464 lines
16 KiB
C++
464 lines
16 KiB
C++
//===- CorrelatedValuePropagation.cpp - Propagate CFG-derived info --------===//
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//
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// The LLVM Compiler Infrastructure
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements the Correlated Value Propagation pass.
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//
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//===----------------------------------------------------------------------===//
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#include "llvm/Transforms/Scalar.h"
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#include "llvm/ADT/Statistic.h"
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#include "llvm/Analysis/GlobalsModRef.h"
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#include "llvm/Analysis/InstructionSimplify.h"
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#include "llvm/Analysis/LazyValueInfo.h"
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#include "llvm/IR/CFG.h"
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#include "llvm/IR/Constants.h"
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#include "llvm/IR/Function.h"
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#include "llvm/IR/Instructions.h"
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#include "llvm/IR/Module.h"
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#include "llvm/Pass.h"
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#include "llvm/Support/Debug.h"
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#include "llvm/Support/raw_ostream.h"
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#include "llvm/Transforms/Utils/Local.h"
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using namespace llvm;
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#define DEBUG_TYPE "correlated-value-propagation"
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STATISTIC(NumPhis, "Number of phis propagated");
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STATISTIC(NumSelects, "Number of selects propagated");
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STATISTIC(NumMemAccess, "Number of memory access targets propagated");
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STATISTIC(NumCmps, "Number of comparisons propagated");
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STATISTIC(NumReturns, "Number of return values propagated");
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STATISTIC(NumDeadCases, "Number of switch cases removed");
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STATISTIC(NumSDivs, "Number of sdiv converted to udiv");
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namespace {
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class CorrelatedValuePropagation : public FunctionPass {
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LazyValueInfo *LVI;
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bool processSelect(SelectInst *SI);
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bool processPHI(PHINode *P);
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bool processMemAccess(Instruction *I);
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bool processCmp(CmpInst *C);
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bool processSwitch(SwitchInst *SI);
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bool processCallSite(CallSite CS);
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bool processSDiv(BinaryOperator *SDI);
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/// Return a constant value for V usable at At and everything it
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/// dominates. If no such Constant can be found, return nullptr.
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Constant *getConstantAt(Value *V, Instruction *At);
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public:
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static char ID;
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CorrelatedValuePropagation(): FunctionPass(ID) {
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initializeCorrelatedValuePropagationPass(*PassRegistry::getPassRegistry());
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}
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bool runOnFunction(Function &F) override;
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void getAnalysisUsage(AnalysisUsage &AU) const override {
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AU.addRequired<LazyValueInfo>();
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AU.addPreserved<GlobalsAAWrapperPass>();
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}
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};
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}
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char CorrelatedValuePropagation::ID = 0;
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INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
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"Value Propagation", false, false)
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INITIALIZE_PASS_DEPENDENCY(LazyValueInfo)
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INITIALIZE_PASS_END(CorrelatedValuePropagation, "correlated-propagation",
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"Value Propagation", false, false)
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// Public interface to the Value Propagation pass
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Pass *llvm::createCorrelatedValuePropagationPass() {
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return new CorrelatedValuePropagation();
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}
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bool CorrelatedValuePropagation::processSelect(SelectInst *S) {
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if (S->getType()->isVectorTy()) return false;
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if (isa<Constant>(S->getOperand(0))) return false;
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Constant *C = LVI->getConstant(S->getOperand(0), S->getParent(), S);
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if (!C) return false;
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ConstantInt *CI = dyn_cast<ConstantInt>(C);
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if (!CI) return false;
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Value *ReplaceWith = S->getOperand(1);
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Value *Other = S->getOperand(2);
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if (!CI->isOne()) std::swap(ReplaceWith, Other);
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if (ReplaceWith == S) ReplaceWith = UndefValue::get(S->getType());
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S->replaceAllUsesWith(ReplaceWith);
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S->eraseFromParent();
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++NumSelects;
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return true;
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}
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bool CorrelatedValuePropagation::processPHI(PHINode *P) {
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bool Changed = false;
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BasicBlock *BB = P->getParent();
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for (unsigned i = 0, e = P->getNumIncomingValues(); i < e; ++i) {
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Value *Incoming = P->getIncomingValue(i);
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if (isa<Constant>(Incoming)) continue;
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Value *V = LVI->getConstantOnEdge(Incoming, P->getIncomingBlock(i), BB, P);
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// Look if the incoming value is a select with a scalar condition for which
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// LVI can tells us the value. In that case replace the incoming value with
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// the appropriate value of the select. This often allows us to remove the
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// select later.
