llvm-6502/lib/Transforms/Scalar/CorrelatedValuePropagation.cpp
Benjamin Kramer e8aa36a4af CVP: If we have a PHI with an incoming select, try to skip the select.
This is a common pattern with dyn_cast and similar constructs, when the
PHI no longer depends on the select it can often be turned into a simpler
construct or even get hoisted out of the loop.

PR15340.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@175995 91177308-0d34-0410-b5e6-96231b3b80d8
2013-02-24 15:34:43 +00:00

322 lines
10 KiB
C++

//===- 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;
}