Make min/max and copysign intrinsics generic by N1ark · Pull Request #162414 · rust-lang/rust

N1ark · 2026-09-07T10:24:24Z

Split off from #162395

Make the following intrinsics generic over the float type:

copysign, with a shared fallback (bit manipulations). Note that this required adding #[rustc_const_unstable] to the whole intrinsic, which copysignf16 and copysignf128 didn't have. This was already FCP'd, see rust-lang/rust#153834 (comment).
minimum_number_nsz, maximum_number_nsz, minimum, maximum, with shared fallbacks.

Because these are generic and require const trait impls to have a fallback, I had to add #[rustc_allow_const_fn_unstable] to them:

copysign needs const_ops, const_trait_impl
minimum_number_nsz and maximum_number_nsz need const_cmp, const_trait_impl
minimum and maximum need const_cmp, const_ops, const_trait_impl

I have also bundled in the addition of the attribute to fabs, which used these features without having a formal approval (see #162409 -- this was introduced in #153834)

See #162395 (comment)

r? RalfJung

rustbot · 2026-09-07T10:24:28Z

Some changes occurred to the intrinsics. Make sure the CTFE / Miri interpreter
gets adapted for the changes, if necessary.

cc @rust-lang/miri, @RalfJung, @oli-obk, @lcnr

rustc_codegen_cranelift is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_cranelift instead.

cc @bjorn3

clippy is developed in its own repository. If possible, consider making this change to rust-lang/rust-clippy instead.

cc @rust-lang/clippy

⚠️ #[miri::intrinsic_fallback_is_spec] must only be used if the function actively checks for all UB cases,
and explores the possible non-determinism of the intrinsic.

cc @rust-lang/miri

rustc_codegen_gcc is developed in its own repository. If possible, consider making this change to rust-lang/rustc_codegen_gcc instead.

cc @antoyo, @GuillaumeGomez

Some changes occurred to the CTFE / Miri interpreter

cc @rust-lang/miri

Any special-casing of Miri in the standard library requires review.

cc @rust-lang/miri

⚠️ #[rustc_intrinsic_const_stable_indirect] controls whether intrinsics can be exposed to stable const
code; adding it needs t-lang approval.

cc @rust-lang/wg-const-eval

⚠️ #[rustc_allow_const_fn_unstable] needs careful audit to avoid accidentally exposing unstable
implementation details on stable.

cc @rust-lang/wg-const-eval

Some changes occurred to the CTFE machinery

cc @RalfJung, @oli-obk, @lcnr

rustbot · 2026-09-07T10:24:31Z

RalfJung is not on the review rotation at the moment.
They may take a while to respond.

RalfJung · 2026-09-07T10:27:10Z

Because these are generic and require const trait impls to have a fallback, I had to add #[rustc_allow_const_fn_unstable] to them:

I just realized that we can maybe just use rustc_do_not_const_check for all of them? They all have built-in implementations right?

N1ark · 2026-09-07T10:33:32Z

doesn't that feel a bit hackier / risk prone that just doing it like this? though i don't mind, we are calling externs from const code in other fallbacks after all ^^'

RalfJung · 2026-09-07T11:52:02Z

doesn't that feel a bit hackier / risk prone that just doing it like this? though i don't mind, we are calling externs from const code in other fallbacks after all ^^'

IMO it makes sense -- in both cases it's a fallback body that we don't want to use for const-eval, just the reason for not using it is different.

RalfJung · 2026-09-07T11:53:06Z

CI is red. Also please add the same kind of test as in your other PR. Yeah that means they will conflict -- sorry for that. Shouldn't be too hard of a conflict to deal with I hope.

@rustbot author

N1ark · 2026-09-07T12:13:07Z

de-constified the trait, as done in #162409, and switched to #[rustc_do_not_const_check]

RalfJung

LGTM on the Rust + LLVM side, but I can't review the other codegen backends. Looks like the GCC changes already got approved -- @bjorn3 could you look at the cranelift changes?

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RalfJung · 2026-09-08T07:48:59Z

+// Check that the float intrinsics carrying `#[rustc_do_not_const_check]` can actually be called in
+// a const context, for every float width. Their bodies are never const-checked and never run by
+// const-eval, which has to implement each of these intrinsics itself.


