use libm for acosh and asinh for f16, f32, and f64 by malezjaa · Pull Request #154051 · rust-lang/rust

malezjaa · 2026-03-18T18:09:00Z

Uses libm for acosh and asinh for f16, f32, and f64.
I didn't change impl for f128 as i couldn't find existing function for these in libm

rustbot · 2026-03-18T18:09:06Z

r? @joboet

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reddevilmidzy · 2026-03-20T14:28:04Z

Hello @malezjaa,
Next time you contribute, please check if someone has been assigned to the issue before contributing.

malezjaa · 2026-03-20T15:27:10Z

I'm really sorry about that. I should've checked If anyone was already assigned to this issue. I'll do that from now on.

rustbot · 2026-03-21T09:01:17Z

This PR was rebased onto a different main commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

tgross35 · 2026-03-26T22:54:00Z

Happy to review this or alternatives, unless you have a preference @joboet

r? tgross35

tgross35 · 2026-03-27T08:37:07Z

this fixes acosh and asinh for large float values using the approximation from libm

Can we just call libm via the cmath moule instead, rather than having the same logic here? Like we do for acos and similar.

Can you also clarify what approximations you are referencing? This isn't what is in https://github.com/rust-lang/compiler-builtins/blob/644346f0541f74fc425070f8e0712f2dac898e11/libm/src/math/acosh.rs.

malezjaa · 2026-03-28T17:05:38Z

Oh yeah that makes sense. I'll update the PR title and description to show that we're now using libm for these functions.
For f128 I kept the existing approximation since I couldn't find the f128 version in libm

reddevilmidzy · 2026-03-31T04:44:56Z

@rustbot author

rustbot · 2026-03-31T04:45:01Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

rustbot · 2026-03-31T06:08:40Z

The Miri subtree was changed

cc @rust-lang/miri

RalfJung · 2026-03-31T08:19:33Z

Oh wait, this was for the i686-pc-windows-msvc target. Not sure why that would matter, but I can reproduce the problem locally with --target i686-pc-windows-msvc.

RalfJung · 2026-03-31T09:09:02Z

It probably has to do with this:

rust/src/tools/miri/tests/pass/float.rs

Lines 1602 to 1605 in 37cfa17

    
           // On i686-pc-windows-msvc , these functions are implemented by calling the `f64` version, 
        
           // which means the little rounding errors Miri introduces are discarded by the cast down to 
        
           // `f32`. Just skip the test for them. 
        
           if !cfg!(all(target_os = "windows", target_env = "msvc", target_arch = "x86")) {

You can move the new cases into that if as well (after checking that the comment indeed applies to the two new functions as well).

malezjaa · 2026-03-31T17:57:30Z

@rustbot ready

tgross35 · 2026-03-31T18:00:46Z

I didn't change impl for f128 as i couldn't find existing function for these in libm

These are supported in glibc, which is pretty much the only libc to support f128 math for now, cfg(target_has_reliable_f128_math) accounts for this. So they are fine to do here as well.

library/coretests/tests/num/floats.rs

tgross35

Thank you! I'll merge after CI passes

View changes since this review

library/coretests/tests/num/floats.rs

tgross35

For reference, I usually use Julia for this quick arbitrary-precision number crunching. Set to MANTISSA_DIGITS then test with MAX.

julia> setprecision(113)
113

julia> x=parse(BigFloat, "1.18973149535723176508575932662800702e+4932")
1.18973149535723176508575932662800702e+4932

julia> asinh(x)
11357.216553474703894801348310092223

julia> asinh(big(1.7976931348623157e+308))
710.475860073943942041640622032115338

julia> setprecision(53)
53

julia> asinh(big(1.7976931348623157e+308))
710.47586007394398

julia> setprecision(24)
24

julia> asinh(big(3.40282347e+38))
89.4159851

julia> setprecision(11)
11

julia> asinh(big(6.5504e+4))
11.781

View changes since this review

library/coretests/tests/num/floats.rs

malezjaa · 2026-04-01T13:06:27Z

Nice. Thank you for the correct values :D

tgross35

Thank you!

@bors r+

View changes since this review

rust-bors · 2026-04-03T10:55:24Z

📌 Commit e83e484 has been approved by tgross35

It is now in the queue for this repository.

…asinh, r=tgross35 use libm for acosh and asinh for f16, f32, and f64 Fixes rust-lang#153878 Uses libm for `acosh` and `asinh` for `f16`, `f32`, and `f64`. I didn't change impl for f128 as i couldn't find existing function for these in libm

Rollup of 5 pull requests Successful merges: - #153286 (various fixes for scalable vectors) - #153592 (Add `min_adt_const_params` gate) - #154051 (use libm for acosh and asinh for f16, f32, and f64) - #154743 (Remove an unused `StableHash` impl.) - #154752 (Add comment to borrow-checker)

matthiaskrgr · 2026-04-03T18:48:48Z

@bors r-
#154757 (comment)

malezjaa · 2026-04-03T19:38:32Z

Should I just increase ASINH_APPROX, ACOSH_APPROX thresholds for x86 or is that not the solution?

tgross35 · 2026-04-03T19:44:18Z

Ah yeah, i586 has all kinds of issues with floating point accuracy. I would do this as:

/// i586 has issues with floating point precision.
const I586: bool = cfg!(target_arch = "x86") && cfg!(not(target_feature = "sse2"));

impl ... {
    const ASINH_APPROX: Self = if i586 { /* relaxed precision */ } else { /* current precision */ };
}

Make the precision larger than what showed up in the tests to leave some flexibility in the future.

malezjaa · 2026-04-03T20:00:34Z

Oh also, is this only for f32 or for other types too?

tgross35 · 2026-04-03T21:27:14Z

Usually that applies to f32 and f64 (not f16/f128 since they don't have the same ABI), but I guess the f64 precision is already relaxed enough.

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malezjaa force-pushed the approximations-acosh-and-asinh branch from 4a67c1e to 90a7256 Compare March 21, 2026 09:01

rustbot assigned tgross35 and unassigned joboet Mar 26, 2026

malezjaa changed the title ~~return approximations for large float values in acosh and asinh~~ use libm for acosh and asinh for f16, f32, and f64 Mar 28, 2026

malezjaa marked this pull request as draft March 28, 2026 16:41

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Mar 28, 2026

malezjaa force-pushed the approximations-acosh-and-asinh branch from 3a210bb to 538e681 Compare March 28, 2026 17:02

malezjaa marked this pull request as ready for review March 28, 2026 17:05

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malezjaa force-pushed the approximations-acosh-and-asinh branch from 538e681 to abd315c Compare March 31, 2026 06:08

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malezjaa force-pushed the approximations-acosh-and-asinh branch 2 times, most recently from 2332619 to 30e7816 Compare March 31, 2026 21:29

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use libm for acosh and asinh

e83e484

malezjaa force-pushed the approximations-acosh-and-asinh branch from 30e7816 to e83e484 Compare April 1, 2026 13:04

tgross35 reviewed Apr 3, 2026

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matthiaskrgr mentioned this pull request Apr 3, 2026

Rollup of 5 pull requests #154757

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