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`

3 files changed, 44 insertions, 1 deletions

diff --git a/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.diff b/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.diff
index 543ddcfc44c..8a8cd896e85 100644
--- a/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.diff
+++ b/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.diff
@@ -34,7 +34,7 @@
 -                                          // + literal: Const { ty: unsafe fn(Option<T>) -> T {Option::<T>::unwrap_unchecked}, val: Value(<ZST>) }
 +         StorageLive(_3);                 // scope 0 at $DIR/unwrap_unchecked.rs:+1:9: +1:27
 +         _4 = discriminant(_2);           // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
-+         switchInt(move _4) -> [0: bb1, 1: bb2, otherwise: bb1]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
++         switchInt(move _4) -> [1: bb2, otherwise: bb1]; // scope 1 at $SRC_DIR/core/src/option.rs:LL:COL
       }
   
       bb1: {
diff --git a/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.rs b/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.rs
new file mode 100644
index 00000000000..09779d789e5
--- /dev/null
+++ b/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.rs
@@ -0,0 +1,16 @@
+// compile-flags: -Zmir-opt-level=2 -Zinline-mir
+// ignore-debug: standard library debug assertions add a panic that breaks this optimization
+#![crate_type = "lib"]
+
+pub enum Thing {
+    A,
+    B,
+}
+
+// EMIT_MIR instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir
+pub unsafe fn ub_if_b(t: Thing) -> Thing {
+    match t {
+        Thing::A => t,
+        Thing::B => std::hint::unreachable_unchecked(),
+    }
+}
diff --git a/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir b/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir
new file mode 100644
index 00000000000..acb7297310f
--- /dev/null
+++ b/tests/mir-opt/instcombine_duplicate_switch_targets_e2e.ub_if_b.PreCodegen.after.mir
@@ -0,0 +1,27 @@
+// MIR for `ub_if_b` after PreCodegen
+
+fn ub_if_b(_1: Thing) -> Thing {
+    debug t => _1;                       // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:23: +0:24
+    let mut _0: Thing;                   // return place in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+0:36: +0:41
+    let mut _2: isize;                   // in scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:9: +2:17
+    scope 1 (inlined unreachable_unchecked) { // at $DIR/instcombine_duplicate_switch_targets_e2e.rs:14:21: 14:55
+        scope 2 {
+            scope 3 (inlined unreachable_unchecked::runtime) { // at $SRC_DIR/core/src/intrinsics.rs:LL:COL
+            }
+        }
+    }
+
+    bb0: {
+        _2 = discriminant(_1);           // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:11: +1:12
+        switchInt(move _2) -> [0: bb2, otherwise: bb1]; // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+1:5: +1:12
+    }
+
+    bb1: {
+        unreachable;                     // scope 2 at $SRC_DIR/core/src/intrinsics.rs:LL:COL
+    }
+
+    bb2: {
+        _0 = move _1;                    // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+2:21: +2:22
+        return;                          // scope 0 at $DIR/instcombine_duplicate_switch_targets_e2e.rs:+5:2: +5:2
+    }
+}