`core::num::f16b` Rust's 16bit Brain Float by Jamesbarford · Pull Request #160859 · rust-lang/rust

Jamesbarford · 2026-08-10T14:27:46Z

Implements the RFC: f16b type. Best reviewed commit by commit, happy to split into separate PRs if that is deemed easier to review. However the line count and surface area is, in my opinion, reasonably small.

Adds;

ABI plumbing for the f16b along with bfloat lang item to work with LLVM, GCC is explicitly unimplemented!(...)
f16b feature gate, page for f16b on libruscdoc and a struct bf16 in core::num
Tests
Treat f16b as a scalar primitive for scalable vectors

Issues;

rustbot · 2026-08-10T14:27:50Z

Some changes occurred in compiler/rustc_attr_ir

cc @jdonszelmann, @JonathanBrouwer

This PR changes rustc_public

cc @oli-obk, @celinval, @ouz-a, @makai410

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

rustbot · 2026-08-10T14:27:54Z

r? @jieyouxu

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jieyouxu · 2026-08-10T14:45:34Z

@rustbot reroll

antoyo · 2026-08-10T14:48:39Z

    }

+    fn type_f16b(&self) -> Type<'gcc> {
+        bug!("f16b is not supported by the GCC codegen backend")


I believe GCC actually supports this type: https://github.com/rust-lang/gccjit.rs/blob/master/src/context.rs#L1482

View changes since the review

Thanks 😄, I will aim to add it in a follow up PR 👍

Actually, as @folkertdev pointed out, it was a doddle. So I've included the implementation in the PR 👍

rustbot · 2026-08-10T14:48:49Z

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

cc @bjorn3

rustbot · 2026-08-12T12:03:56Z

Some changes occurred in cfg and check-cfg configuration

cc @Urgau

miri is developed in its own repository. If the Miri part of this change can be broken out, consider making this change to rust-lang/miri instead. However, if Miri needs adjusting for rustc changes, just ignore this message.

cc @rust-lang/miri

mati865 · 2026-08-12T13:36:14Z

I'm not a good reviewer for this change. Can somebody here pick it up rather than blindly rerolling?

folkertdev · 2026-08-12T13:54:02Z

r? me

`core::num::f16b` Rust's 16bit Brain Float try-job: test-various try-job: aarch64-apple-* try-job: *-gnu-nopt-* try-job: x86_64-mingw-* try-job: aarch64-msvc-* try-job: arm-android

rust-bors · 2026-09-02T12:23:37Z

💔 Test for f6d6327 failed: CI. Failed jobs:

try - aarch64-apple-1 (web logs, enhanced plaintext logs)
try - aarch64-apple-macos-26-1 (web logs, enhanced plaintext logs)
try - aarch64-msvc-1 (web logs, enhanced plaintext logs)
try - arm-android (web logs, enhanced plaintext logs)
try - i686-gnu-nopt-1 (web logs, enhanced plaintext logs)
(and 2 other jobs)

@folkertdev

…lkertdev Make sin, cos, exp, exp2, log, log2, log10 generic Rebased and smaller version of rust-lang/rust#153934 Following `fabs`, make the `sin`, `cos`, `exp`, `exp2`, `log`, `log2` and `log10` intrinsics generic over the float type, rather than having four variants per float type. The first two commits are purely stylistic: - reorganised Cranelift code to make following changes simpler - moved a misplaced comment in `compiler/rustc_codegen_llvm/src/intrinsic.rs` that caused `x fmt` to give up The last commit actually makes them generic! Most code is a bit simpler, ~~and this will also hopefully simplify adding support for these intrinsics for the future [`bf16` type](rust-lang/rust#160859 :) Unfortunately both GCC and Cranelift backend changes are a bit churny. Their code is a bit, opaque, to put it kindly, and I didn't want to refactor those here. r? @folkertdev cc @RalfJung

@folkertdev

…lkertdev Make sin, cos, exp, exp2, log, log2, log10 generic Rebased and smaller version of rust-lang/rust#153934 Following `fabs`, make the `sin`, `cos`, `exp`, `exp2`, `log`, `log2` and `log10` intrinsics generic over the float type, rather than having four variants per float type. The first two commits are purely stylistic: - reorganised Cranelift code to make following changes simpler - moved a misplaced comment in `compiler/rustc_codegen_llvm/src/intrinsic.rs` that caused `x fmt` to give up The last commit actually makes them generic! Most code is a bit simpler, ~~and this will also hopefully simplify adding support for these intrinsics for the future [`bf16` type](rust-lang/rust#160859 :) Unfortunately both GCC and Cranelift backend changes are a bit churny. Their code is a bit, opaque, to put it kindly, and I didn't want to refactor those here. r? @folkertdev cc @RalfJung

folkertdev

some nits

I diagnosed the arm/aarch64 issue to be about what types are valid in a homogeneous float aggregate. Not much we can do about that here.

