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> } >=20 > 42 > } >=20 > fn f2(a: i32) -> i32 { > if likely(a < 0) { > return 124; > } >=20 > 42 > } >=20 > fn f3(a: i32) -> i32 { > if unlikely(a < 0) { > return 125; > } >=20 > 42 > } >=20 > LLVM emits the same code it would for similar C functions: >=20 > f1: > test %edi,%edi > mov $0x7b,%ecx > mov $0x2a,%eax > cmovs %ecx,%eax > ret >=20 > f2: > mov $0x7c,%eax > test %edi,%edi > /-- jns > | ret > \-> mov $0x2a,%eax > ret >=20 > f3: > test %edi,%edi > /-- js > | mov $0x2a,%eax > | ret > \-> mov $0x7d,%eax > ret I haven't checked the assembly, but otherwise: Reviewed-by: Gary Guo >=20 > The feature itself, `feature(cold_path)`, was added in Rust 1.86.0 [2]. >=20 > Previously, a PR in Rust 1.84.0 [3] fixed a number of issues with the > `likely()` and `unlikely()` intrinsics (by implementing them on top of > the new `cold_path()` intrinsic). So we could use that, in principle, > for Rust 1.85.0. However, it is just a single version, and it is simpler > to avoid intrinsics. Instead, approximate it with `#[cold]` [4]. >=20 > Thus add support for `cold_path()` by applying several approaches: >=20 > - For Rust >=3D 1.86.0, `use` directly `core`'s `cold_path()`. >=20 > - For Rust 1.85, provide a `#[cold]` no-op, and vendor `core`s > documentation. >=20 > And, for all versions, simply provide `likely()` and `unlikely()` based > on `cold_path()`, by vendoring `core`'s unstable ones (the one from > `intrinsics`, not the `hint` wrapper, to save a layer). >=20 > In the future, if `likely()` and `unlikely()` become stable [5], we may > want to use them directly as well. >=20 > Now, in the C side, the `likely()` and `unlikely()` macros come > from `compiler.h`, which means it is pretty much available everywhere > directly. Thus just add these to the prelude (instead of e.g. re-exportin= g > them in the root or in a new `hint` module). >=20 > This will also mean less churn when we can remove the `cold_path()` > version from `std_vendor` (and potentially the other two too). >=20 > I tested that Rust 1.93.0 generate the code above and, in a previous > version of the patch, that Rust 1.83.0 and 1.78.0 do not, as expected. >=20 > Link: https://github.com/rust-lang/rust/pull/151576 [1] > Link: https://github.com/rust-lang/rust/pull/133695 [2] > Link: https://github.com/rust-lang/rust/pull/120370 [3] > Link: https://lore.kernel.org/rust-for-linux/DGA6GUR58QJ7.1XZZ0P4VZNW86@g= aryguo.net/ [4] > Link: https://github.com/rust-lang/rust/issues/151619 [5] > Signed-off-by: Miguel Ojeda > --- > v1: https://lore.kernel.org/rust-for-linux/20260208224659.18406-3-ojeda@k= ernel.org/ > v2: > - Drop the intrinsics and use `#[cold]` for Rust 1.85.0. >=20 > - Sorted after `feature(file_with_nul)` since now we know which was > the stable version when it happened (Rust 1.92.0). >=20 > - Rebased and reworded accordingly. >=20 > init/Kconfig | 3 + > rust/kernel/lib.rs | 4 ++ > rust/kernel/prelude.rs | 5 ++ > rust/kernel/std_vendor.rs | 142 ++++++++++++++++++++++++++++++++++++++ > 4 files changed, 154 insertions(+)