Auto merge of #112001 - saethlin:enable-matchbranchsimplification, r=cjgillot

Enable MatchBranchSimplification

This pass is one of the small number of benefits from `-Zmir-opt-level=3` that has motivated rustc_codegen_cranelift to use it:

https://github.com/rust-lang/rust/blob/19ed0aade60e1c1038fe40554bcd9d01b717effa/compiler/rustc_codegen_cranelift/build_system/build_sysroot.rs#L244-L246

Cranelift's motivation for this is _runtime_ performance improvements in debug builds. Lifting this pass all the way to `-Zmir-opt-level=1` seems to come without significant perf overhead, so that's what I'm suggesting here.

1 files changed, 7 insertions, 2 deletions

diff --git a/compiler/rustc_mir_transform/src/match_branches.rs b/compiler/rustc_mir_transform/src/match_branches.rs
index 59942dc76f9..6eb48498274 100644
--- a/compiler/rustc_mir_transform/src/match_branches.rs
+++ b/compiler/rustc_mir_transform/src/match_branches.rs
@@ -41,7 +41,7 @@ pub struct MatchBranchSimplification;
 
 impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 3
+        sess.mir_opt_level() >= 1
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
@@ -62,7 +62,12 @@ impl<'tcx> MirPass<'tcx> for MatchBranchSimplification {
                     ..
                 } if targets.iter().len() == 1 => {
                     let (value, target) = targets.iter().next().unwrap();
-                    if target == targets.otherwise() {
+                    // We require that this block and the two possible target blocks all be
+                    // distinct.
+                    if target == targets.otherwise()
+                        || bb_idx == target
+                        || bb_idx == targets.otherwise()
+                    {
                         continue;
                     }
                     (discr, value, target, targets.otherwise())