View changes since this review

@folkertdev

…lkertdev Make sin, cos, exp, exp2, log, log2, log10 generic Rebased and smaller version of rust-lang/rust#153934 Following `fabs`, make the `sin`, `cos`, `exp`, `exp2`, `log`, `log2` and `log10` intrinsics generic over the float type, rather than having four variants per float type. The first two commits are purely stylistic: - reorganised Cranelift code to make following changes simpler - moved a misplaced comment in `compiler/rustc_codegen_llvm/src/intrinsic.rs` that caused `x fmt` to give up The last commit actually makes them generic! Most code is a bit simpler, ~~and this will also hopefully simplify adding support for these intrinsics for the future [`bf16` type](rust-lang/rust#160859 :) Unfortunately both GCC and Cranelift backend changes are a bit churny. Their code is a bit, opaque, to put it kindly, and I didn't want to refactor those here. r? @folkertdev cc @RalfJung

update tests and remove major >= 21 check

… in the documentation

rustbot · 2026-09-07T12:24:09Z

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.

@folkertdev

…lkertdev Make sin, cos, exp, exp2, log, log2, log10 generic Rebased and smaller version of rust-lang/rust#153934 Following `fabs`, make the `sin`, `cos`, `exp`, `exp2`, `log`, `log2` and `log10` intrinsics generic over the float type, rather than having four variants per float type. The first two commits are purely stylistic: - reorganised Cranelift code to make following changes simpler - moved a misplaced comment in `compiler/rustc_codegen_llvm/src/intrinsic.rs` that caused `x fmt` to give up The last commit actually makes them generic! Most code is a bit simpler, ~~and this will also hopefully simplify adding support for these intrinsics for the future [`bf16` type](rust-lang/rust#160859 :) Unfortunately both GCC and Cranelift backend changes are a bit churny. Their code is a bit, opaque, to put it kindly, and I didn't want to refactor those here. r? @folkertdev cc @RalfJung

rust-bors · 2026-09-11T13:44:54Z

☔ The latest upstream changes (presumably #162631) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

folkertdev · 2026-09-13T15:29:39Z

+use minicore::From;
+use minicore::num::f16b;
+
+// CHECK-LABEL: {{^"?[#_]?identity_f16b"?:}}


what is this regex for? just CHECK-LABEL: identity_f16b should suffice?

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rustbot added A-attributes Area: Attributes (`#[…]`, `#![…]`) A-LLVM Area: Code generation parts specific to LLVM. Both correctness bugs and optimization-related issues. S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Aug 10, 2026

rustbot assigned jieyouxu Aug 10, 2026

rustbot added T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. T-libs Relevant to the library team, which will review and decide on the PR/issue. labels Aug 10, 2026

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rustbot assigned mati865 and unassigned jieyouxu Aug 10, 2026

antoyo reviewed Aug 10, 2026

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Jamesbarford force-pushed the feat/fb16-pt1 branch from bf10f8a to 01c5c1b Compare August 10, 2026 14:48

bjorn3 reviewed Aug 10, 2026

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folkertdev reviewed Aug 11, 2026

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Comment thread library/core/src/num/f16b.rs Outdated

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tgross35 reviewed Aug 11, 2026

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Comment thread library/core/src/num/bfloat.rs

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Jamesbarford force-pushed the feat/fb16-pt1 branch from 53d3660 to 40d3f6e Compare August 12, 2026 08:19

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rustbot added the A-test-infra-minicore Area: `minicore` test auxiliary and `//@ add-core-stubs` label Aug 12, 2026

Jamesbarford force-pushed the feat/fb16-pt1 branch from da620d0 to 0ed984f Compare August 12, 2026 10:08

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Jamesbarford force-pushed the feat/fb16-pt1 branch from c7fcc75 to 39a366b Compare September 2, 2026 13:25

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folkertdev reviewed Sep 4, 2026

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Jamesbarford added 12 commits September 7, 2026 13:19

Create f16b struct and documentation

a706555

Add F16B to Float & wireup trivial matches

6c3742a

Use cfg's in backends

5d548ec

Wire up backends

ac1c383

Create tests

df1f7a0

cfg test on target_has_reliable_f16b

d7f1871

Consolidate assembly-llvm tests into one, factor out ignore's,

048da1f

update tests and remove major >= 21 check

Test changes for Tidy, hard cfg check and include the use core...…

256ab0a

… in the documentation

add optimisation revisions for scalable-vectors/bf16-intrinsic.rs test

96b2e54

rename brain_float bfloat

1881b6e

Apply nits

0a2cc1c

Update hygine test by removing $f

2f22e82

Jamesbarford force-pushed the feat/fb16-pt1 branch from d8ba332 to 2f22e82 Compare September 7, 2026 12:24

folkertdev reviewed Sep 13, 2026

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