[llvm-commits] [PATCH][FAST-MATH] recognize fp negation

[Duncan Sands]

Hi Shuxin,

>      Currently the compiler only considers "-0.0 - x" as a negation of quantity
> x regardless the
>   fast-math flags associated with the expr or the fast-math flags derived from
> the supper-expr.
>   This patch is to fix this problem.
>
>     With this change, "0.0 - x" will be considered as negation of x as well so
> long as we don't care
> "signed-zero".
>
>     It is about two weeks old. Unfortunately, as of I write this mail, I'm not
> able to update the latest revision
> and re-test it.

how about getting rid of isFNeg from InstrTypes, and move users over to the
version in llvm/Support/PatternMatch.h instead?  I'm not sure it makes much
sense to have these pattern matching predicates in InstrTypes.h any more -
I suspect they are something of a historical relic nowadays.

Ciao, Duncan.

PS: Otherwise your patch looks fine, except for:

> --- lib/VMCore/Instructions.cpp	(revision 170838)
> +++ lib/VMCore/Instructions.cpp	(working copy)
> @@ -1928,11 +1928,11 @@
>    return false;
>  }
>
> -bool BinaryOperator::isFNeg(const Value *V) {
> +bool BinaryOperator::isFNeg(const Value *V, bool IgnoreZeroSign) {
>    if (const BinaryOperator *Bop = dyn_cast<BinaryOperator>(V))
>      if (Bop->getOpcode() == Instruction::FSub)
>        if (Constant* C = dyn_cast<Constant>(Bop->getOperand(0)))
> -        return C->isNegativeZeroValue();
> +        return !IgnoreZeroSign ? C->isNegativeZeroValue() : C->isZeroValue();

If IgnoreZeroSign is true, don't you want to match both ordinary and negative
zero?  Right now it only matches ordinary zero.