FWIW there's no good reason to only cover rustc_do_not_const_check intrinsics here. All const intrinsics without const-capable fallback body should ideally be covered somewhere. Maybe we already test some of them somewhere? I don't know.

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N1ark · 2026-10-22T13:55:32Z

@rustbot ready i think i addressed all outstanding comments!

RalfJung · 2026-10-22T16:30:05Z

The LLVM and const-eval part is all good.
r? @bjorn3
for the cranelift parts

@RalfJung

…rtdev Make more float intrinsics generic Another (big) step of rust-lang#153834 + rust-lang#160989 Makemore remaining float intrinsics generic! Each commit is self-contained. In order: - `ceil`, `floor`, `trunc`, `round`, `round_ties_even`, with const libm fallbacks (using `#[rustc_do_not_const_check]`, see rust-lang#150961, cc @RalfJung @tgross35) - `sqrt`, with libm fallbacks - `powf`, with libm fallbacks - `fma` and `fmuladd`, with libm fallbacks (+`#[rustc_do_not_const_check]`) and a shared fallback, respectively. - `powi`, with no fallback (it already didn't have any) The first commit just removes the f16/f128 fallbacks for `fabs`, as mentioned [here](rust-lang#160989 (comment)). I didn't try making the backends use more fallbacks, to make the PR less risky since it's quite large. Happy to do changes there though (cc @folkertdev) `copysign`, `minimum_number_nsz`, `maximum_number_nsz`, `minimum`, `maximum` will be handled separately as they require `rustc_allow_const_fn_unstable`: rust-lang#162414

@RalfJung

Rollup merge of #162395 - N1ark:more-float-generics, r=folkertdev Make more float intrinsics generic Another (big) step of #153834 + #160989 Makemore remaining float intrinsics generic! Each commit is self-contained. In order: - `ceil`, `floor`, `trunc`, `round`, `round_ties_even`, with const libm fallbacks (using `#[rustc_do_not_const_check]`, see #150961, cc @RalfJung @tgross35) - `sqrt`, with libm fallbacks - `powf`, with libm fallbacks - `fma` and `fmuladd`, with libm fallbacks (+`#[rustc_do_not_const_check]`) and a shared fallback, respectively. - `powi`, with no fallback (it already didn't have any) The first commit just removes the f16/f128 fallbacks for `fabs`, as mentioned [here](#160989 (comment)). I didn't try making the backends use more fallbacks, to make the PR less risky since it's quite large. Happy to do changes there though (cc @folkertdev) `copysign`, `minimum_number_nsz`, `maximum_number_nsz`, `minimum`, `maximum` will be handled separately as they require `rustc_allow_const_fn_unstable`: #162414

rustbot · 2026-10-23T15:28:51Z

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.

rustbot added the A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. label Sep 7, 2026

rustbot assigned RalfJung Sep 7, 2026

N1ark mentioned this pull request Sep 7, 2026

Make more float intrinsics generic #162395

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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 Sep 7, 2026

N1ark force-pushed the const-unstable-float-generics branch from 1514b9c to 101eb51 Compare September 7, 2026 12:12

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Comment thread compiler/rustc_codegen_gcc/src/intrinsic/mod.rs

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RalfJung reviewed Sep 8, 2026

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Comment thread compiler/rustc_codegen_cranelift/src/intrinsics/mod.rs

N1ark force-pushed the const-unstable-float-generics branch from 101eb51 to 4b1bf47 Compare October 22, 2026 12:32

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N1ark force-pushed the const-unstable-float-generics branch from 4b1bf47 to 37ea318 Compare October 22, 2026 13:47

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. and removed S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. labels Oct 22, 2026

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N1ark added 7 commits October 23, 2026 08:08

cranelift: use fallbacks for abs f16, f128

3a7772b

Make copysign generic

13c2c2d

Make minimum/maximum generic

5a64aca

Remove rustc_const_unstable from fabs

177b621

Replace float bound impls with a macro

3aefc62

De-constify the FloatPrimitive sealed trait

6fdb0dd

Add test

4fb0643

N1ark force-pushed the const-unstable-float-generics branch from 37ea318 to 4fb0643 Compare October 23, 2026 15:28

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