//===- 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. // //===----------------------------------------------------------------------===// #include "llvm/Transforms/Scalar.h" #include "llvm/ADT/Statistic.h" #include "llvm/Analysis/GlobalsModRef.h" #include "llvm/Analysis/InstructionSimplify.h" #include "llvm/Analysis/LazyValueInfo.h" #include "llvm/IR/CFG.h" #include "llvm/IR/Constants.h" #include "llvm/IR/Function.h" #include "llvm/IR/Instructions.h" #include "llvm/IR/Module.h" #include "llvm/Pass.h" #include "llvm/Support/Debug.h" #include "llvm/Support/raw_ostream.h" #include "llvm/Transforms/Utils/Local.h" using namespace llvm; #define DEBUG_TYPE "correlated-value-propagation" 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(NumReturns, "Number of return values 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); bool processCallSite(CallSite CS); /// Return a constant value for V usable at At and everything it /// dominates. If no such Constant can be found, return nullptr. Constant *getConstantAt(Value *V, Instruction *At); public: static char ID; CorrelatedValuePropagation(): FunctionPass(ID) { initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry()); } bool runOnFunction(Function &F) override; void getAnalysisUsage(AnalysisUsage &AU) const override { AU.addRequired<LazyValueInfo>(); AU.addPreserved<GlobalsAAWrapperPass>(); } }; } 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(), S); 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, P); // Look if the incoming value is a select with a scalar condition for which // LVI can tells us the value. In that case replace the incoming value with // the appropriate value of the select. This often allows us to remove the // select later. if (!V) { SelectInst *SI = dyn_cast<SelectInst>(Incoming); if (!SI) continue; Value *Condition = SI->getCondition(); if (!Condition->getType()->isVectorTy()) { if (Constant *C = LVI->getConstantOnEdge( Condition, P->getIncomingBlock(i), BB, P)) { if (C->isOneValue()) { V = SI->getTrueValue(); } else if (C->isZeroValue()) { V = SI->getFalseValue(); } // Once LVI learns to handle vector types, we could also add support // for vector type constants that are not all zeroes or all ones. } } // Look if the select has 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) { Constant *C = dyn_cast<Constant>(SI->getFalseValue()); if (!C) continue; if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C, P->getIncomingBlock(i), BB, P) != LazyValueInfo::False) continue; V = SI->getTrueValue(); } DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n'); } P->setIncomingValue(i, V); Changed = true; } // FIXME: Provide TLI, DT, AT to SimplifyInstruction. const DataLayout &DL = BB->getModule()->getDataLayout(); if (Value *V = SimplifyInstruction(P, DL)) { P->replaceAllUsesWith(V); P->eraseFromParent(); Changed = true; } if (Changed) ++NumPhis; return Changed; } bool CorrelatedValuePropagation::processMemAccess(Instruction *I) { Value *Pointer = nullptr; 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(), I); if (!C) return false; ++NumMemAccess; I->replaceUsesOfWith(Pointer, C); return true; } /// processCmp - See if LazyValueInfo's ability to exploit edge conditions, /// or range information is sufficient to prove this comparison. Even for /// local conditions, this can sometimes prove conditions instcombine can't by /// exploiting range information. bool CorrelatedValuePropagation::processCmp(CmpInst *C) { Value *Op0 = C->getOperand(0); Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); if (!Op1) return false; // As a policy choice, we choose not to waste compile time on anything where // the comparison is testing local values. While LVI can sometimes reason // about such cases, it's not its primary purpose. We do make sure to do // the block local query for uses from terminator instructions, but that's // handled in the code for each terminator. auto *I = dyn_cast<Instruction>(Op0); if (I && I->getParent() == C->getParent()) return false; LazyValueInfo::Tristate Result = LVI->getPredicateAt(C->getPredicate(), Op0, Op1, C); if (Result == LazyValueInfo::Unknown) return false; ++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, SI); // 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; } /// processCallSite - Infer nonnull attributes for the arguments at the /// specified callsite. bool CorrelatedValuePropagation::processCallSite(CallSite CS) { SmallVector<unsigned, 4> Indices; unsigned ArgNo = 0; for (Value *V : CS.args()) { PointerType *Type = dyn_cast<PointerType>(V->getType()); if (Type && !CS.paramHasAttr(ArgNo + 1, Attribute::NonNull) && LVI->getPredicateAt(ICmpInst::ICMP_EQ, V, ConstantPointerNull::get(Type), CS.getInstruction()) == LazyValueInfo::False) Indices.push_back(ArgNo + 1); ArgNo++; } assert(ArgNo == CS.arg_size() && "sanity check"); if (Indices.empty()) return false; AttributeSet AS = CS.getAttributes(); LLVMContext &Ctx = CS.getInstruction()->getContext(); AS = AS.addAttribute(Ctx, Indices, Attribute::get(Ctx, Attribute::NonNull)); CS.setAttributes(AS); return true; } Constant *CorrelatedValuePropagation::getConstantAt(Value *V, Instruction *At) { if (Constant *C = LVI->getConstant(V, At->getParent(), At)) return C; // TODO: The following really should be sunk inside LVI's core algorithm, or // at least the outer shims around such. auto *C = dyn_cast<CmpInst>(V); if (!C) return nullptr; Value *Op0 = C->getOperand(0); Constant *Op1 = dyn_cast<Constant>(C->getOperand(1)); if (!Op1) return nullptr; LazyValueInfo::Tristate Result = LVI->getPredicateAt(C->getPredicate(), Op0, Op1, At); if (Result == LazyValueInfo::Unknown) return nullptr; return (Result == LazyValueInfo::True) ? ConstantInt::getTrue(C->getContext()) : ConstantInt::getFalse(C->getContext()); } bool CorrelatedValuePropagation::runOnFunction(Function &F) { if (skipOptnoneFunction(F)) return false; 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; case Instruction::Call: case Instruction::Invoke: BBChanged |= processCallSite(CallSite(II)); break; } } Instruction *Term = FI->getTerminator(); switch (Term->getOpcode()) { case Instruction::Switch: BBChanged |= processSwitch(cast<SwitchInst>(Term)); break; case Instruction::Ret: { auto *RI = cast<ReturnInst>(Term); // Try to determine the return value if we can. This is mainly here to // simplify the writing of unit tests, but also helps to enable IPO by // constant folding the return values of callees. auto *RetVal = RI->getReturnValue(); if (!RetVal) break; // handle "ret void" if (isa<Constant>(RetVal)) break; // nothing to do if (auto *C = getConstantAt(RetVal, RI)) { ++NumReturns; RI->replaceUsesOfWith(RetVal, C); BBChanged = true; } } }; FnChanged |= BBChanged; } return FnChanged; }