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if (!V) {
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SelectInst *SI = dyn_cast<SelectInst>(Incoming);
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if (!SI) continue;
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Value *Condition = SI->getCondition();
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if (!Condition->getType()->isVectorTy()) {
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if (Constant *C = LVI->getConstantOnEdge(
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Condition, P->getIncomingBlock(i), BB, P)) {
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if (C->isOneValue()) {
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V = SI->getTrueValue();
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} else if (C->isZeroValue()) {
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V = SI->getFalseValue();
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}
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// Once LVI learns to handle vector types, we could also add support
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// for vector type constants that are not all zeroes or all ones.
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}
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}
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// Look if the select has a constant but LVI tells us that the incoming
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// value can never be that constant. In that case replace the incoming
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// value with the other value of the select. This often allows us to
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// remove the select later.
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if (!V) {
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Constant *C = dyn_cast<Constant>(SI->getFalseValue());
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if (!C) continue;
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if (LVI->getPredicateOnEdge(ICmpInst::ICMP_EQ, SI, C,
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P->getIncomingBlock(i), BB, P) !=
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LazyValueInfo::False)
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continue;
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V = SI->getTrueValue();
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}
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DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
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}
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P->setIncomingValue(i, V);
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Changed = true;
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}
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// FIXME: Provide TLI, DT, AT to SimplifyInstruction.
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const DataLayout &DL = BB->getModule()->getDataLayout();
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if (Value *V = SimplifyInstruction(P, DL)) {
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P->replaceAllUsesWith(V);
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P->eraseFromParent();
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Changed = true;
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}
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if (Changed)
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++NumPhis;
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return Changed;
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}
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bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
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Value *Pointer = nullptr;
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if (LoadInst *L = dyn_cast<LoadInst>(I))
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Pointer = L->getPointerOperand();
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else
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Pointer = cast<StoreInst>(I)->getPointerOperand();
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if (isa<Constant>(Pointer)) return false;
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Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
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if (!C) return false;
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++NumMemAccess;
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I->replaceUsesOfWith(Pointer, C);
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return true;
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}
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/// processCmp - See if LazyValueInfo's ability to exploit edge conditions,
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/// or range information is sufficient to prove this comparison. Even for
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/// local conditions, this can sometimes prove conditions instcombine can't by
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/// exploiting range information.
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bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
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Value *Op0 = C->getOperand(0);
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Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
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if (!Op1) return false;
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// As a policy choice, we choose not to waste compile time on anything where
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// the comparison is testing local values. While LVI can sometimes reason
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// about such cases, it's not its primary purpose. We do make sure to do
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// the block local query for uses from terminator instructions, but that's
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// handled in the code for each terminator.
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auto *I = dyn_cast<Instruction>(Op0);
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if (I && I->getParent() == C->getParent())
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return false;
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LazyValueInfo::Tristate Result =
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LVI->getPredicateAt(C->getPredicate(), Op0, Op1, C);
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if (Result == LazyValueInfo::Unknown) return false;
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++NumCmps;
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if (Result == LazyValueInfo::True)
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C->replaceAllUsesWith(ConstantInt::getTrue(C->getContext()));
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else
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C->replaceAllUsesWith(ConstantInt::getFalse(C->getContext()));
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C->eraseFromParent();
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return true;
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}
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/// processSwitch - Simplify a switch instruction by removing cases which can
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/// never fire. If the uselessness of a case could be determined locally then
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/// constant propagation would already have figured it out. Instead, walk the
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/// predecessors and statically evaluate cases based on information available
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/// on that edge. Cases that cannot fire no matter what the incoming edge can
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/// safely be removed. If a case fires on every incoming edge then the entire
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/// switch can be removed and replaced with a branch to the case destination.
