Auto merge of #36181 - seanmonstar:likely, r=nikomatsakis

core: add likely and unlikely intrinsics

I'm no good at reading assembly, but I have tried a stage1 compiler with this patch, and it does cause different asm output. Additionally, testing this compiler on my httparse crate with some `likely` usage added in to the branches does affect benchmarks. However, I'm sure a codegen test should be included, if anyone knows what it should look like.

There isn't an entry in `librustc_trans/context.rs` in this diff, because it already exists (`llvm.expect.i1` is used for array indices).

----

Even though this does affect httparse benchmarks, it doesn't seem to affect it the same way GCC's `__builtin_expect` affects picohttpparser. I was confused that the deviation on the benchmarks grew hugely when testing this, especially since I'm absolutely certain that the branchs where I added `likely` were always `true`. I chalk that up to GCC and LLVM handle branch prediction differently.

cc #26179

1 files changed, 14 insertions, 0 deletions

diff --git a/src/libcore/intrinsics.rs b/src/libcore/intrinsics.rs
index 06b506ab014..22abe7a99b1 100644
--- a/src/libcore/intrinsics.rs
+++ b/src/libcore/intrinsics.rs
@@ -194,6 +194,20 @@ extern "rust-intrinsic" {
     /// own, or if it does not enable any significant optimizations.
     pub fn assume(b: bool);
 
+    #[cfg(not(stage0))]
+    /// Hints to the compiler that branch condition is likely to be true.
+    /// Returns the value passed to it.
+    ///
+    /// Any use other than with `if` statements will probably not have an effect.
+    pub fn likely(b: bool) -> bool;
+
+    #[cfg(not(stage0))]
+    /// Hints to the compiler that branch condition is likely to be false.
+    /// Returns the value passed to it.
+    ///
+    /// Any use other than with `if` statements will probably not have an effect.
+    pub fn unlikely(b: bool) -> bool;
+
     /// Executes a breakpoint trap, for inspection by a debugger.
     pub fn breakpoint();