Imprecise floating point operations (fast-math)

[mpdn]

There should be a way to use imprecise floating point operations like GCC's and Clang's -ffast-math. The simplest way to do this would be to do like GCC and Clang and implement a command line flag, but I think a better way to do this would be to create a f32fast and f64fast type that would then call the fast LLVM math functions. This way you can easily mix fast and "slow" floating point operations.

I think this could be implemented as a library if LLVM assembly could be used in the asm macro.

[kmcallister]

Inline IR was discussed on #15180. Another option is extern "llvm-intrinsic" { ... } which I vaguely think we had at some point. If we added more intrinsics to std::intrinsics would that be sufficient?

[mpdn]

Yeah, adding it as a function in std::intrinsics could definitely work as well.

There are a few different fast math flags, but the fast flag is probably the most important as it implies all the other flags. Adding all of them would be unreasonable if using intrinsic functions, but I don't think all of them are necessary.

[bluss]

This forum thread has examples of loops that llvm can vectorize well for integers, but doesn't for floats (a dot product).

[kornelski]

I've prototyped it using a newtype: https://gitlab.com/kornelski/ffast-math (https://play.rust-lang.org/?gist=d516771d1d002f740cc9bf6eb5cacdf0&version=nightly&backtrace=0)

It works in simple cases, but the newtype solution is insufficient:

• it doesn't work with floating-point literals. That's a huge pain when converting programs to this newtype.
• it doesn't work with the as operator, and a trait to make that possible has been rejected before.
• the wrapper type and extra level of indirection affects inlining of code using it. I've found some large functions where the newtype was slower than regular float, but not because of float math, but because other structs and calls around it weren't as optimized. I wasn't able to reproduce it in simple cases, so I'm not sure what exactly is going on.

So I'm very keen on seeing it supported natively in Rust.

[bluss]

@pornel The issue #24963 had a test case where a newtype impacted vectorization. That example was fixed (great!), sounds like the bug is probably still visible in similar code.

[pedrocr]

I've tried -ffast-math in my C vs Rust benchmark of some graphics code:

https://github.com/pedrocr/rustc-math-bench

In the C code it's a ~20% improvement in clang but no benefit with GCC. In both cases it returns a wrong result and the math is extremely simple (multiplying a vector by a matrix). According to this:

https://stackoverflow.com/questions/38978951/can-ffast-math-be-safely-used-on-a-typical-project#38981307

-ffast-math is generally too unsafe for normal usage as it implies some strange things (e.g., NaN checks always return false). So it seems sensible to have a way to opt-in only to the more benign ones.

[kornelski]

@pedrocr Your benchmark has a loss of precision in sum regardless of fast-math mode. Both slow and fast give wrong result compared to summation using double sum.

With double for the sum and you'll get correct result, even with -ffast-math.

You get significantly different sum with float sum, because fast-math gives you a small systemic rounding error, which accumulates over 100 million additions.

All values from matrix multiplication are the same to at least 6 digits (I've diffed printf("%f", out[i]) of all values and they're all the same).

[pedrocr]

@pornel thanks, fixed here:

pedrocr/rustc-math-bench@8169fa3

The benchmark results are fine though, the sum is only used as a checksum. Here are the averages of three runs in ms/megapixel:

Compiler	-O3 -march=native	-O3 -march=native -ffast-math
clang 3.8.0-2ubuntu4	6,91	5,40 (-22%)
gcc 5.4.0-6ubuntu1~16.04.4	5,71	5,85 (+2%)

So as I mentioned before clang/llvm gets a good benefit from ffast-math but not gcc. I'd say making sure things like is_normal() still work is very important but at least on llvm it helps to be able to enable ffast-math.