[Bug fortran/104535] New: don't use fmod?

[fx at gnu dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=104535

            Bug ID: 104535
           Summary: don't use fmod?
           Product: gcc
           Version: unknown
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: fortran
          Assignee: unassigned at gcc dot gnu.org
          Reporter: fx at gnu dot org
  Target Milestone: ---

I was reminded by comments on the report I made about poor fmod performance on
x86 that I should have commented on the original observation.

I'd looked at one of the Polyhedron benchmarks which suffers badly from a
simple random number routine that calls DMOD.  That gets compiled to fmod,
which is only inlined, albeit poorly on x86, with the relevant component(s) of
-ffast-math.  It seems to me that MOD should compile to the arithmetical
expression in the standard, which doesn't have the complication of having to
treat errors.  (When I defined DMOD as a statement function for it in that
routine, I got performance much closer to ifort.  I should have kept the
profiles I compared, but could regenerate them.

Is there a good reason not to do that (and maybe similarly with other
intrinsics I haven't checked)?  I could probably have a go at implementing it
if appropriate, though I don't know my way around now.