[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"]

Abe Skolnik via llvm-dev llvm-dev at lists.llvm.org
Fri Sep 9 15:48:20 PDT 2016

[Stephen Canon wrote:]
>>> Gating this on -Owhatever is dangerous, .  We should simply default to the pragma “on” state universally.

[Abe wrote:]
>> 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".

[Steve Canon wrote:]
> 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).

I strongly agree with that philosophy.  I tried arguing that GCC should change its policy, but 
I was rebuffed ["RESOLVED INVALID"].

For reference:   https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77515



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