Re: Concerns regarding the -ffp-contract=fast default

[Florian Weimer <fweimer@redhat.com>]

* Richard Biener:

>> > > GCC contraction behavior is rather inconsistent.  It does not contract x
>> > > + x - x without -ffast-math, for example, although I believe it would be
>> > > permissible under the rules that enable FMA contraction.  This whole
>
> Is that really just x + x - x?  We currently gate simplifying x - x to zero
> on no-signed-zeros and round-to-nearest and have no special
> handling for x + x - x.

It's just an example I made up, and checked that GCC wouldn't optimize
it by default (without -ffast-math et al.).  I believe the rule that GCC
invokes to introduce FMA under -ffp-contract=standard (and
-ffp-contract=fast) would apply to these contractions as well.

Or put differently, -ffp-contract= seems to be exclusively about FMA,
while the option name and description is more generic.

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.

>> The "correctness trumps performance" principle still applies, and
>> -ffp-contract=fast (the current default outside of -std=cNN) is
>> known to cause correctness issues and violates the C language standard.
>> And -ffast[-and-loose]-math for is not going away.
>
> I think that changing the default to =standard without -ffast-math is
> reasonable.  IIRC the standard allows such default if it's indicated,
> so it doesn't require =off anywhere.

How much numerical code is compatible with that?  For example, David
Goldberg's What Every Computer Scientist Should Know About
Floating-Point Arithmetic (revised) contains this sentence:

| A language definition that does not require parentheses to be honored
| is useless for floating-point calculations.

<https://docs.oracle.com/cd/E19957-01/806-3568/ncg_goldberg.html>

Few people have access to the IEEE 754 standard, and Goldberg's article
is therefore widely quoted as gospel.  If I understand this correctly,
according to Goldberg, contractions make C “useless”.  But I'm not a
floating point person, and I nowadays regret that I looked down upon the
numerical methods people at university.

Thanks,
Florian