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Fix a layering violation: hasConstantValue, which is part of the PHINode

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class, uses DominatorTree which is an analysis.  This change moves all of
the tricky hasConstantValue logic to SimplifyInstruction, and replaces it
with a very simple literal implementation.  I already taught users of
hasConstantValue that need tricky stuff to use SimplifyInstruction instead.
I didn't update InlineFunction because the IR looks like it might be in a
funky state at the point it calls hasConstantValue, which makes calling
SimplifyInstruction dangerous since it can in theory do a lot of tricky
reasoning.  This may be a pessimization, for example in the case where
all phi node operands are either undef or a fixed constant.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@119459 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Duncan Sands 2010-11-17 04:30:22 +00:00
parent a0c5244e85
commit ff10341183
4 changed files with 47 additions and 69 deletions
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9
include/llvm/Instructions.h
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@ -29,7 +29,6 @@ class ConstantInt;
class ConstantRange;
class APInt;
class LLVMContext;
class DominatorTree;
//===----------------------------------------------------------------------===//
// AllocaInst Class
@ -1946,13 +1945,7 @@ public:
/// hasConstantValue - If the specified PHI node always merges together the
/// same value, return the value, otherwise return null.
///
/// If the PHI has undef operands, but all the rest of the operands are
/// some unique value, return that value if it can be proved that the
/// value dominates the PHI. If DT is null, use a conservative check,
/// otherwise use DT to test for dominance.
///
Value *hasConstantValue(const DominatorTree *DT = 0) const;
Value *hasConstantValue() const;
/// Methods for support type inquiry through isa, cast, and dyn_cast:
static inline bool classof(const PHINode *) { return true; }

40
lib/Analysis/InstructionSimplify.cpp
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@ -173,7 +173,7 @@ static Value *ThreadBinOpOverPHI(unsigned Opcode, Value *LHS, Value *RHS,
Value *CommonValue = 0;
for (unsigned i = 0, e = PI->getNumIncomingValues(); i != e; ++i) {
Value *Incoming = PI->getIncomingValue(i);
// If the incoming value is the phi node itself, it can be safely skipped.
// If the incoming value is the phi node itself, it can safely be skipped.
if (Incoming == PI) continue;
Value *V = PI == LHS ?
SimplifyBinOp(Opcode, Incoming, RHS, TD, DT, MaxRecurse) :
@ -211,7 +211,7 @@ static Value *ThreadCmpOverPHI(CmpInst::Predicate Pred, Value *LHS, Value *RHS,
Value *CommonValue = 0;
for (unsigned i = 0, e = PI->getNumIncomingValues(); i != e; ++i) {
Value *Incoming = PI->getIncomingValue(i);
// If the incoming value is the phi node itself, it can be safely skipped.
// If the incoming value is the phi node itself, it can safely be skipped.
if (Incoming == PI) continue;
Value *V = SimplifyCmpInst(Pred, Incoming, RHS, TD, DT, MaxRecurse);
// If the operation failed to simplify, or simplified to a different value
@ -663,6 +663,40 @@ Value *llvm::SimplifyGEPInst(Value *const *Ops, unsigned NumOps,
(Constant *const*)Ops+1, NumOps-1);
}
/// SimplifyPHINode - See if we can fold the given phi. If not, returns null.
static Value *SimplifyPHINode(PHINode *PN, const DominatorTree *DT) {
// If all of the PHI's incoming values are the same then replace the PHI node
// with the common value.
Value *CommonValue = 0;
bool HasUndefInput = false;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *Incoming = PN->getIncomingValue(i);
// If the incoming value is the phi node itself, it can safely be skipped.
if (Incoming == PN) continue;
if (isa<UndefValue>(Incoming)) {
// Remember that we saw an undef value, but otherwise ignore them.
HasUndefInput = true;
continue;
}
if (CommonValue && Incoming != CommonValue)
return 0; // Not the same, bail out.
CommonValue = Incoming;
}
// If CommonValue is null then all of the incoming values were either undef or
// equal to the phi node itself.
if (!CommonValue)
return UndefValue::get(PN->getType());
// If we have a PHI node like phi(X, undef, X), where X is defined by some
// instruction, we cannot return X as the result of the PHI node unless it
// dominates the PHI block.
if (HasUndefInput)
return ValueDominatesPHI(CommonValue, PN, DT) ? CommonValue : 0;
return CommonValue;
}
//=== Helper functions for higher up the class hierarchy.
@ -748,7 +782,7 @@ Value *llvm::SimplifyInstruction(Instruction *I, const TargetData *TD,
return SimplifyGEPInst(&Ops[0], Ops.size(), TD, DT);
}
case Instruction::PHI:
return cast<PHINode>(I)->hasConstantValue(DT);
return SimplifyPHINode(cast<PHINode>(I), DT);
}
}

