On Thu, 14 Sep 2023, Florian Weimer via Gcc wrote:
> While rebuilding CentOS Stream with -march=x86-64-v3, I rediscovered
> several packages had test suite failures because x86-64 suddenly gained
> FMA support. I say “rediscovered” because these issues were already
> visible on other architectures with FMA.
>
> So far, our package/architecture maintainers had just disabled test
> suites or had built the package with -fp-contract=off because the
> failures did not reproduce on x86-64. I'm not sure if this is the right
> course of action.
>
> 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
> thing looks suspiciously like a quick hack to get a performance
> improvement from FMA instructions (sorry).
>
> I know that GCC 14 has -fp-contract=standard. Would it make sense to
> switch the default to that? If it fixes those package test suites, it
> probably has an observable performance impact. 8-/
Note that with =standard FMA contraction is still allowed within an
expression: the compiler will transform 'x * y + z' to 'fma(x, y, z)'.
The difference between =fast and =standard is contraction across
statement boundaries. So I'd expect some test suite failures you speak of
to remain with =standard as opposed to =off.
I think it's better to switch both C and C++ defaults to =standard,
matching Clang, but it needs a bit of leg work to avoid regressing
our own testsuite for targets that have FMA in the base ISA.
(personally I'd be on board with switching to =off even)
See https://gcc.gnu.org/PR106902 for a worked example where -ffp-contract=fast
caused a correctness issue in a widely used FOSS image processing application
that was quite hard to debug.
Obviously -Ofast and -ffast-math will still imply -ffp-contract=fast if we
make the change, so SPEC scores won't be affected.
Thanks.
Alexander
