CorrelatedValuePropagation.cpp revision 243830
1212793Sdim//===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===// 2212793Sdim// 3212793Sdim// The LLVM Compiler Infrastructure 4212793Sdim// 5212793Sdim// This file is distributed under the University of Illinois Open Source 6212793Sdim// License. See LICENSE.TXT for details. 7212793Sdim// 8212793Sdim//===----------------------------------------------------------------------===// 9212793Sdim// 10212793Sdim// This file implements the Correlated Value Propagation pass. 11212793Sdim// 12212793Sdim//===----------------------------------------------------------------------===// 13212793Sdim 14212793Sdim#define DEBUG_TYPE "correlated-value-propagation" 15212793Sdim#include "llvm/Transforms/Scalar.h" 16221345Sdim#include "llvm/Constants.h" 17212793Sdim#include "llvm/Function.h" 18212793Sdim#include "llvm/Instructions.h" 19212793Sdim#include "llvm/Pass.h" 20218893Sdim#include "llvm/Analysis/InstructionSimplify.h" 21212793Sdim#include "llvm/Analysis/LazyValueInfo.h" 22212793Sdim#include "llvm/Support/CFG.h" 23212793Sdim#include "llvm/Transforms/Utils/Local.h" 24212793Sdim#include "llvm/ADT/Statistic.h" 25212793Sdimusing namespace llvm; 26212793Sdim 27212793SdimSTATISTIC(NumPhis, "Number of phis propagated"); 28212793SdimSTATISTIC(NumSelects, "Number of selects propagated"); 29212793SdimSTATISTIC(NumMemAccess, "Number of memory access targets propagated"); 30212793SdimSTATISTIC(NumCmps, "Number of comparisons propagated"); 31234353SdimSTATISTIC(NumDeadCases, "Number of switch cases removed"); 32212793Sdim 33212793Sdimnamespace { 34212793Sdim class CorrelatedValuePropagation : public FunctionPass { 35212793Sdim LazyValueInfo *LVI; 36218893Sdim 37212793Sdim bool processSelect(SelectInst *SI); 38212793Sdim bool processPHI(PHINode *P); 39212793Sdim bool processMemAccess(Instruction *I); 40212793Sdim bool processCmp(CmpInst *C); 41234353Sdim bool processSwitch(SwitchInst *SI); 42218893Sdim 43212793Sdim public: 44212793Sdim static char ID; 45218893Sdim CorrelatedValuePropagation(): FunctionPass(ID) { 46218893Sdim initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry()); 47218893Sdim } 48218893Sdim 49212793Sdim bool runOnFunction(Function &F); 50218893Sdim 51212793Sdim virtual void getAnalysisUsage(AnalysisUsage &AU) const { 52212793Sdim AU.addRequired<LazyValueInfo>(); 53212793Sdim } 54212793Sdim }; 55212793Sdim} 56212793Sdim 57212793Sdimchar CorrelatedValuePropagation::ID = 0; 58218893SdimINITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation", 59218893Sdim "Value Propagation", false, false) 60218893SdimINITIALIZE_PASS_DEPENDENCY(LazyValueInfo) 61218893SdimINITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation", 62218893Sdim "Value Propagation", false, false) 63212793Sdim 64212793Sdim// Public interface to the Value Propagation pass 65212793SdimPass *llvm::createCorrelatedValuePropagationPass() { 66212793Sdim return new CorrelatedValuePropagation(); 67212793Sdim} 68212793Sdim 69212793Sdimbool CorrelatedValuePropagation::processSelect(SelectInst *S) { 70212793Sdim if (S->getType()->isVectorTy()) return false; 71212793Sdim if (isa<Constant>(S->getOperand(0))) return false; 72218893Sdim 73212793Sdim Constant *C = LVI->getConstant(S->getOperand(0), S->getParent()); 74212793Sdim if (!C) return false; 75218893Sdim 76212793Sdim ConstantInt *CI = dyn_cast<ConstantInt>(C); 77212793Sdim if (!CI) return false; 78218893Sdim 79218893Sdim Value *ReplaceWith = S->getOperand(1); 80218893Sdim Value *Other = S->getOperand(2); 81218893Sdim if (!CI->isOne()) std::swap(ReplaceWith, Other); 82218893Sdim if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType()); 83218893Sdim 84218893Sdim S->replaceAllUsesWith(ReplaceWith); 85212793Sdim S->eraseFromParent(); 86212793Sdim 87212793Sdim ++NumSelects; 88218893Sdim 89212793Sdim return true; 90212793Sdim} 91212793Sdim 92212793Sdimbool CorrelatedValuePropagation::processPHI(PHINode *P) { 93212793Sdim bool Changed = false; 94218893Sdim 95212793Sdim BasicBlock *BB = P->getParent(); 96212793Sdim for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) { 97212793Sdim Value *Incoming = P->getIncomingValue(i); 98212793Sdim if (isa<Constant>(Incoming)) continue; 99218893Sdim 100212793Sdim Constant *C = LVI->getConstantOnEdge(P->getIncomingValue(i), 101212793Sdim P->getIncomingBlock(i), 102212793Sdim BB); 103212793Sdim if (!C) continue; 104218893Sdim 105212793Sdim P->setIncomingValue(i, C); 106212793Sdim Changed = true; 107212793Sdim } 108218893Sdim 109218893Sdim if (Value *V = SimplifyInstruction(P)) { 110218893Sdim P->replaceAllUsesWith(V); 111212793Sdim P->eraseFromParent(); 112212793Sdim Changed = true; 113212793Sdim } 114218893Sdim 115234353Sdim if (Changed) 116234353Sdim ++NumPhis; 117218893Sdim 118212793Sdim return Changed; 119212793Sdim} 120212793Sdim 121212793Sdimbool CorrelatedValuePropagation::processMemAccess(Instruction *I) { 122212793Sdim Value *Pointer = 0; 123212793Sdim if (LoadInst *L = dyn_cast<LoadInst>(I)) 124212793Sdim Pointer = L->getPointerOperand(); 125212793Sdim else 126212793Sdim Pointer = cast<StoreInst>(I)->getPointerOperand(); 127218893Sdim 128212793Sdim if (isa<Constant>(Pointer)) return false; 129218893Sdim 130212793Sdim Constant *C = LVI->getConstant(Pointer, I->getParent()); 131212793Sdim if (!C) return false; 132218893Sdim 133212793Sdim ++NumMemAccess; 134212793Sdim I->replaceUsesOfWith(Pointer, C); 135212793Sdim return true; 136212793Sdim} 137212793Sdim 138212793Sdim/// processCmp - If the value of this comparison could be determined locally, 139212793Sdim/// constant propagation would already have figured it out. Instead, walk 140212793Sdim/// the predecessors and statically evaluate the comparison based on information 141212793Sdim/// available on that edge. If a given static evaluation is true on ALL 142212793Sdim/// incoming edges, then it's true universally and we can simplify the compare. 143212793Sdimbool CorrelatedValuePropagation::processCmp(CmpInst *C) { 144212793Sdim Value *Op0 = C->getOperand(0); 145212793Sdim if (isa<Instruction>(Op0) && 146212793Sdim cast<Instruction>(Op0)->getParent() == C->getParent()) 147212793Sdim return false; 148218893Sdim 149212793Sdim Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); 150212793Sdim if (!Op1) return false; 151218893Sdim 152212793Sdim pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent()); 153212793Sdim if (PI == PE) return false; 154218893Sdim 155218893Sdim LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(), 156212793Sdim C->getOperand(0), Op1, *PI, C->getParent()); 157212793Sdim if (Result == LazyValueInfo::Unknown) return false; 158212793Sdim 159212793Sdim ++PI; 160212793Sdim while (PI != PE) { 161218893Sdim LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(), 162212793Sdim C->getOperand(0), Op1, *PI, C->getParent()); 163212793Sdim if (Res != Result) return false; 164212793Sdim ++PI; 165212793Sdim } 166218893Sdim 167212793Sdim ++NumCmps; 168218893Sdim 169212793Sdim if (Result == LazyValueInfo::True) 170212793Sdim C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext())); 171212793Sdim else 172212793Sdim C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext())); 173218893Sdim 174212793Sdim C->eraseFromParent(); 175212793Sdim 176212793Sdim return true; 177212793Sdim} 178212793Sdim 179234353Sdim/// processSwitch - Simplify a switch instruction by removing cases which can 180234353Sdim/// never fire. If the uselessness of a case could be determined locally then 181234353Sdim/// constant propagation would already have figured it out. Instead, walk the 182234353Sdim/// predecessors and statically evaluate cases based on information available 183234353Sdim/// on that edge. Cases that cannot fire no matter what the incoming edge can 184234353Sdim/// safely be removed. If a case fires on every incoming edge then the entire 185234353Sdim/// switch can be removed and replaced with a branch to the case destination. 186234353Sdimbool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) { 187234353Sdim Value *Cond = SI->getCondition(); 188234353Sdim BasicBlock *BB = SI->getParent(); 189234353Sdim 190234353Sdim // If the condition was defined in same block as the switch then LazyValueInfo 191234353Sdim // currently won't say anything useful about it, though in theory it could. 192234353Sdim if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB) 193234353Sdim return false; 194234353Sdim 195234353Sdim // If the switch is unreachable then trying to improve it is a waste of time. 196234353Sdim pred_iterator PB = pred_begin(BB), PE = pred_end(BB); 197234353Sdim if (PB == PE) return false; 198234353Sdim 199234353Sdim // Analyse each switch case in turn. This is done in reverse order so that 200234353Sdim // removing a case doesn't cause trouble for the iteration. 