[llvm-dev] fp-contract=fast and pragmas

Kaylor, Andrew via llvm-dev llvm-dev at lists.llvm.org
Fri Jun 25 14:30:21 PDT 2021

Hi everyone,

I've been exploring clang's fp-contract behavior recently, and I see that when '-ffp-contract=fast' is used it can't be overridden with a pragma. I would have regarded this as a bug (and in fact, a bug has been filed https://bugs.llvm.org/show_bug.cgi?id=39679). However, I've found some discussions on the mailing lists that described this as expected behavior.

Last October, Sam Liu added support for a new setting ('fp-contract=fast-honor-pragmas') and updated the clang documentation to reflect the behavior of fp-contract=fast. See https://reviews.llvm.org/D90174. I feel very strongly that this should have been done the other way around -- fp-contract=fast should honor pragmas and if we need an option that doesn't that could be added.

In the above review, John McCall asked what "other compilers" do. Steve Canon showed that GCC doesn't honor the pragma. If I may humbly offer another  "other compiler", ICC (which doesn't distinguish between 'on' and 'fast' for fp-contract) does respect the pragma (https://godbolt.org/z/x5r9WdYb4). I'm not saying that ICC should be treated as a reference implementation over GCC or anything like that, but I am saying that its behavior strikes me as more correct than what GCC or clang currently do.

Thoughts and opinions?


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