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git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@207196 91177308-0d34-0410-b5e6-96231b3b80d8
326 lines
10 KiB
C++
326 lines
10 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/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/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(NumDeadCases, "Number of switch cases removed");
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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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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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}
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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());
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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);
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// Look if the incoming value is a select with a constant but LVI tells us
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// that the incoming value can never be that constant. In that case replace
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// the incoming value with the other value of the select. This often allows
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// us to remove the 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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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) !=
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LazyValueInfo::False)
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continue;
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DEBUG(dbgs() << "CVP: Threading PHI over " << *SI << '\n');
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V = SI->getTrueValue();
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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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if (Value *V = SimplifyInstruction(P)) {
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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());
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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 - If the value of this comparison could be determined locally,
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/// constant propagation would already have figured it out. Instead, walk
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/// the predecessors and statically evaluate the comparison based on information
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/// available on that edge. If a given static evaluation is true on ALL
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/// incoming edges, then it's true universally and we can simplify the compare.
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bool CorrelatedValuePropagation::processCmp(CmpInst *C) {
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Value *Op0 = C->getOperand(0);
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if (isa<Instruction>(Op0) &&
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cast<Instruction>(Op0)->getParent() == C->getParent())
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return false;
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Constant *Op1 = dyn_cast<Constant>(C->getOperand(1));
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if (!Op1) return false;
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pred_iterator PI = pred_begin(C->getParent()), PE = pred_end(C->getParent());
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if (PI == PE) return false;
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LazyValueInfo::Tristate Result = LVI->getPredicateOnEdge(C->getPredicate(),
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C->getOperand(0), Op1, *PI, C->getParent());
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if (Result == LazyValueInfo::Unknown) return false;
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++PI;
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while (PI != PE) {
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LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
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C->getOperand(0), Op1, *PI, C->getParent());
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if (Res != Result) return false;
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++PI;
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}
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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, BB);
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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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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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}
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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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}
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FnChanged |= BBChanged;
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}
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return FnChanged;
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}
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