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Clarify that constant folding of instructions applies when all operands

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are constant.  There was in fact one exception to this (phi nodes) - so
remove that exception (InstructionSimplify handles this so there should
be no loss).

llvm-svn: 120015
This commit is contained in:
Duncan Sands 2010-11-23 10:16:18 +00:00
parent af4cd15f2a
commit 809b3e3634
2 changed files with 18 additions and 16 deletions
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13
include/llvm/Analysis/ConstantFolding.h
View File

@ -7,7 +7,8 @@
//
//===----------------------------------------------------------------------===//
//
// This file declares routines for folding instructions into constants.
// This file declares routines for folding instructions into constants when all
// operands are constants, for example "sub i32 1, 0" -> "1".
//
// Also, to supplement the basic VMCore ConstantExpr simplifications,
// this file declares some additional folding routines that can make use of
@ -27,11 +28,11 @@ namespace llvm {
class Function;
class Type;
/// ConstantFoldInstruction - Attempt to constant fold the specified
/// instruction. If successful, the constant result is returned, if not, null
/// is returned. Note that this function can only fail when attempting to fold
/// instructions like loads and stores, which have no constant expression form.
///
/// ConstantFoldInstruction - Try to constant fold the specified instruction.
/// If successful, the constant result is returned, if not, null is returned.
/// Note that this fails if not all of the operands are constant. Otherwise,
/// this function can only fail when attempting to fold instructions like loads
/// and stores, which have no constant expression form.
Constant *ConstantFoldInstruction(Instruction *I, const TargetData *TD = 0);
/// ConstantFoldConstantExpression - Attempt to fold the constant expression

21
lib/Analysis/ConstantFolding.cpp
View File

@ -695,22 +695,23 @@ static Constant *SymbolicallyEvaluateGEP(Constant *const *Ops, unsigned NumOps,
// Constant Folding public APIs
//===----------------------------------------------------------------------===//
/// ConstantFoldInstruction - Attempt to constant fold the specified
/// instruction. If successful, the constant result is returned, if not, null
/// is returned. Note that this function can only fail when attempting to fold
/// instructions like loads and stores, which have no constant expression form.
///
/// ConstantFoldInstruction - Try to constant fold the specified instruction.
/// If successful, the constant result is returned, if not, null is returned.
/// Note that this fails if not all of the operands are constant. Otherwise,
/// this function can only fail when attempting to fold instructions like loads
/// and stores, which have no constant expression form.
Constant *llvm::ConstantFoldInstruction(Instruction *I, const TargetData *TD) {
// Handle PHI nodes specially here...
// Handle PHI nodes quickly here...
if (PHINode *PN = dyn_cast<PHINode>(I)) {
Constant *CommonValue = 0;
for (unsigned i = 0, e = PN->getNumIncomingValues(); i != e; ++i) {
Value *Incoming = PN->getIncomingValue(i);
// If the incoming value is equal to the phi node itself or is undef then
// skip it.
if (Incoming == PN || isa<UndefValue>(Incoming))
// If the incoming value is undef then skip it. Note that while we could
// skip the value if it is equal to the phi node itself we choose not to
// because that would break the rule that constant folding only applies if
// all operands are constants.
if (isa<UndefValue>(Incoming))
continue;
// If the incoming value is not a constant, or is a different constant to
// the one we saw previously, then give up.