[PATCH] D50913: [FPEnv] Don't need copysign/fabs/fneg constrained intrinsics

[Kevin P. Neal via Phabricator via llvm-commits]

kpn added a comment.

I just got done talking to one of our floating point math guys here.

In https://reviews.llvm.org/D50913#1205990, @cameron.mcinally wrote:

> > If the fsub that led to the FNEG node was not constrained in the first place then this seems like a valid transform.
>
> Bah, sorry. My comment was ambiguous...
>
> I meant that the FNEG->FSUB xform is "bad" in the context Ulrich proposed. For example:
>
> The user writes a unary minus, -x, which is a bitwise operation and will never trap. The frontend would then convert -x to an *unconstrained* -0-x. InstCombine catches this pattern and canonicalizes the -0-x to a FNEG. But, if the target expands the FNEG, it will be converted back into an FSUB, which is arithmetic and could trap. We don't want to introduce traps where none were originally intended.


This sounds like the case for a constrained fneg intrinsic.

We can't convert -x into an fsub, constrained or not, because we can't risk introducing or removing traps. We need an unambiguous way of making a distinction between "-0-x" and "-x". The constrained fneg intrinsic would allow that distinction.


Repository:
  rL LLVM

https://reviews.llvm.org/D50913