[orlp]

> By default, in GNU mode, gcc will happily emit FMA at -O3.

I had no idea that gcc was in 'GNU mode' by default, and that specifying -std would turn that off. I always assumed it just had a default standard version that is (very) irregularly incremented.

> I would argue that it would have been fortunate if FMA was disabled by default.

I agree, and (outside of my earlier ignorance of GNU mode) it is most everywhere.

My 'unfortunate' wasn't aimed at compilers per se, but rather at (unavoidable) the non-commutative and associative nature of floating point. I do wish that it was easier to specify at a per-file or per-function level that emitting FMA / performing algebraic and other non-bit-for-bit reproducible optimizations is ok.

[adgjlsfhk1]

This is a major portion of why I use Julia as my primary math programming language. By default it doesn't re-associate (unlike C) which makes it much easier to write error compensating arithmetic, and it has macros to make it easy on to give any function/expression fastmath semantics (or narrower re-association only semantics without the NaN/Inf/subnormal effects of fastmath. It also has a reinterpret function which makes it much simpler to do bitcasts than C where there are 100 different ways, 99% of which are technically UB.

[gpderetta]

There are parallel strict C or C++ standard version flags and the corresponding gnu variants (for example -std=gnu++17) which enable gnu extensions for each corresponding standard. The gnu mode (of whatever standard is default for a specific gcc version) is enabled by default.