| //===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===// |
| // |
| // The LLVM Compiler Infrastructure |
| // |
| // This file is distributed under the University of Illinois Open Source |
| // License. See LICENSE.TXT for details. |
| // |
| //===----------------------------------------------------------------------===// |
| // |
| // This file implements the Correlated Value Propagation pass. |
| // |
| //===----------------------------------------------------------------------===// |
| |
| #define DEBUG_TYPE "correlated-value-propagation" |
| #include "llvm/Transforms/Scalar.h" |
| #include "llvm/ADT/Statistic.h" |
| #include "llvm/Analysis/InstructionSimplify.h" |
| #include "llvm/Analysis/LazyValueInfo.h" |
| #include "llvm/IR/Constants.h" |
| #include "llvm/IR/Function.h" |
| #include "llvm/IR/Instructions.h" |
| #include "llvm/Pass.h" |
| #include "llvm/Support/CFG.h" |
| #include "llvm/Support/Debug.h" |
| #include "llvm/Support/raw_ostream.h" |
| #include "llvm/Transforms/Utils/Local.h" |
| using namespace llvm; |
| |
| STATISTIC(NumPhis, "Number of phis propagated"); |
| STATISTIC(NumSelects, "Number of selects propagated"); |
| STATISTIC(NumMemAccess, "Number of memory access targets propagated"); |
| STATISTIC(NumCmps, "Number of comparisons propagated"); |
| STATISTIC(NumDeadCases, "Number of switch cases removed"); |
| |
| namespace { |
| class CorrelatedValuePropagation : public FunctionPass { |
| LazyValueInfo *LVI; |
| |
| bool processSelect(SelectInst *SI); |
| bool processPHI(PHINode *P); |
| bool processMemAccess(Instruction *I); |
| bool processCmp(CmpInst *C); |
| bool processSwitch(SwitchInst *SI); |
| |
| public: |
| static char ID; |
| CorrelatedValuePropagation(): FunctionPass(ID) { |
| initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry()); |
| } |
| |
| bool runOnFunction(Function &F); |
| |
| virtual void getAnalysisUsage(AnalysisUsage &AU) const { |
| AU.addRequired<LazyValueInfo>(); |
| } |
| }; |
| } |
| |
| char CorrelatedValuePropagation::ID = 0; |
| INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation", |
| "Value Propagation", false, false) |
| INITIALIZE_PASS_DEPENDENCY(LazyValueInfo) |
| INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation", |
| "Value Propagation", false, false) |
| |
| // Public interface to the Value Propagation pass |
| Pass *llvm::createCorrelatedValuePropagationPass() { |
| return new CorrelatedValuePropagation(); |
| } |
| |
| bool CorrelatedValuePropagation::processSelect(SelectInst *S) { |
| if (S->getType()->isVectorTy()) return false; |
| if (isa<Constant>(S->getOperand(0))) return false; |
| |
| Constant *C = LVI->getConstant(S->getOperand(0), S->getParent()); |
| if (!C) return false; |
| |
| ConstantInt *CI = dyn_cast<ConstantInt>(C); |
| if (!CI) return false; |
| |
| Value *ReplaceWith = S->getOperand(1); |
| Value *Other = S->getOperand(2); |
| if (!CI->isOne()) std::swap(ReplaceWith, Other); |
| if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType()); |
| |
| S->replaceAllUsesWith(ReplaceWith); |
| S->eraseFromParent(); |
| |
| ++NumSelects; |
| |
| return true; |
| } |
| |
| bool CorrelatedValuePropagation::processPHI(PHINode *P) { |
| bool Changed = false; |
| |
| BasicBlock *BB = P->getParent(); |
| for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) { |
| Value *Incoming = P->getIncomingValue(i); |
| if (isa<Constant>(Incoming)) continue; |
| |
| Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB); |
| |
| // Look if the incoming value is a select with a constant but LVI tells us |
| // that the incoming value can never be that constant. In that case replace |
| // the incoming value with the other value of the select. This often allows |
| // us to remove the select later. |
| if (!V) { |
| SelectInst *SI = dyn_cast<SelectInst>(Incoming); |
| if (!SI) continue; |
| |
| Constant *C = dyn_cast<Constant>(SI->getFalseValue()); |
| if (!C) continue; |
| |
| if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, |
| P->getIncomingBlock(i), BB) != |
| LazyValueInfo::False) |
| continue; |
| |
| DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n'); |
| V = SI->getTrueValue(); |
| } |
| |
| P->setIncomingValue(i, V); |
| Changed = true; |
| } |
| |
| if (Value *V = SimplifyInstruction(P)) { |
| P->replaceAllUsesWith(V); |
| P->eraseFromParent(); |
| Changed = true; |
| } |
| |
| if (Changed) |
| ++NumPhis; |
| |
| return Changed; |
| } |
| |
| bool CorrelatedValuePropagation::processMemAccess(Instruction *I) { |
| Value *Pointer = 0; |
| if (LoadInst *L = dyn_cast<LoadInst>(I)) |
| Pointer = L->getPointerOperand(); |
| else |
| Pointer = cast<StoreInst>(I)->getPointerOperand(); |
| |
| if (isa<Constant>(Pointer)) return false; |
| |
| Constant *C = LVI->getConstant(Pointer, I->getParent()); |
| if (!C) return false; |
| |
| ++NumMemAccess; |
| I->replaceUsesOfWith(Pointer, C); |
| return true; |
| } |
| |
| /// processCmp - If the value of this comparison could be determined locally, |
| /// constant propagation would already have figured it out. Instead, walk |
| /// the predecessors and statically evaluate the comparison based on information |
| /// available on that edge. If a given static evaluation is true on ALL |
| /// incoming edges, then it's true universally and we can simplify the compare. |
