Re: [PATCH v2] Fix -ffast-math flags handling inconsistencies

[Segher Boessenkool <segher@kernel.crashing.org>]

Hi Ulrich,

On Tue, Nov 24, 2020 at 07:42:47PM +0100, Ulrich Weigand wrote:
> This did handle flag_excess_precision incorrectly, causing in particular
> https://gcc.gnu.org/bugzilla/show_bug.cgi?id=97970
> 
> I've reverted for now and will post a modified patch later.
> Sorry for the breakage!
> 
> At a first glance, it appears the problem is that -fexcess-precision=fast
> is already the default, so it does not make sense for -fno-fast-math to
> "reset" this back to default.  Probably the best is for fast-math to
> simply not touch excess precision at all:
> - if it is set explicitly on the command line, fast-math should not
>   affect it in any case;
> - if it is not set explicitly on the command line, the default either
>   with fast-math or no-fast-math is excess-precision=fast, so again
>   it should not be touched.
> 
> I'll still need to look into the language-specific handling of the
> excess precision setting to make sure this works for all languages.

At least C uses "standard" (instead of "fast") for the strict ISO C
dialects.


Segher