[llvm-commits] CVS: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
Chris Lattner
lattner at cs.uiuc.edu
Wed Jul 20 11:49:39 PDT 2005
Changes in directory llvm/lib/Transforms/Scalar:
InstructionCombining.cpp updated: 1.358 -> 1.359
---
Log message:
Do not let MaskedValueIsZero consider undef to be zero, for reasons
explained in the comment.
This fixes UnitTests/2003-09-18-BitFieldTest on darwin
---
Diffs of the changes: (+8 -2)
InstructionCombining.cpp | 10 ++++++++--
1 files changed, 8 insertions(+), 2 deletions(-)
Index: llvm/lib/Transforms/Scalar/InstructionCombining.cpp
diff -u llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.358 llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.359
--- llvm/lib/Transforms/Scalar/InstructionCombining.cpp:1.358 Mon Jul 18 18:07:33 2005
+++ llvm/lib/Transforms/Scalar/InstructionCombining.cpp Wed Jul 20 13:49:28 2005
@@ -1317,7 +1317,13 @@
/// this predicate to simplify operations downstream. V and Mask are known to
/// be the same type.
static bool MaskedValueIsZero(Value *V, ConstantIntegral *Mask) {
- if (isa<UndefValue>(V) || Mask->isNullValue())
+ // Note, we cannot consider 'undef' to be "IsZero" here. The problem is that
+ // we cannot optimize based on the assumption that it is zero without changing
+ // to to an explicit zero. If we don't change it to zero, other code could
+ // optimized based on the contradictory assumption that it is non-zero.
+ // Because instcombine aggressively folds operations with undef args anyway,
+ // this won't lose us code quality.
+ if (Mask->isNullValue())
return true;
if (ConstantIntegral *CI = dyn_cast<ConstantIntegral>(V))
return ConstantExpr::getAnd(CI, Mask)->isNullValue();
@@ -3134,7 +3140,7 @@
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
}
if (isa<UndefValue>(Op1)) {
- if (isLeftShift || I.getType()->isUnsigned())
+ if (isLeftShift || I.getType()->isUnsigned())// X << undef, X >>u undef -> 0
return ReplaceInstUsesWith(I, Constant::getNullValue(I.getType()));
else
return ReplaceInstUsesWith(I, Op0); // X >>s undef -> X
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