| bool CorrelatedValuePropagation::processCmp(CmpInst *C) { |
| Value *Op0 = C->getOperand(0); |
| if (isa<Instruction>(Op0) && |
| cast<Instruction>(Op0)->getParent() == C->getParent()) |
| return false; |
| |
| Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); |
| if (!Op1) return false; |
| |
| pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent()); |
| if (PI == PE) return false; |
| |
| LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(), |
| C->getOperand(0), Op1, *PI, C->getParent()); |
| if (Result == LazyValueInfo::Unknown) return false; |
| |
| ++PI; |
| while (PI != PE) { |
| LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(), |
| C->getOperand(0), Op1, *PI, C->getParent()); |
| if (Res != Result) return false; |
| ++PI; |
| } |
| |
| ++NumCmps; |
| |
| if (Result == LazyValueInfo::True) |
| C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext())); |
| else |
| C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext())); |
| |
| C->eraseFromParent(); |
| |
| return true; |
| } |
| |
| /// processSwitch - Simplify a switch instruction by removing cases which can |
| /// never fire. If the uselessness of a case could be determined locally then |
| /// constant propagation would already have figured it out. Instead, walk the |
| /// predecessors and statically evaluate cases based on information available |
| /// on that edge. Cases that cannot fire no matter what the incoming edge can |
| /// safely be removed. If a case fires on every incoming edge then the entire |
| /// switch can be removed and replaced with a branch to the case destination. |
| bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) { |
| Value *Cond = SI->getCondition(); |
| BasicBlock *BB = SI->getParent(); |
| |
| // If the condition was defined in same block as the switch then LazyValueInfo |
| // currently won't say anything useful about it, though in theory it could. |
| if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB) |
| return false; |
| |
| // If the switch is unreachable then trying to improve it is a waste of time. |
| pred_iterator PB = pred_begin(BB), PE = pred_end(BB); |
| if (PB == PE) return false; |
| |
| // Analyse each switch case in turn. This is done in reverse order so that |
| // removing a case doesn't cause trouble for the iteration. |
| bool Changed = false; |
| for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE; |
| ) { |
| ConstantInt *Case = CI.getCaseValue(); |
| |
| // Check to see if the switch condition is equal to/not equal to the case |
| // value on every incoming edge, equal/not equal being the same each time. |
| LazyValueInfo::Tristate State = LazyValueInfo::Unknown; |
| for (pred_iterator PI = PB; PI != PE; ++PI) { |
| // Is the switch condition equal to the case value? |
| LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ, |
| Cond, Case, *PI, BB); |
| // Give up on this case if nothing is known. |
| if (Value == LazyValueInfo::Unknown) { |
| State = LazyValueInfo::Unknown; |
| break; |
| } |
| |
| // If this was the first edge to be visited, record that all other edges |
| // need to give the same result. |
| if (PI == PB) { |
| State = Value; |
| continue; |
| } |
| |
| // If this case is known to fire for some edges and known not to fire for |
| // others then there is nothing we can do - give up. |
| if (Value != State) { |
| State = LazyValueInfo::Unknown; |
| break; |
| } |
| } |
| |
| if (State == LazyValueInfo::False) { |
| // This case never fires - remove it. |
| CI.getCaseSuccessor()->removePredecessor(BB); |
| SI->removeCase(CI); // Does not invalidate the iterator. |
| |
| // The condition can be modified by removePredecessor's PHI simplification |
| // logic. |
| Cond = SI->getCondition(); |
| |
| ++NumDeadCases; |
| Changed = true; |
| } else if (State == LazyValueInfo::True) { |
| // This case always fires. Arrange for the switch to be turned into an |
| // unconditional branch by replacing the switch condition with the case |
| // value. |
| SI->setCondition(Case); |
| NumDeadCases += SI->getNumCases(); |
| Changed = true; |
| break; |
| } |
| } |
| |
| if (Changed) |
| // If the switch has been simplified to the point where it can be replaced |
| // by a branch then do so now. |
| ConstantFoldTerminator(BB); |
| |
| return Changed; |
| } |
| |
| bool CorrelatedValuePropagation::runOnFunction(Function &F) { |
| LVI = &getAnalysis<LazyValueInfo>(); |
| |
| bool FnChanged = false; |
| |
| for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) { |
| bool BBChanged = false; |
| for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) { |
| Instruction *II = BI++; |
| switch (II->getOpcode()) { |
| case Instruction::Select: |
| BBChanged |= processSelect(cast<SelectInst>(II)); |
| break; |
| case Instruction::PHI: |
| BBChanged |= processPHI(cast<PHINode>(II)); |
| break; |
| case Instruction::ICmp: |
| case Instruction::FCmp: |
| BBChanged |= processCmp(cast<CmpInst>(II)); |
| break; |
| case Instruction::Load: |
| case Instruction::Store: |
| BBChanged |= processMemAccess(II); |
| break; |
| } |
| } |
| |
| Instruction *Term = FI->getTerminator(); |
| switch (Term->getOpcode()) { |
| case Instruction::Switch: |
| BBChanged |= processSwitch(cast<SwitchInst>(Term)); |
| break; |
| } |
| |
| FnChanged |= BBChanged; |
| } |
| |
| return FnChanged; |
| } |