Suggestions for additional floating-point types

[aaronfranke]

I noticed that, like other languages, the only floating-point types built-in are f32 and f64. However, only having these can be limiting. I propose adding f128, and as mentioned in this thread f16 would likely be very useful for some workloads.

f128 would not be needed in most programs, but there are use cases for them, and it'd be nice to have it as a language built-in type. RISC-V is able to hardware accelerate them using the Q extension.

f16 is a more efficient type for workloads where you need tons of floats at low precision, like machine learning. Hardware using this is already widespread in Apple's neural engine and in mobile graphics.

Also, if covering IEEE-754 is desired, then there's also f256.

Original text:

Details

I noticed that, like other languages, the only floating-point types built-in are f32 and f64. However, I often have limitations with just these. I propose the following: fsize, freal, and f128

fsize would be like isize but for floats. Basically, use the version that's most efficient for your processor. On modern 64-bit processors with wide FPUs and/or 256-bit SIMD this would become f64.

Sometimes I want to be able to have a variable for real numbers, or I don't know what precision I want yet. In C++ I can do the following to have an abstract precision that I control via compiler flags:

#ifdef REAL_T_IS_DOUBLE
typedef double real_t;
#else
typedef float real_t;
#endif

I propose something similar in Rust, where you can just write freal or something and be able to change the precision later with compiler flags. The default would probably be f32.

Finally, it would be nice to have 128-bit floats (f128) in the language. These are not normally needed, but there are use cases for them, and it'd be nice to have it as a language built-in type. Some newer processors have 512-bit SIMD chipsets that can process these efficiently, though most don't.

If you only implement some of these proposals, that's fine too. Originally posted at rust-lang/rust#57928

[sfackler]

fsize would be like isize but for floats. Basically, use the version that's most efficient for your processor.

isize is not the integer type that's most efficient for your processors - it's the integer type that's the same size as a pointer. It's like ptrdiff_t, not int.

I propose something similar in Rust, where you can just write freal or something and be able to change the precision later with compiler flags. The default would probably be f32.

#[cfg(feature = "real_t_is_double")]
type real_t = f64;
#[cfg(not(feature = "real_t_is_double")]
type real_t = f32;

[moonheart08]

A better suggestion would be f16 support, as it is common in graphics.

[shingtaklam1324]

@moonheart08

Are f16 used much in intermediate calculations? I know it is used commonly as a storage format, but that last time I checked this (I wrote a Pre-RFC on this on internals a while back, but I'm a bit fuzzy on the details), a lot of the calculations involving f16 on most platforms is done by casting to f32, performing the op, the cast back to f16. If that is the case then having native f16 support may not be that important.

Adding the ability to use the F16C instructions may be useful to have in core::arch though, perhaps something like __m128h which has 8 "f16"s.

[jacob-greenfield]

How about long double and 128-bit floats? I could be wrong, but I'm 99% sure that we currently unavoidably lose precision when using long doubles from C. On my computer (macOS), bindgen outputs f64, but sizeof(long double) in C outputs 16 bytes. (128 bits; for alignment I guess?).

(On a side note, is that even safe behavior? What about C functions that take long double *?)

[aaronfranke]

@Coder-256 In C++, long double is 64-bit on Windows, 80-bit in MinGW, and 128-bit on Mac and Linux (probably indeed for alignment, as I don't think anyone implements it as quadruple precision).

[jacob-greenfield]

@aaronfranke Could you please clarify what you mean? What I was trying to say is that Rust currently does not have any support for floats larger than 64 bits (8 bytes), for example, long double on certain platforms. I was also trying to point out that in addition to having limited precision within Rust code, this makes it difficult to interact with native code that uses large floats, such as using FFI with C code that uses floats larger than 64 bits.

There was also a separate issue with bindgen that caused float sizes to be incorrect for large floats, but that has been fixed (in rust-lang/rust-bindgen@ed6e1bb).

[aaronfranke]

I wasn't disagreeing with you, I was just adding information. Sorry if I wasn't clear. f128 would be great.

[jacob-greenfield]

@aaronfranke I absolutely agree, both f128 and f80 would be very useful, especially for FFI (for example, Swift already has Float80 mainly for communicating with old C code, just an example to show how it could help)

[lygstate]

old things never be gone, I wanna push this. rust is a system language not a script language, need compat old things.

[lygstate]

I wanna push add support for fp80 and fp128... any help need?

[lygstate]

Like rust-lang/rust#38482 does

[thomcc]

Basically, use the version that's most efficient for your processor. On modern 64-bit processors with wide FPUs and/or 256-bit SIMD this would become f64.

Even on modern x86 which has similar or equal speed between most f32 and f64 ops, f32 is still very much the fastest for your processor because it cuts cache misses in half.

Sometimes I want to be able to have a variable for real numbers, or I don't know what precision I want yet. In C++ I can do the following to have an abstract precision that I control via compiler flags:

#[cfg(real_is_f64)]
type real = f64;
#[cfg(not(real_is_f64))]
type real = f32;

then you can control via RUSTFLAGS="--cfg real_is_f64" (you can also use cargo features, but they're not a great fit for cases where enabling a feature can cause compile errors like this)

... Regarding suggestions of f80

What would f80 do on platforms that aren't x86? Noting else has native 80bit floats. It's not even part of IEEE754 (even though it's largely natural extension of it... although it has a lot of quirks). This is something that would be viable in core::arch::{x86,x86_64} but isn't portable. We don't want to have to implement these as software floats on other platforms.

I'd be in favor of a std::os::raw::c_long_double type but it would have to be carefully designed. Note that PPC's long double is exceptionally cursed, as it's a pair of doubles that are summed together...

I'd be in favor of f16, and tentatively f128 since binary128 is part of IEEE754 2019, at least.

EDIT: I hadn't noticed that sfalker said the exact same thing as my first point >_>