Auto merge of #119675 - cjgillot:set-no-discriminant, r=tmiasko

Skip threading over no-op SetDiscriminant.

Fixes https://github.com/rust-lang/rust/issues/119674

3 files changed, 130 insertions, 0 deletions

diff --git a/tests/mir-opt/set_no_discriminant.f.JumpThreading.diff b/tests/mir-opt/set_no_discriminant.f.JumpThreading.diff
new file mode 100644
index 00000000000..bc28e81c9a8
--- /dev/null
+++ b/tests/mir-opt/set_no_discriminant.f.JumpThreading.diff
@@ -0,0 +1,26 @@
+- // MIR for `f` before JumpThreading
++ // MIR for `f` after JumpThreading
+  
+  fn f() -> usize {
+      let mut _0: usize;
+      let mut _1: isize;
+      let mut _2: E<char>;
+  
+      bb0: {
+          _2 = E::<char>::A;
+          discriminant(_2) = 1;
+          _1 = discriminant(_2);
+          switchInt(_1) -> [0: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          _0 = const 0_usize;
+          return;
+      }
+  
+      bb2: {
+          _0 = const 1_usize;
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/set_no_discriminant.generic.JumpThreading.diff b/tests/mir-opt/set_no_discriminant.generic.JumpThreading.diff
new file mode 100644
index 00000000000..78bfeef3c64
--- /dev/null
+++ b/tests/mir-opt/set_no_discriminant.generic.JumpThreading.diff
@@ -0,0 +1,26 @@
+- // MIR for `generic` before JumpThreading
++ // MIR for `generic` after JumpThreading
+  
+  fn generic() -> usize {
+      let mut _0: usize;
+      let mut _1: isize;
+      let mut _2: E<T>;
+  
+      bb0: {
+          _2 = E::<T>::A;
+          discriminant(_2) = 1;
+          _1 = discriminant(_2);
+          switchInt(_1) -> [0: bb1, otherwise: bb2];
+      }
+  
+      bb1: {
+          _0 = const 0_usize;
+          return;
+      }
+  
+      bb2: {
+          _0 = const 1_usize;
+          return;
+      }
+  }
+  
diff --git a/tests/mir-opt/set_no_discriminant.rs b/tests/mir-opt/set_no_discriminant.rs
new file mode 100644
index 00000000000..8ffb9a2910a
--- /dev/null
+++ b/tests/mir-opt/set_no_discriminant.rs
@@ -0,0 +1,78 @@
+// `SetDiscriminant` does not actually write anything if the chosen variant is the untagged variant
+// of a niche encoding. Verify that we do not thread over this case.
+// unit-test: JumpThreading
+
+#![feature(custom_mir)]
+#![feature(core_intrinsics)]
+
+use std::intrinsics::mir::*;
+
+enum E<T> {
+    A,
+    B(T),
+}
+
+// EMIT_MIR set_no_discriminant.f.JumpThreading.diff
+#[custom_mir(dialect = "runtime")]
+pub fn f() -> usize {
+    // CHECK-LABEL: fn f(
+    // CHECK-NOT: goto
+    // CHECK: switchInt(
+    // CHECK-NOT: goto
+    mir!(
+        let a: isize;
+        let e: E<char>;
+        {
+            e = E::A;
+            SetDiscriminant(e, 1);
+            a = Discriminant(e);
+            match a {
+                0 => bb0,
+                _ => bb1,
+            }
+        }
+        bb0 = {
+            RET = 0;
+            Return()
+        }
+        bb1 = {
+            RET = 1;
+            Return()
+        }
+    )
+}
+
+// EMIT_MIR set_no_discriminant.generic.JumpThreading.diff
+#[custom_mir(dialect = "runtime")]
+pub fn generic<T>() -> usize {
+    // CHECK-LABEL: fn generic(
+    // CHECK-NOT: goto
+    // CHECK: switchInt(
+    // CHECK-NOT: goto
+    mir!(
+        let a: isize;
+        let e: E<T>;
+        {
+            e = E::A;
+            SetDiscriminant(e, 1);
+            a = Discriminant(e);
+            match a {
+                0 => bb0,
+                _ => bb1,
+            }
+        }
+        bb0 = {
+            RET = 0;
+            Return()
+        }
+        bb1 = {
+            RET = 1;
+            Return()
+        }
+    )
+}
+
+fn main() {
+    assert_eq!(f(), 0);
+    assert_eq!(generic::<char>(), 0);
+}