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//===- 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/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(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);
static char ID;
CorrelatedValuePropagation(): FunctionPass(ID) {
bool runOnFunction(Function &F) override;
void getAnalysisUsage(AnalysisUsage &AU) const override {
char CorrelatedValuePropagation::ID = 0;
INITIALIZE_PASS_BEGIN(CorrelatedValuePropagation, "correlated-propagation",
"Value Propagation", false, false)
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());
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) !=
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)) {
Changed = true;
if (Changed)
return Changed;
bool CorrelatedValuePropagation::processMemAccess(Instruction *I) {
Value *Pointer = nullptr;
if (LoadInst *L = dyn_cast<LoadInst>(I))
Pointer = L->getPointerOperand();
Pointer = cast<StoreInst>(I)->getPointerOperand();
if (isa<Constant>(Pointer)) return false;
Constant *C = LVI->getConstant(Pointer, I->getParent(), I);
if (!C) return false;
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(), C);
if (Result == LazyValueInfo::Unknown) return false;
while (PI != PE) {
LazyValueInfo::Tristate Res = LVI->getPredicateOnEdge(C->getPredicate(),
C->getOperand(0), Op1, *PI,
C->getParent(), C);
if (Res != Result) return false;
if (Result == LazyValueInfo::True)
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;
// 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;
// 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;
if (State == LazyValueInfo::False) {
// This case never fires - remove it.
SI->removeCase(CI); // Does not invalidate the iterator.
// The condition can be modified by removePredecessor's PHI simplification
// logic.
Cond = SI->getCondition();
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.
NumDeadCases += SI->getNumCases();
Changed = true;
if (Changed)
// If the switch has been simplified to the point where it can be replaced
// by a branch then do so now.
return Changed;
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));
case Instruction::PHI:
BBChanged |= processPHI(cast<PHINode>(II));
case Instruction::ICmp:
case Instruction::FCmp:
BBChanged |= processCmp(cast<CmpInst>(II));
case Instruction::Load:
case Instruction::Store:
BBChanged |= processMemAccess(II);
Instruction *Term = FI->getTerminator();
switch (Term->getOpcode()) {
case Instruction::Switch:
BBChanged |= processSwitch(cast<SwitchInst>(Term));
FnChanged |= BBChanged;
return FnChanged;