4
lib/Analysis/Lint.cpp
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@ -582,7 +582,7 @@ Value *Lint::findValueImpl(Value *V, bool OffsetOk,
BBI = BB->end();
}
} else if (PHINode *PN = dyn_cast<PHINode>(V)) {
if (Value *W = PN->hasConstantValue(DT))
if (Value *W = PN->hasConstantValue())
return findValueImpl(W, OffsetOk, Visited);
} else if (CastInst *CI = dyn_cast<CastInst>(V)) {
if (CI->isNoopCast(TD ? TD->getIntPtrType(V->getContext()) :
@ -615,7 +615,7 @@ Value *Lint::findValueImpl(Value *V, bool OffsetOk,
// As a last resort, try SimplifyInstruction or constant folding.
if (Instruction *Inst = dyn_cast<Instruction>(V)) {
if (Value *W = SimplifyInstruction(Inst, TD))
if (Value *W = SimplifyInstruction(Inst, TD, DT))
if (W != Inst)
return findValueImpl(W, OffsetOk, Visited);
} else if (ConstantExpr *CE = dyn_cast<ConstantExpr>(V)) {

63
lib/VMCore/Instructions.cpp
View File

@ -19,7 +19,6 @@
#include "llvm/Instructions.h"
#include "llvm/Module.h"
#include "llvm/Operator.h"
#include "llvm/Analysis/Dominators.h"
#include "llvm/Support/ErrorHandling.h"
#include "llvm/Support/CallSite.h"
#include "llvm/Support/ConstantRange.h"
@ -164,61 +163,13 @@ void PHINode::resizeOperands(unsigned NumOps) {
/// hasConstantValue - If the specified PHI node always merges together the same
/// value, return the value, otherwise return null.
///
/// If the PHI has undef operands, but all the rest of the operands are
/// some unique value, return that value if it can be proved that the
/// value dominates the PHI. If DT is null, use a conservative check,
/// otherwise use DT to test for dominance.
///
Value *PHINode::hasConstantValue(const DominatorTree *DT) const {
// If the PHI node only has one incoming value, eliminate the PHI node.
if (getNumIncomingValues() == 1) {
if (getIncomingValue(0) != this) // not X = phi X
return getIncomingValue(0);
return UndefValue::get(getType()); // Self cycle is dead.
}
// Otherwise if all of the incoming values are the same for the PHI, replace
// the PHI node with the incoming value.
//
Value *InVal = 0;
bool HasUndefInput = false;
for (unsigned i = 0, e = getNumIncomingValues(); i != e; ++i)
if (isa<UndefValue>(getIncomingValue(i))) {
HasUndefInput = true;
} else if (getIncomingValue(i) != this) { // Not the PHI node itself...
if (InVal && getIncomingValue(i) != InVal)
return 0; // Not the same, bail out.
InVal = getIncomingValue(i);
}
// The only case that could cause InVal to be null is if we have a PHI node
// that only has entries for itself. In this case, there is no entry into the
// loop, so kill the PHI.
//
if (InVal == 0) InVal = UndefValue::get(getType());
// If we have a PHI node like phi(X, undef, X), where X is defined by some
// instruction, we cannot always return X as the result of the PHI node. Only
// do this if X is not an instruction (thus it must dominate the PHI block),
// or if the client is prepared to deal with this possibility.
if (!HasUndefInput || !isa<Instruction>(InVal))
return InVal;
Instruction *IV = cast<Instruction>(InVal);
if (DT) {
// We have a DominatorTree. Do a precise test.
if (!DT->dominates(IV, this))
return 0;
} else {
// If it is in the entry block, it obviously dominates everything.
if (IV->getParent() != &IV->getParent()->getParent()->getEntryBlock() ||
isa<InvokeInst>(IV))
return 0; // Cannot guarantee that InVal dominates this PHINode.
}
// All of the incoming values are the same, return the value now.
return InVal;
Value *PHINode::hasConstantValue() const {
// Exploit the fact that phi nodes always have at least one entry.
Value *ConstantValue = getIncomingValue(0);
for (unsigned i = 1, e = getNumIncomingValues(); i != e; ++i)
if (getIncomingValue(i) != ConstantValue)
return 0; // Incoming values not all the same.
return ConstantValue;
}