Auto merge of rust-lang#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

Author: bors

compiler/rustc_codegen_ssa/src/mir/block.rs

@@ -649,8 +649,8 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             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);