201234353Sdim bool Changed = false; 202234353Sdim for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE; 203234353Sdim ) { 204234353Sdim ConstantInt *Case = CI.getCaseValue(); 205234353Sdim 206234353Sdim // Check to see if the switch condition is equal to/not equal to the case 207234353Sdim // value on every incoming edge, equal/not equal being the same each time. 208234353Sdim LazyValueInfo::Tristate State = LazyValueInfo::Unknown; 209234353Sdim for (pred_iterator PI = PB; PI != PE; ++PI) { 210234353Sdim // Is the switch condition equal to the case value? 211234353Sdim LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ, 212234353Sdim Cond, Case, *PI, BB); 213234353Sdim // Give up on this case if nothing is known. 214234353Sdim if (Value == LazyValueInfo::Unknown) { 215234353Sdim State = LazyValueInfo::Unknown; 216234353Sdim break; 217234353Sdim } 218234353Sdim 219234353Sdim // If this was the first edge to be visited, record that all other edges 220234353Sdim // need to give the same result. 221234353Sdim if (PI == PB) { 222234353Sdim State = Value; 223234353Sdim continue; 224234353Sdim } 225234353Sdim 226234353Sdim // If this case is known to fire for some edges and known not to fire for 227234353Sdim // others then there is nothing we can do - give up. 228234353Sdim if (Value != State) { 229234353Sdim State = LazyValueInfo::Unknown; 230234353Sdim break; 231234353Sdim } 232234353Sdim } 233234353Sdim 234234353Sdim if (State == LazyValueInfo::False) { 235234353Sdim // This case never fires - remove it. 236234353Sdim CI.getCaseSuccessor()->removePredecessor(BB); 237234353Sdim SI->removeCase(CI); // Does not invalidate the iterator. 238243830Sdim 239243830Sdim // The condition can be modified by removePredecessor's PHI simplification 240243830Sdim // logic. 241243830Sdim Cond = SI->getCondition(); 242243830Sdim 243234353Sdim ++NumDeadCases; 244234353Sdim Changed = true; 245234353Sdim } else if (State == LazyValueInfo::True) { 246234353Sdim // This case always fires. Arrange for the switch to be turned into an 247234353Sdim // unconditional branch by replacing the switch condition with the case 248234353Sdim // value. 249234353Sdim SI->setCondition(Case); 250234353Sdim NumDeadCases += SI->getNumCases(); 251234353Sdim Changed = true; 252234353Sdim break; 253234353Sdim } 254234353Sdim } 255234353Sdim 256234353Sdim if (Changed) 257234353Sdim // If the switch has been simplified to the point where it can be replaced 258234353Sdim // by a branch then do so now. 259234353Sdim ConstantFoldTerminator(BB); 260234353Sdim 261234353Sdim return Changed; 262234353Sdim} 263234353Sdim 264212793Sdimbool CorrelatedValuePropagation::runOnFunction(Function &F) { 265212793Sdim LVI = &getAnalysis<LazyValueInfo>(); 266218893Sdim 267212793Sdim bool FnChanged = false; 268218893Sdim 269212793Sdim for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { 270212793Sdim bool BBChanged = false; 271212793Sdim for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) { 272212793Sdim Instruction *II = BI++; 273212793Sdim switch (II->getOpcode()) { 274212793Sdim case Instruction::Select: 275212793Sdim BBChanged |= processSelect(cast<SelectInst>(II)); 276212793Sdim break; 277212793Sdim case Instruction::PHI: 278212793Sdim BBChanged |= processPHI(cast<PHINode>(II)); 279212793Sdim break; 280212793Sdim case Instruction::ICmp: 281212793Sdim case Instruction::FCmp: 282212793Sdim BBChanged |= processCmp(cast<CmpInst>(II)); 283212793Sdim break; 284212793Sdim case Instruction::Load: 285212793Sdim case Instruction::Store: 286212793Sdim BBChanged |= processMemAccess(II); 287212793Sdim break; 288212793Sdim } 289212793Sdim } 290218893Sdim 291234353Sdim Instruction *Term = FI->getTerminator(); 292234353Sdim switch (Term->getOpcode()) { 293234353Sdim case Instruction::Switch: 294234353Sdim BBChanged |= processSwitch(cast<SwitchInst>(Term)); 295234353Sdim break; 296234353Sdim } 297234353Sdim 298212793Sdim FnChanged |= BBChanged; 299212793Sdim } 300218893Sdim 301212793Sdim return FnChanged; 302212793Sdim} 303