-ffast-math as a floating-point type

IEEE floats have NaNs and lots of guarantees about precision and strictly defined behavior to the point they’re weird (e.g. not associative) and annoying (e.g. std::cmp::max is not defined for floats).

How about adding another floating point type (could be called “real” r32 and r64) that doesn’t give any guarantees about rounding modes, NaN, Inf, +/-0. It’d be associative and have total order.

It could be used for things like particle effects in games (where precision is not as important as speed) and graphics algorithms that manipulate pixels as floating point numbers (finite values with usually plenty of bits of precision to spare, and would benefit greatly from SIMD).


Personally I’ve never used floats for their “scientific” features, and the only case for which I’ve deliberately used NaN is better handled in Rust by Option.

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My (old) thought on this was to introduce some way of narrowing the set of valid bit patterns for a type. Use this to construct r32/r64 types with “holes” in them where the NaNs would be. Those holes could be used for differentiating between Some(x) and None, much like you can mark a value as being NonZero.

Then, all operations that cannot result in NaN for non-NaN inputs would return r32/r64, everything else would return Option<r32>/Option<r64>, which would be bit-compatible with f32/f64 (i.e. you could transmute between them).

However, I think “fast math” is an independent problem. There was a proposal a while ago to add an annotation that could be used to turn range checking on or off in a block of code (i.e. detect overflow and underflow). It seems like that might be a better approach for speed optimisation. Allow programmers to say “I need speed for this bit of code, so #[fast_math(3)] and feel free to sacrifice up to 3 bits of precision”. If you want it crate-wide, just apply to the crate, then override if you need precision with #[fast_math(0)].


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