`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 👍

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

bjorn3 · 2026-08-10T14:54:16Z

What is the calling convention of other targets?

View changes since the review

I believe only these files were changed because they have an exhaustive match on Float.

However, my version of abi-cafe found two interesting failures: GCC and Clang are inconsistent on aarch64 and armv7

// callee, compiled with GCC 12 #include <inttypes.h> #include <string.h> #include <stdio.h> #include <stdbool.h> typedef struct Many1 { __bf16 f0; } Many1; void struct_in_1(Many1 arg0) { printf("%d", arg0.f0); }

// caller, compiled with clang 23 #include <inttypes.h> #include <string.h> #include <stdio.h> #include <stdbool.h> typedef struct Many1 { __bf16 f0; } Many1; void struct_in_1(Many1 arg0); void do_test(void) { { Many1 arg0 = { .f0 = (((union { uint16_t bits; __bf16 value; }){ .bits = 49600 }).value) }; printf("%d", arg0.f0); struct_in_1(arg0); } }

hits

func struct_in_1's values differed values (native-endian hex bytes): expect: C0 C1 caller: C0 C1 callee: 04 00 the value was arg0.f0: rustarithmeticty(f16b) whose arg was arg0: Many1

The current Rust implementation matches clang, and is hence incompatible with GCC

armv7 with hardware floats also runs into incompatibilities

[target.armv7-unknown-linux-gnueabihf."f16::conv_c::repr_c::clang-nightly_calls_distro-gcc"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16::conv_c::repr_c::distro-gcc_calls_clang-nightly"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16b::conv_c::repr_c::clang-nightly_calls_distro-gcc"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16b::conv_c::repr_c::distro-gcc_calls_clang-nightly"] busted = "check"

Finally, you can let this ICE on many targets, e.g. mips, powerpc, s390x, sparc

https://godbolt.org/z/ox3G88hEW

You can also (e.g. on loongarch64 https://godbolt.org/z/Y3hdhG4e6) emit a __truncsfbf2 libcall that is not provided (probably needs to be added to compiler-builtins).

I think the ICEs are probably a blocker? That needs a mechanism similar to has_reliable_f128.

Yes I added it because of the exhaustive match statement in mips64.rs and sparc64.rs I've removed it; 40d3f6e and put in a panic!(...).

With regard to has_reliable_f128, are you envisaging a has_reliable_f16b entry on TargetConfig?

With regard to has_reliable_f128, are you envisaging a has_reliable_f16b entry on TargetConfig?

Exactly

What's your take on those ABI mismatches? We should track that somewhere.

I'm not particularly certain what mips64 should do.

It doesn't, at least to my knowledge, have hardware support. Given we aren't implementing scalar arithmetic and an f16b can only be created through a bit pattern or vendor intrinsics, I can't immediately see a practical application? Hence a panic! seems like a pragmatic choice for the time being.

No comment on whether this is the right approach, but always use span_bug! or at least bug! where possible in the compiler, rather than panic. It gives much more useful info.

folkertdev · 2026-08-11T13:50:33Z

I believe only these files were changed because they have an exhaustive match on Float.

However, my version of abi-cafe found two interesting failures: GCC and Clang are inconsistent on aarch64 and armv7

// callee, compiled with GCC 12 #include <inttypes.h> #include <string.h> #include <stdio.h> #include <stdbool.h> typedef struct Many1 { __bf16 f0; } Many1; void struct_in_1(Many1 arg0) { printf("%d", arg0.f0); }

// caller, compiled with clang 23 #include <inttypes.h> #include <string.h> #include <stdio.h> #include <stdbool.h> typedef struct Many1 { __bf16 f0; } Many1; void struct_in_1(Many1 arg0); void do_test(void) { { Many1 arg0 = { .f0 = (((union { uint16_t bits; __bf16 value; }){ .bits = 49600 }).value) }; printf("%d", arg0.f0); struct_in_1(arg0); } }

hits

func struct_in_1's values differed values (native-endian hex bytes): expect: C0 C1 caller: C0 C1 callee: 04 00 the value was arg0.f0: rustarithmeticty(f16b) whose arg was arg0: Many1

The current Rust implementation matches clang, and is hence incompatible with GCC

armv7 with hardware floats also runs into incompatibilities

[target.armv7-unknown-linux-gnueabihf."f16::conv_c::repr_c::clang-nightly_calls_distro-gcc"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16::conv_c::repr_c::distro-gcc_calls_clang-nightly"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16b::conv_c::repr_c::clang-nightly_calls_distro-gcc"] busted = "check" [target.armv7-unknown-linux-gnueabihf."f16b::conv_c::repr_c::distro-gcc_calls_clang-nightly"] busted = "check"

Finally, you can let this ICE on many targets, e.g. mips, powerpc, s390x, sparc

https://godbolt.org/z/ox3G88hEW

folkertdev · 2026-08-11T14:03:54Z

        },
        Primitive::Float(float) => match float {
-            Float::F16 | Float::F32 => "f32",
+            Float::F16 | Float::F16B | Float::F32 => "f32",


is that right? LLVM just crashes on bf16 right now, so it's probably at least untested?

