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-/ Thanks, Florian
