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