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bool CorrelatedValuePropagation::processSwitch(SwitchInst *SI) {
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Value *Cond = SI->getCondition();
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BasicBlock *BB = SI->getParent();
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// If the condition was defined in same block as the switch then LazyValueInfo
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// currently won't say anything useful about it, though in theory it could.
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if (isa<Instruction>(Cond) && cast<Instruction>(Cond)->getParent() == BB)
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return false;
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// If the switch is unreachable then trying to improve it is a waste of time.
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pred_iterator PB = pred_begin(BB), PE = pred_end(BB);
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if (PB == PE) return false;
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// Analyse each switch case in turn. This is done in reverse order so that
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// removing a case doesn't cause trouble for the iteration.
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bool Changed = false;
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for (SwitchInst::CaseIt CI = SI->case_end(), CE = SI->case_begin(); CI-- != CE;
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) {
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ConstantInt *Case = CI.getCaseValue();
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// Check to see if the switch condition is equal to/not equal to the case
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// value on every incoming edge, equal/not equal being the same each time.
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LazyValueInfo::Tristate State = LazyValueInfo::Unknown;
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for (pred_iterator PI = PB; PI != PE; ++PI) {
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// Is the switch condition equal to the case value?
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LazyValueInfo::Tristate Value = LVI->getPredicateOnEdge(CmpInst::ICMP_EQ,
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Cond, Case, *PI,
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BB, SI);
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// Give up on this case if nothing is known.
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if (Value == LazyValueInfo::Unknown) {
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State = LazyValueInfo::Unknown;
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break;
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}
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// If this was the first edge to be visited, record that all other edges
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// need to give the same result.
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if (PI == PB) {
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State = Value;
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continue;
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}
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// If this case is known to fire for some edges and known not to fire for
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// others then there is nothing we can do - give up.
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if (Value != State) {
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State = LazyValueInfo::Unknown;
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break;
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}
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}
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if (State == LazyValueInfo::False) {
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// This case never fires - remove it.
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CI.getCaseSuccessor()->removePredecessor(BB);
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SI->removeCase(CI); // Does not invalidate the iterator.
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// The condition can be modified by removePredecessor's PHI simplification
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// logic.
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Cond = SI->getCondition();
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++NumDeadCases;
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Changed = true;
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} else if (State == LazyValueInfo::True) {
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// This case always fires. Arrange for the switch to be turned into an
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// unconditional branch by replacing the switch condition with the case
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// value.
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SI->setCondition(Case);
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NumDeadCases += SI->getNumCases();
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Changed = true;
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break;
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}
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}
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if (Changed)
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// If the switch has been simplified to the point where it can be replaced
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// by a branch then do so now.
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ConstantFoldTerminator(BB);
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return Changed;
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}
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/// processCallSite - Infer nonnull attributes for the arguments at the
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/// specified callsite.
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bool CorrelatedValuePropagation::processCallSite(CallSite CS) {
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SmallVector<unsigned, 4> Indices;
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unsigned ArgNo = 0;
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for (Value *V : CS.args()) {
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PointerType *Type = dyn_cast<PointerType>(V->getType());
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// Try to mark pointer typed parameters as non-null. We skip the
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// relatively expensive analysis for constants which are obviously either
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// null or non-null to start with.
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if (Type && !CS.paramHasAttr(ArgNo + 1, Attribute::NonNull) &&
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!isa<Constant>(V) &&
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LVI->getPredicateAt(ICmpInst::ICMP_EQ, V,
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ConstantPointerNull::get(Type),
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CS.getInstruction()) == LazyValueInfo::False)
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Indices.push_back(ArgNo + 1);
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ArgNo++;
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}
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assert(ArgNo == CS.arg_size() && "sanity check");
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if (Indices.empty())
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return false;
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AttributeSet AS = CS.getAttributes();
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LLVMContext &Ctx = CS.getInstruction()->getContext();
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AS = AS.addAttribute(Ctx, Indices, Attribute::get(Ctx, Attribute::NonNull));
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CS.setAttributes(AS);
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return true;
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}
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/// See if LazyValueInfo's ability to exploit edge conditions, or range
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/// information is sufficient to prove the both operands of this SDiv are
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/// positive. If this is the case, replace the SDiv with a UDiv. Even for local
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/// conditions, this can sometimes prove conditions instcombine can't by
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/// exploiting range information.
