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- Remove the buggy -X/C -> X/-C transform. This isn't valid when X isn't a

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constant. If X is a constant, then this is folded elsewhere.

- Added a note to Target/README.txt to indicate that we'd like to implement
  this when we're able.


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@60399 91177308-0d34-0410-b5e6-96231b3b80d8
This commit is contained in:
Bill Wendling 2008-12-02 05:12:47 +00:00
parent a8bb13f989
commit 3bdcda82a5
2 changed files with 10 additions and 12 deletions
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10
lib/Target/README.txt
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@ -1242,3 +1242,13 @@ Should combine to "20 * (((unsigned)x) & -2)". Currently not
optimized with "clang -emit-llvm-bc | opt -std-compile-opts".
//===---------------------------------------------------------------------===//
We would like to do the following transform in the instcombiner:
-X/C -> X/-C
However, this isn't valid if (-X) overflows. We can implement this when we
have the concept of a "C signed subtraction" operator that which is undefined
on overflow.
//===---------------------------------------------------------------------===//

12
lib/Transforms/Scalar/InstructionCombining.cpp
View File

@ -2929,18 +2929,6 @@ Instruction *InstCombiner::visitSDiv(BinaryOperator &I) {
// sdiv X, -1 == -X
if (RHS->isAllOnesValue())
return BinaryOperator::CreateNeg(Op0);
// -X/C -> X/-C, if and only if negation doesn't overflow.
if (Value *LHSNeg = dyn_castNegVal(Op0)) {
if (ConstantInt *CI = dyn_cast<ConstantInt>(LHSNeg)) {
Constant *RHSNeg = ConstantExpr::getNeg(RHS);
if (RHS != RHSNeg) { // Check that there is no overflow.
Constant *CINeg = ConstantExpr::getNeg(CI);
if (CI != CINeg) // Check that there is no overflow.
return BinaryOperator::CreateSDiv(LHSNeg, RHSNeg);
}
}
}
}
// If the sign bits of both operands are zero (i.e. we can prove they are