`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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… and backend handling. GCC and Cranelift explicitly unsupported

jieyouxu · 2026-08-10T14:45:34Z

@rustbot reroll

…long with traits agreed on in the RFC

…6b` to be treated as a primitive scalar

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 · 2026-08-10T14:53:26Z

+                let bfloat = scalar_unit(Primitive::Float(abi::Float::F16B));
+                assert_eq!(layout.size, abi::Float::F16B.size());
+                assert_eq!(layout.align, abi::Float::F16B.align(cx));
+                layout.backend_repr = BackendRepr::Scalar(bfloat);


You probably also want to add a check that you can't take a reference to the inner field just like for #[repr(simd)].

View changes since the review

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

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 · 2026-08-11T13:51:27Z

+#[lang = "bfloat"]
+#[allow(non_camel_case_types)]
+#[repr(transparent)]
+#[unstable(feature = "f16b", issue = "160630")]
+pub struct f16b(u16);


can you also add this to minicore.rs?

View changes since the review

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

- Add documentation aliases to `f16b` - Add Wikipedia link to `bfloat16` - Remove `@ only-x86_64` flag from test

- ICE on `sparc64`, `mips64` & WASM

rust-log-analyzer · 2026-08-12T08:58:10Z

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)

diff of stdout:

15     /*
16     0#0
17     */ {
-     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0 */>)
-     =>
+     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0
+     */>) =>
20     { fn /* 0#0 */ $f /* 0#0 */<$a /* 0#0 */, 'a /* 0#0 */>() {} }
21 }
22 fn f /* 0#0 */<'a /* 0#0 */, 'a /* 0#1 */>() {}

Note: some mismatched output was normalized before being compared
-     ($f /* 2355#0 */:ident /* 1001#0 */<$a /* 2350#0 */:lifetime /* 1098#0
+     ($f /* 0#0 */:ident /* 0#0 */<$a /* 0#0 */:lifetime /* 0#0
+     */>) =>


The actual stdout differed from the expected stdout
To update references, rerun the tests and pass the `--bless` flag
To only update this specific test, also pass `--test-args hygiene/unpretty-debug-lifetimes.rs`

error: 1 errors occurred comparing output.
status: exit status: 0
command: env -u RUSTC_LOG_COLOR RUSTC_ICE="0" RUST_BACKTRACE="short" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2/bin/rustc" "/checkout/tests/ui/hygiene/unpretty-debug-lifetimes.rs" "-Zsimulate-remapped-rust-src-base=/rustc/FAKE_PREFIX" "-Ztranslate-remapped-path-to-local-path=no" "-Z" "ignore-directory-in-diagnostics-source-blocks=/cargo" "-Z" "ignore-directory-in-diagnostics-source-blocks=/checkout/vendor" "--sysroot" "/checkout/obj/build/aarch64-unknown-linux-gnu/stage2" "--target=aarch64-unknown-linux-gnu" "--check-cfg" "cfg(test,FALSE)" "--error-format" "json" "--json" "future-incompat" "-Ccodegen-units=1" "-Zui-testing" "-Zdeduplicate-diagnostics=no" "-Zwrite-long-types-to-disk=no" "-Cstrip=debuginfo" "--emit" "metadata" "-C" "prefer-dynamic" "--out-dir" "/checkout/obj/build/aarch64-unknown-linux-gnu/test/ui/hygiene/unpretty-debug-lifetimes" "-A" "unused" "-W" "unused_attributes" "-A" "internal_features" "-A" "incomplete_features" "-A" "unused_parens" "-A" "unused_braces" "-Crpath" "-Cdebuginfo=0" "-Lnative=/checkout/obj/build/aarch64-unknown-linux-gnu/native/rust-test-helpers" "-Zunpretty=expanded,hygiene"
--- stdout -------------------------------
//@ check-pass
//@ compile-flags: -Zunpretty=expanded,hygiene

// Regression test for lifetime hygiene annotations in -Zunpretty=expanded,hygiene
// Previously, lifetimes were missing the #N syntax context suffix.

// Don't break whenever Symbol numbering changes
//@ normalize-stdout: "\d+#" -> "0#"

#![feature /* 861#0 */(decl_macro /* 695#0 */)]
#![feature /* 861#0 */(no_core /* 1334#0 */)]
#![no_core /* 1334#0 */]

macro lifetime_hygiene
    /*
    2942#0
    */ {
    ($f /* 2355#0 */:ident /* 1001#0 */<$a /* 2350#0 */:lifetime /* 1098#0
    */>) =>
    { fn /* 12#0 */ $f /* 2355#0 */<$a /* 2350#0 */, 'a /* 2943#0 */>() {} }
}
fn f /* 2355#0 */<'a /* 2943#0 */, 'a /* 2943#1 */>() {}


/*
Expansions:
crate0::{{expn0}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Root
crate0::{{expn1}}: parent: crate0::{{expn0}}, call_site_ctxt: #0, def_site_ctxt: #0, kind: Macro(Bang, "lifetime_hygiene")

SyntaxContexts:
#0: parent: #0, outer_mark: (crate0::{{expn0}}, Opaque)
#1: parent: #0, outer_mark: (crate0::{{expn1}}, Opaque)
*/
------------------------------------------
stderr: none

---- [ui] tests/ui/hygiene/unpretty-debug-lifetimes.rs stdout end ----

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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ABI for F16B, the BFloat lang item, the ADT-to-backend layout bridge,…

02fd145

… and backend handling. GCC and Cranelift explicitly unsupported

rustbot assigned mati865 and unassigned jieyouxu Aug 10, 2026

Jamesbarford added 3 commits August 10, 2026 15:45

Add #[lang = "bfloat"] and pub f16b struct (as a u16 wrapper) a…

cc57894

…long with traits agreed on in the RFC

f16b API and LLVM codegen tests

e2f100f

Add test for an AArch64 bf16 intrinsics, fixed code to allow an `f1…

01c5c1b

…6b` to be treated as a primitive scalar

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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tgross35 reviewed Aug 11, 2026

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

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

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

Jamesbarford added 2 commits August 12, 2026 08:59

Pr Feedback;

adf8e90

- Add documentation aliases to `f16b` - Add Wikipedia link to `bfloat16` - Remove `@ only-x86_64` flag from test

Pr Feedback

40d3f6e

- ICE on `sparc64`, `mips64` & WASM

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

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