On Mon, Sep 18, 2023 at 12:10 PM Florian Weimer <fwei...@redhat.com> wrote: > > * 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>
I suppose that applies to re-association not honoring parens. With C you either have no re-association (fine) or re-association but globally, at the expense of violating the standard. FP contraction is _not_ about honoring parens, the 'error' introduced by is subtle. Fortran for example allows re-association by default, but honors parens. GCC can do this just fine. > 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. No, I don't think it says that. Fortran also allows contraction (but not across parens). Richard. > Thanks, > Florian >
