Auto merge of #110569 - saethlin:mir-pass-cooperation, r=cjgillot

Deduplicate unreachable blocks, for real this time

In https://github.com/rust-lang/rust/pull/106428 (in particular https://github.com/rust-lang/rust/pull/106428/commits/41eda69516dd3ee217ae07c0efa369d31f630405) we noticed that inlining `unreachable_unchecked` can produce duplicate unreachable blocks. So we improved two MIR optimizations: `SimplifyCfg` was given a simplify to deduplicate unreachable blocks, then `InstCombine` was given a combiner to deduplicate switch targets that point at the same block. The problem is that change doesn't actually work.

Our current pass order is
```
SimplifyCfg (does nothing relevant to this situation)
Inline (produces multiple unreachable blocks)
InstCombine (doesn't do anything here, oops)
SimplifyCfg (produces the duplicate SwitchTargets that InstCombine is looking for)
```

So in here, I have factored out the specific function from `InstCombine` and placed it inside the simplify that produces the case it is looking for. This should ensure that it runs in the scenario it was designed for.

Fixes https://github.com/rust-lang/rust/issues/110551
r? `@cjgillot`

2 files changed, 17 insertions, 18 deletions

diff --git a/compiler/rustc_mir_transform/src/instcombine.rs b/compiler/rustc_mir_transform/src/instcombine.rs
index 3d06a0a495f..432852a1fdd 100644
--- a/compiler/rustc_mir_transform/src/instcombine.rs
+++ b/compiler/rustc_mir_transform/src/instcombine.rs
@@ -1,11 +1,9 @@
 //! Performs various peephole optimizations.
 
+use crate::simplify::combine_duplicate_switch_targets;
 use crate::MirPass;
 use rustc_hir::Mutability;
-use rustc_middle::mir::{
-    BinOp, Body, CastKind, Constant, ConstantKind, LocalDecls, Operand, Place, ProjectionElem,
-    Rvalue, SourceInfo, Statement, StatementKind, SwitchTargets, Terminator, TerminatorKind, UnOp,
-};
+use rustc_middle::mir::*;
 use rustc_middle::ty::layout::ValidityRequirement;
 use rustc_middle::ty::util::IntTypeExt;
 use rustc_middle::ty::{self, ParamEnv, SubstsRef, Ty, TyCtxt};
@@ -46,7 +44,7 @@ impl<'tcx> MirPass<'tcx> for InstCombine {
                 &mut block.terminator.as_mut().unwrap(),
                 &mut block.statements,
             );
-            ctx.combine_duplicate_switch_targets(&mut block.terminator.as_mut().unwrap());
+            combine_duplicate_switch_targets(block.terminator.as_mut().unwrap());
         }
     }
 }
@@ -264,19 +262,6 @@ impl<'tcx> InstCombineContext<'tcx, '_> {
         terminator.kind = TerminatorKind::Goto { target: destination_block };
     }
 
-    fn combine_duplicate_switch_targets(&self, terminator: &mut Terminator<'tcx>) {
-        let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind
-        else { return };
-
-        let otherwise = targets.otherwise();
-        if targets.iter().any(|t| t.1 == otherwise) {
-            *targets = SwitchTargets::new(
-                targets.iter().filter(|t| t.1 != otherwise),
-                targets.otherwise(),
-            );
-        }
-    }
-
     fn combine_intrinsic_assert(
         &self,
         terminator: &mut Terminator<'tcx>,
diff --git a/compiler/rustc_mir_transform/src/simplify.rs b/compiler/rustc_mir_transform/src/simplify.rs
index 88574addaa0..e1ca7c107b9 100644
--- a/compiler/rustc_mir_transform/src/simplify.rs
+++ b/compiler/rustc_mir_transform/src/simplify.rs
@@ -278,6 +278,18 @@ impl<'a, 'tcx> CfgSimplifier<'a, 'tcx> {
     }
 }
 
+pub fn combine_duplicate_switch_targets(terminator: &mut Terminator<'_>) {
+    if let TerminatorKind::SwitchInt { targets, .. } = &mut terminator.kind {
+        let otherwise = targets.otherwise();
+        if targets.iter().any(|t| t.1 == otherwise) {
+            *targets = SwitchTargets::new(
+                targets.iter().filter(|t| t.1 != otherwise),
+                targets.otherwise(),
+            );
+        }
+    }
+}
+
 pub fn remove_duplicate_unreachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     struct OptApplier<'tcx> {
         tcx: TyCtxt<'tcx>,
@@ -298,6 +310,8 @@ pub fn remove_duplicate_unreachable_blocks<'tcx>(tcx: TyCtxt<'tcx>, body: &mut B
                 }
             }
 
+            combine_duplicate_switch_targets(terminator);
+
             self.super_terminator(terminator, location);
         }
     }