Nikita LisitsaNew blog post! I really hope I'll get a decent amount of people mad with this one 😈😈😈
It's OK to compare floating-points for equality:
https://lisyarus.github.io/blog/posts/its-ok-to-compare-floating-points-for-equality.html
Daniel Gibson@lisyarus
> In reality it is a pretty deterministic (modulo compiler options, CPU flags, etc)
and then det(v, v) suddenly doesn't return 0.0 anymore because the compiler now targets a newer CPU with FMA and uses regular multiplication for `a.y * b.x` and FMA for `a.x * b.y - prev_result`, and FMA uses infinite precision so `a.x * b.y` can be slightly different from the result of the regular multiplication -_-
It annoys me that IEEE754 allows this and compilers (ab)use it
Arseny Kapoulkine@Doomed_Daniel @lisyarus This is not an IEEE754 problem; IEEE754 defines FMA and unfused mul/add. It's up to each language to figure out how to compile compound expressions and what is permissible; C/C++ allow contraction by default, so you do get different results based on the platform or the underlying model - but you can also turn contraction off to fix this, or use languages that don't allow this implicit conversion.
@Doomed_Daniel @lisyarus I agree that contraction should not be enabled unconditionally; Rust is an example of a language that doesn't do that - it follows IEEE 754 strictly. IEEE 754 does not explicitly allow contractions. In fact it has language that prohibits these by default, which C doesn't follow (clang will synthesize fma unconditionally without optimizations when supported)
@zeux @lisyarus
IEEE 754 at least seems to allow this as long as it can be switched off, doesn't require it to be disabled by default - or at least that's what the C/C++ standards and/or compilerwriters think
(GCC is extra fun because it only allows using -ffp-contract but ignores the FP_CONTRACT pragma)
Alexander Monakov@Doomed_Daniel @zeux @lisyarus as far as I'm aware, C++ standards do not have anything analogous to C's Annex F that would say how the language binds to IEEE 754 / IEC 60559.