llvm dev - Sep 2016 - [PATCH] D24481: make “#pragma STDC FP_CONTRACT” on by default

[Abe wrote in an earlier msg.:]
>> Some people -- my team included -- want Clang to remain competitive
with GCC
>> and for Clang to gain ground where possible without breaking
correct/valid code.
[Renato wrote:]
> That's hard to keep when we're breaking FP contracts. ;)
[...]
> Adding that to -ffast-math wouldn't have been so bad, and I think
that's
> a fair game. But no-fast-math O3 should still have a strict FP contract,
IMHO.

I agree.  That`s at least the main reason why my first choice was to try to
convince the GCC
people to make GCC less aggressive, FP-wise.  By myself, I was unable to
convince the relevant
people of such a suggestion.
[<https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77515>]  Therefor,
I started on the work that resulted in the patch under discussion in this
thread.

If anybody reading this has influence over the decisions made in the GCC
community, by all
means please try to convince them that fusing should be _off_ by default and
enabled only by
"-ffast-math", "-ffp-contract=fast", "-Ofast", and
maybe also {"-O<x>" where x>3} if there is
[still?] any such thing in GCC.  [IIRC GCC-for-SPARC at least used to have a
meaningful "-O4",
but that might be "ancient history" by now.]

I think a discussion could be had about how "correct/valid" are
programs that break [e.g.
regression tests fail] when the only change was that the FP results became
_more_ accurate.


[Renato wrote:]
> By making it faster in one arch, you may be destroying performance in
another,
>  and we really don't want that to happen.
Of course not.

> I think even non-compiler-savvy programmers knows about -ffast-math.
I think "-Ofast" is more clear/obvious, but since Clang already has
that and since [AFAIK]
Clang`s implementation of "-ffast-math" enables FP fusion on all
FP-fusion-capable targets, I
don`t see that any change needs to be made along that particular dimension.

Regards,

Abe



* note to non-{Abe|Renato} readers: Renato is offline/infrequent-email-checking
for travel
reasons for about a week.