Concerns regarding the -ffp-contract=fast default

[Alexander Monakov]

On Mon, 18 Sep 2023, Florian Weimer via Gcc wrote:

> x - x is different because replacing it with 0 doesn't seem to be a
> valid contraction because it's incorrect for NaNs.  x + x - x seems to
> be different in this regard, but in our implementation, there might be a
> quirk about sNaNs and qNaNs.

Sorry, do you mean contracting 'x + x - x' to 'x'? That is not allowed
due to different result and lack of FP exception for infinities.

Contracting 'x + x - x' to fma(x, 2, -x) would be fine.

Alexander