Res != Result) return false; 184 ++PI; 185 } 186 187 ++NumCmps; 188 189 if (Result == LazyValueInfo::True) 190 C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext())); 191 else 192 C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext())); 193 194 C->eraseFromParent(); 195 196 return true; 197 } 198 199 /// processSwitch - Simplify a switch instruction by removing cases which can 200 /// never fire. If the uselessness of a case could be determined locally then 201 /// constant propagation would already have figured it out. Instead, walk the 202 /// predecessors and statically evaluate cases based on information available 203 /// on that edge. Cases that cannot fire no matter what the incoming edge can 204 /// safely be removed. If a case fires on every incoming edge then the entire 205 /// switch can be removed and replaced with a branch to the case destination. 206 bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) { 207 Value *Cond = SI->getCondition(); 208 BasicBlock *BB = SI->getParent(); 209 210 // If the condition was defined in same block as the switch then LazyValueInfo 211 // currently won't say anything useful about it, though in theory it could. 212 if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB) 213 return false; 214 215 // If the switch is unreachable then trying to improve it is a waste of time. 216 pred_iterator PB = pred_begin(BB), PE = pred_end(BB); 217 if (PB == PE) return false; 218 219 // Analyse each switch case in turn. This is done in reverse order so that 220 // removing a case doesn't cause trouble for the iteration. 221 bool Changed = false; 222 for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE; 223 ) { 224 ConstantInt *Case = CI.getCaseValue(); 225 226 // Check to see if the switch condition is equal to/not equal to the case 227 // value on every incoming edge, equal/not equal being the same each time. 228 LazyValueInfo::Tristate State = LazyValueInfo::Unknown; 229 for (pred_iterator PI = PB; PI != PE; ++PI) { 230 // Is the switch condition equal to the case value? 231 LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ, 232 Cond, Case, *PI, BB); 233 // Give up on this case if nothing is known. 234 if (Value == LazyValueInfo::Unknown) { 235 State = LazyValueInfo::Unknown; 236 break; 237 } 238 239 // If this was the first edge to be visited, record that all other edges 240 // need to give the same result. 241 if (PI == PB) { 242 State = Value; 243 continue; 244 } 245 246 // If this case is known to fire for some edges and known not to fire for 247 // others then there is nothing we can do - give up. 248 if (Value != State) { 249 State = LazyValueInfo::Unknown; 250 break; 251 } 252 } 253 254 if (State == LazyValueInfo::False) { 255 // This case never fires - remove it. 256 CI.getCaseSuccessor()->removePredecessor(BB); 257 SI->removeCase(CI); // Does not invalidate the iterator. 258 259 // The condition can be modified by removePredecessor's PHI simplification 260 // logic. 261 Cond = SI->getCondition(); 262 263 ++NumDeadCases; 264 Changed = true; 265 } else if (State == LazyValueInfo::True) { 266 // This case always fires. Arrange for the switch to be turned into an 267 // unconditional branch by replacing the switch condition with the case 268 // value. 269 SI->setCondition(Case); 270 NumDeadCases += SI->getNumCases(); 271 Changed = true; 272 break; 273 } 274 } 275 276 if (Changed) 277 // If the switch has been simplified to the point where it can be replaced 278 // by a branch then do so now. 279 ConstantFoldTerminator(BB); 280 281 return Changed; 282 } 283 284 bool CorrelatedValuePropagation::runOnFunction(Function &F) { 285 if (skipOptnoneFunction(F)) 286 return false; 287 288 LVI = &getAnalysis<LazyValueInfo>(); 289 290 bool FnChanged = false; 291 292 for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 293 bool BBChanged = false; 294 for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) { 295 Instruction *II = BI++; 296 switch (II->getOpcode()) { 297 case Instruction::Select: 298 BBChanged |= processSelect(cast<SelectInst>(II)); 299 break; 300 case Instruction::PHI: 301 BBChanged |= processPHI(cast<PHINode>(II)); 302 break; 303 case Instruction::ICmp: 304 case Instruction::FCmp: 305 BBChanged |= processCmp(cast<CmpInst>(II)); 306 break; 307 case Instruction::Load: 308 case Instruction::Store: 309 BBChanged |= processMemAccess(II); 310 break; 311 } 312 } 313 314 Instruction *Term = FI->getTerminator(); 315 switch (Term->getOpcode()) { 316 case Instruction::Switch: 317 BBChanged |= processSwitch(cast<SwitchInst>(Term)); 318 break; 319 } 320 321 FnChanged |= BBChanged; 322 } 323 324 return FnChanged; 325 } 326