core::num::f16b Rust's 16bit Brain Float - #160859
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Some changes occurred in compiler/rustc_attr_ir cc @jdonszelmann, @JonathanBrouwer This PR changes rustc_public cc @oli-obk, @celinval, @ouz-a, @makai410
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r? @jieyouxu rustbot has assigned @jieyouxu. Use Why was this reviewer chosen?The reviewer was selected based on:
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@rustbot reroll |
antoyo
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Aug 10, 2026
| } | ||
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| fn type_f16b(&self) -> Type<'gcc> { | ||
| bug!("f16b is not supported by the GCC codegen backend") |
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I believe GCC actually supports this type: https://github.com/rust-lang/gccjit.rs/blob/master/src/context.rs#L1482
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Thanks 😄, I will aim to add it in a follow up PR 👍
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cc @bjorn3 |
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What is the calling convention of other targets?
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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
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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.
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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?
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With regard to
has_reliable_f128, are you envisaging ahas_reliable_f16bentry onTargetConfig?
Exactly
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What's your take on those ABI mismatches? We should track that somewhere.
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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.
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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.
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folkertdev
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Aug 11, 2026
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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
| }, | ||
| Primitive::Float(float) => match float { | ||
| Float::F16 | Float::F32 => "f32", | ||
| Float::F16 | Float::F16B | Float::F32 => "f32", |
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is that right? LLVM just crashes on bf16 right now, so it's probably at least untested?
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I could be mistaken, however I don't think WASM supports bf16? I've made it panic!(...) for now; 40d3f6e
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Some changes occurred in cfg and check-cfg configuration cc @Urgau
cc @rust-lang/miri |
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I'm not a good reviewer for this change. Can somebody here pick it up rather than blindly rerolling? |
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r? me |
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| #[cfg_attr(feature = "nightly", derive(StableHash))] | ||
| pub enum Float { | ||
| F16, | ||
| F16B, |
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| F16B, | |
| /// `f16b`. This is not a builtin type in Rust (it is exposed as a lang item), | |
| /// but it is a builtin type in LLVM so needs to be explicitly represented | |
| /// in the backend. | |
| F16B, |
… and backend handling. GCC and Cranelift explicitly unsupported
…long with traits agreed on in the RFC
…6b` to be treated as a primitive scalar
- Add documentation aliases to `f16b` - Add Wikipedia link to `bfloat16` - Remove `@ only-x86_64` flag from test
- ICE on `sparc64`, `mips64` & WASM
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The job Click to see the possible cause of the failure (guessed by this bot)Important For more information how to resolve CI failures of this job, visit this link. |
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re
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? |
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| PartialEq, sym::eq, eq_trait, Target::Trait, GenericRequirement::Exact(1); | ||
| PartialOrd, sym::partial_ord, partial_ord_trait, Target::Trait, GenericRequirement::Exact(1); | ||
| CVoid, sym::c_void, c_void, Target::Enum, GenericRequirement::None; | ||
| BFloat, sym::bfloat, bfloat_type, Target::Struct, GenericRequirement::Exact(0); |
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| BFloat, sym::bfloat, bfloat_type, Target::Struct, GenericRequirement::Exact(0); | |
| F16B, sym::f16b, f16b, Target::Struct, GenericRequirement::Exact(0); |
May as well keep the lang item name consistent with the type name.
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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;
f16balong withbfloatlang item to work with LLVM, GCC is explicitlyunimplemented!(...)f16bfeature gate, page forf16bon libruscdoc and astruct bf16incore::numf16bas a scalar primitive for scalable vectorsIssues;