View changes since the review

I could be mistaken, however I don't think WASM supports bf16? I've made it panic!(...) for now; 40d3f6e

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

…t updates

…umentation

folkertdev · 2026-08-18T15:23:51Z

re

##[error]error: internal compiler error: compiler/rustc_codegen_gcc/src/type_.rs:160:9: f16b is not supported by the GCC codegen backend

by delaying adding support you now exclude a bunch of tests from the GCC run that we later need to enable again. I believe hooking it up is actually kind of trivial, so that may be the better call?

rust-log-analyzer · 2026-08-19T10:04:25Z

The job aarch64-gnu-llvm-21-1 failed! Check out the build log: (web) (plain enhanced) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[RUSTC-TIMING] thread_local test:false 0.201
   Compiling unicase v2.8.1
[RUSTC-TIMING] pulldown_cmark_escape test:false 0.064
   Compiling nu-ansi-term v0.50.3
error[E0004]: non-exhaustive patterns: `Primitive::Float(Float::F16B)` not covered
    --> compiler/rustc_abi/src/layout/ty.rs:321:19
     |
 321 |             match primitive {
     |                   ^^^^^^^^^ pattern `Primitive::Float(Float::F16B)` not covered
     |
note: `Primitive` defined here
    --> compiler/rustc_abi/src/lib.rs:1457:10
     |
1457 | pub enum Primitive {
     |          ^^^^^^^^^
...
1466 |     Float(Float),
     |     ----- not covered
     = note: the matched value is of type `Primitive`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
     |
 328 ~                 Primitive::Pointer(..) => None,
 329 ~                 Primitive::Float(Float::F16B) => todo!(),
     |

For more information about this error, try `rustc --explain E0004`.
[RUSTC-TIMING] rustc_abi test:false 0.555
error: could not compile `rustc_abi` (lib) due to 1 previous error

folkertdev · 2026-08-19T10:30:11Z

re the CI error

error[E0004]: non-exhaustive patterns: `Primitive::Float(Float::F16B)` not covered
    --> compiler/rustc_abi/src/layout/ty.rs:321:19
     |
 321 |             match primitive {
     |                   ^^^^^^^^^ pattern `Primitive::Float(Float::F16B)` not covered
     |
note: `Primitive` defined here
    --> compiler/rustc_abi/src/lib.rs:1457:10
     |
1457 | pub enum Primitive {
     |          ^^^^^^^^^
...
1466 |     Float(Float),
     |     ----- not covered
     = note: the matched value is of type `Primitive`
help: ensure that all possible cases are being handled by adding a match arm with a wildcard pattern or an explicit pattern as shown
     |
 328 ~                 Primitive::Pointer(..) => None,
 329 ~                 Primitive::Float(Float::F16B) => todo!(),
     |

C compilers do not support _Complex bf16, so we shouldn't either. So that branch should return None.

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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Comment thread compiler/rustc_ty_utils/src/layout.rs

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

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

Comment thread tests/codegen-llvm/float/f16b.rs Outdated

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 added 9 commits August 17, 2026 14:59

weave in target_has_reliable_f16b

a6d66d0

update condition for cfg.has_reliable_f16b = ..., along with commen…

3bc0032

…t updates

documentation feedback

d88e0d4

version >= 21 for AArch64, x86_64 and RISC-V, along with improved doc…

d1c5eee

…umentation

Add tests checking assembly

8a51190

more similar tests to the f16 for feature gating

42162a9

update documentation on struct

f9b0cce

update error message on WASM

f76c208

add loongarch64 & tests

b5705ce

Jamesbarford force-pushed the feat/fb16-pt1 branch from 4ff6c4b to fc04ded Compare August 17, 2026 13:59

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Jamesbarford force-pushed the feat/fb16-pt1 branch from fc04ded to 6754cda Compare August 17, 2026 14:50

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Jamesbarford force-pushed the feat/fb16-pt1 branch from 6754cda to b5496fb Compare August 17, 2026 17:24

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Jamesbarford force-pushed the feat/fb16-pt1 branch from b5496fb to 505a38a Compare August 18, 2026 07:13

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CI pipeline work; use a neon example and minicore for intrinsic test

8b18e08

Jamesbarford force-pushed the feat/fb16-pt1 branch from 505a38a to 8b18e08 Compare August 18, 2026 14:33

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beetrees reviewed Aug 18, 2026

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Comment thread compiler/rustc_attr_ir/src/lang_items.rs Outdated

pr-feedback: comments and renaming

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Attempt wiring up f16b for GCC

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davidtwco mentioned this pull request Aug 19, 2026

Tracking Issue for f16b #160630

Open

11 tasks

cfg out minicore's f16b to get GCC purring along

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