Auto merge of #124606 - scottmcm:less-expect, r=cjgillot
Stop `llvm.expect`ing assert terminators
We're putting `llvm.expect` calls before the <https://doc.rust-lang.org/nightly/nightly-rustc/rustc_middle/mir/enum.TerminatorKind.html#variant.Assert> terminators.
But we don't need them. One of the arms is always to a panic function that's marked `#[cold]`, which is `cold` <https://llvm.org/docs/LangRef.html#function-attributes> in LLVM, which
> When computing edge weights, basic blocks post-dominated by a cold function call are also considered to be cold; and, thus, given low weight.
So even without us emitting the extra intrinsic call, LLVM knows what to expect for the `br`. Thus we can save the (small) effort of emitting it and then LLVM optimizing it out.
r? compiler
diff --git a/compiler/rustc_codegen_ssa/src/mir/block.rs b/compiler/rustc_codegen_ssa/src/mir/block.rs
index 26e4290..2f57695 100644
--- a/compiler/rustc_codegen_ssa/src/mir/block.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/block.rs
@@ -649,8 +649,8 @@
return helper.funclet_br(self, bx, target, mergeable_succ);
}
- // Pass the condition through llvm.expect for branch hinting.
- let cond = bx.expect(cond, expected);
+ // Because we're branching to a panic block (either a `#[cold]` one
+ // or an inlined abort), there's no need to `expect` it.
// Create the failure block and the conditional branch to it.
let lltarget = helper.llbb_with_cleanup(self, target);
