[cfe-dev] question about fused multiply add and Clang GNU modes

Tim Northover via cfe-dev cfe-dev at lists.llvm.org
Sun Sep 20 21:42:56 PDT 2015

Thanks Steve, an excellent summary of where IEEE-754 stands. I knew
about the recommendation for languages to provide controls, but hadn't
tracked down the one for the default.

> Now, it *does* appear to me that we do not default to having STDC
> FP_CONTRACT ON, which is inhibiting fma formation *even within an
> expression*.  Given that we support STDC FP_CONTRACT OFF, we could certainly
> choose to make ON the default, and I would encourage doing so.

I doubt we really supoprt FP_CONTRACT ON: I've never seen any attempt
to track C expressions in the IR, which seems like a necessity. So the
choice is probably between a conformant OFF and a buggy ON for



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