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bool CorrelatedValuePropagation::processSDiv(BinaryOperator *SDI) {
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if (SDI->getType()->isVectorTy())
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return false;
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for (Value *O : SDI->operands()) {
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// As a policy choice, we choose not to waste compile time on anything where
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// the operands are local defs. While LVI can sometimes reason about such
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// cases, it's not its primary purpose.
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auto *I = dyn_cast<Instruction>(O);
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if (I && I->getParent() == SDI->getParent())
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return false;
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}
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Constant *Zero = ConstantInt::get(SDI->getType(), 0);
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for (Value *O : SDI->operands()) {
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LazyValueInfo::Tristate Result =
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LVI->getPredicateAt(ICmpInst::ICMP_SGE, O, Zero, SDI);
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if (Result != LazyValueInfo::True)
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return false;
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}
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++NumSDivs;
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auto *BO = BinaryOperator::CreateUDiv(SDI->getOperand(0), SDI->getOperand(1),
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SDI->getName(), SDI);
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BO->setIsExact(SDI->isExact());
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SDI->replaceAllUsesWith(BO);
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SDI->eraseFromParent();
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return true;
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}
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Constant *CorrelatedValuePropagation::getConstantAt(Value *V, Instruction *At) {
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if (Constant *C = LVI->getConstant(V, At->getParent(), At))
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return C;
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// TODO: The following really should be sunk inside LVI's core algorithm, or
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// at least the outer shims around such.
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auto *C = dyn_cast<CmpInst>(V);
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if (!C) return nullptr;
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Value *Op0 = C->getOperand(0);
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Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
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if (!Op1) return nullptr;
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LazyValueInfo::Tristate Result =
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LVI->getPredicateAt(C->getPredicate(), Op0, Op1, At);
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if (Result == LazyValueInfo::Unknown)
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return nullptr;
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return (Result == LazyValueInfo::True) ?
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ConstantInt::getTrue(C->getContext()) :
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ConstantInt::getFalse(C->getContext());
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}
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bool CorrelatedValuePropagation::runOnFunction(Function &F) {
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if (skipOptnoneFunction(F))
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return false;
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LVI = &getAnalysis<LazyValueInfo>();
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bool FnChanged = false;
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for (Function::iterator FI = F.begin(), FE = F.end(); FI != FE; ++FI) {
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bool BBChanged = false;
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for (BasicBlock::iterator BI = FI->begin(), BE = FI->end(); BI != BE; ) {
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Instruction *II = &*BI++;
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switch (II->getOpcode()) {
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case Instruction::Select:
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BBChanged |= processSelect(cast<SelectInst>(II));
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break;
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case Instruction::PHI:
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BBChanged |= processPHI(cast<PHINode>(II));
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break;
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case Instruction::ICmp:
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case Instruction::FCmp:
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BBChanged |= processCmp(cast<CmpInst>(II));
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break;
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case Instruction::Load:
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case Instruction::Store:
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BBChanged |= processMemAccess(II);
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break;
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case Instruction::Call:
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case Instruction::Invoke:
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BBChanged |= processCallSite(CallSite(II));
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break;
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case Instruction::SDiv:
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BBChanged |= processSDiv(cast<BinaryOperator>(II));
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break;
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}
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}
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Instruction *Term = FI->getTerminator();
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switch (Term->getOpcode()) {
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case Instruction::Switch:
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BBChanged |= processSwitch(cast<SwitchInst>(Term));
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break;
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case Instruction::Ret: {
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auto *RI = cast<ReturnInst>(Term);
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// Try to determine the return value if we can. This is mainly here to
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// simplify the writing of unit tests, but also helps to enable IPO by
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// constant folding the return values of callees.
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auto *RetVal = RI->getReturnValue();
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if (!RetVal) break; // handle "ret void"
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if (isa<Constant>(RetVal)) break; // nothing to do
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if (auto *C = getConstantAt(RetVal, RI)) {
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++NumReturns;
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RI->replaceUsesOfWith(RetVal, C);
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BBChanged = true;
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}
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}
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};
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FnChanged |= BBChanged;
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}
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return FnChanged;
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}
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