[llvm-dev] defaults for FP contraction [e.g. fused multiply-add]: suggestion and patch to be slightly more aggressive and to make Clang`s optimization settings closer to having the same meaning as when they are given to GCC [at least for "-O3"]

Steve Canon via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 9 15:27:32 PDT 2016

Sent from my iPhone

> On Sep 9, 2016, at 6:21 PM, Abe Skolnik <a.skolnik at samsung.com> wrote:
>> On 09/09/2016 04:31 PM, Stephen Canon wrote:
>> Gating this on -Owhatever is dangerous, .  We should simply default to the pragma “on” state universally.
> Why so?  [honestly asking, not arguing]
> My guess: b/c we don`t want programs to give different results when compiled at different "-O<...>" settings with the exception of "-Ofast".

Pretty much.  In particular, imagine a user trying to debug an unexpected floating point result caused by conversion of a*b + c into fma(a, b, c).
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