Auto merge of rust-lang#121397 - DianQK:early_otherwise_branch_sound, r=<try>

 Re-enable the early otherwise branch optimization

Fixes rust-lang#95162. Fixes rust-lang#119014. Fixes rust-lang#117970.

An invalid enum discriminant can come from anywhere. We have to check to see if all successors contain the discriminant statement.

It should not be UB that we pass in an invalid enum discriminant when calling a function, this is more like LLVM's poison value. UB only when used. Although miri would consider the following code to be UB. (It's fine for miri.)

https://play.rust-lang.org/?version=stable&mode=debug&edition=2021&gist=18602870aaeb07cbdf7dfcd2c28961a2

I extended the scenario with scalars and the same target values.

r? compiler

Author: bors

compiler/rustc_mir_transform/src/early_otherwise_branch.rs

@@ -0,0 +1,25 @@
+//@ compile-flags: -O
+
+#![crate_type = "lib"]
+
+pub enum Enum {
+    A(u32),
+    B(u32),
+    C(u32),
+}
+
+#[no_mangle]
+pub fn foo(lhs: &Enum, rhs: &Enum) -> bool {
+    // CHECK-LABEL: define{{.*}}i1 @foo(
+    // CHECK-NOT: switch
+    // CHECK-NOT: br
+    // CHECK: [[SELECT:%.*]] = select
+    // CHECK-NEXT: ret i1 [[SELECT]]
+    // CHECK-NEXT: }
+    match (lhs, rhs) {
+        (Enum::A(lhs), Enum::A(rhs)) => lhs == rhs,
+        (Enum::B(lhs), Enum::B(rhs)) => lhs == rhs,
+        (Enum::C(lhs), Enum::C(rhs)) => lhs == rhs,
+        _ => false,
+    }
+}

tests/codegen/enum/enum-early-otherwise-branch.rs

@@ -12,8 +12,6 @@
       let mut _7: isize;
       let _8: u32;
       let _9: u32;
-+     let mut _10: isize;
-+     let mut _11: bool;
       scope 1 {
           debug a => _8;
           debug b => _9;
@@ -29,48 +27,33 @@
           StorageDead(_5);
           StorageDead(_4);
           _7 = discriminant((_3.0: std::option::Option<u32>));
--         switchInt(move _7) -> [1: bb2, otherwise: bb1];
-+         StorageLive(_10);
-+         _10 = discriminant((_3.1: std::option::Option<u32>));
-+         StorageLive(_11);
-+         _11 = Ne(_7, move _10);
-+         StorageDead(_10);
-+         switchInt(move _11) -> [0: bb4, otherwise: bb1];
+          switchInt(move _7) -> [1: bb2, otherwise: bb1];
       }
 
       bb1: {
-+         StorageDead(_11);
           _0 = const 1_u32;
--         goto -> bb4;
-+         goto -> bb3;
+          goto -> bb4;
       }
 
       bb2: {
--         _6 = discriminant((_3.1: std::option::Option<u32>));
--         switchInt(move _6) -> [1: bb3, otherwise: bb1];
--     }
-- 
--     bb3: {
+          _6 = discriminant((_3.1: std::option::Option<u32>));
+          switchInt(move _6) -> [1: bb3, otherwise: bb1];
+      }
+  
+      bb3: {
           StorageLive(_8);
           _8 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
           StorageLive(_9);
           _9 = (((_3.1: std::option::Option<u32>) as Some).0: u32);
           _0 = const 0_u32;
           StorageDead(_9);
           StorageDead(_8);
--         goto -> bb4;
-+         goto -> bb3;
+          goto -> bb4;
       }
 
--     bb4: {
-+     bb3: {
+      bb4: {
           StorageDead(_3);
           return;
-+     }
-+ 
-+     bb4: {
-+         StorageDead(_11);
-+         switchInt(_7) -> [1: bb2, otherwise: bb1];
       }
   }
 

tests/mir-opt/early_otherwise_branch.opt1.EarlyOtherwiseBranch.diff

@@ -0,0 +1,90 @@
+- // MIR for `opt10` before EarlyOtherwiseBranch
++ // MIR for `opt10` after EarlyOtherwiseBranch
+  
+  fn opt10(_1: E8, _2: E16) -> u32 {
+      debug x => _1;
+      debug y => _2;
+      let mut _0: u32;
+      let mut _3: (E8, E16);
+      let mut _4: E8;
+      let mut _5: E16;
+      let mut _6: u16;
+      let mut _7: u16;
+      let mut _8: u16;
+      let mut _9: u8;
++     let mut _10: u16;
+  
+      bb0: {
+          StorageLive(_3);
+          StorageLive(_4);
+          _4 = move _1;
+          StorageLive(_5);
+          _5 = move _2;
+          _3 = (move _4, move _5);
+          StorageDead(_5);
+          StorageDead(_4);
+          _9 = discriminant((_3.0: E8));
+-         switchInt(move _9) -> [0: bb2, 1: bb3, 2: bb4, otherwise: bb9];
++         StorageLive(_10);
++         _10 = discriminant((_3.1: E16));
++         switchInt(move _10) -> [0: bb7, otherwise: bb1];
+      }
+  
+      bb1: {
++         StorageDead(_10);
+          _0 = const 0_u32;
+-         goto -> bb8;
++         goto -> bb5;
+      }
+  
+      bb2: {
+-         _6 = discriminant((_3.1: E16));
+-         switchInt(move _6) -> [0: bb5, otherwise: bb1];
++         _0 = const 1_u32;
++         goto -> bb5;
+      }
+  
+      bb3: {
+-         _7 = discriminant((_3.1: E16));
+-         switchInt(move _7) -> [0: bb6, otherwise: bb1];
++         _0 = const 2_u32;
++         goto -> bb5;
+      }
+  
+      bb4: {
+-         _8 = discriminant((_3.1: E16));
+-         switchInt(move _8) -> [0: bb7, otherwise: bb1];
++         _0 = const 3_u32;
++         goto -> bb5;
+      }
+  
+      bb5: {
+-         _0 = const 1_u32;
+-         goto -> bb8;
++         StorageDead(_3);
++         return;
+      }
+  
+      bb6: {
+-         _0 = const 2_u32;
+-         goto -> bb8;
++         unreachable;
+      }
+  
+      bb7: {
+-         _0 = const 3_u32;
+-         goto -> bb8;
+-     }
+- 
+-     bb8: {
+-         StorageDead(_3);
+-         return;
+-     }
+- 
+-     bb9: {
+-         unreachable;
++         StorageDead(_10);
++         switchInt(_9) -> [0: bb2, 1: bb3, 2: bb4, otherwise: bb6];
+      }
+  }
+  

tests/mir-opt/early_otherwise_branch.opt10.EarlyOtherwiseBranch.diff

@@ -30,13 +30,13 @@
           StorageDead(_5);
           StorageDead(_4);
           _8 = discriminant((_3.0: std::option::Option<u32>));
--         switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb1];
+-         switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb7];
 +         StorageLive(_11);
 +         _11 = discriminant((_3.1: std::option::Option<u32>));
 +         StorageLive(_12);
 +         _12 = Ne(_8, move _11);
 +         StorageDead(_11);
-+         switchInt(move _12) -> [0: bb5, otherwise: bb1];
++         switchInt(move _12) -> [0: bb6, otherwise: bb1];
       }
 
       bb1: {
@@ -70,7 +70,7 @@
 
 -     bb5: {
 +     bb3: {
-          _0 = const 0_u32;
+          _0 = const 2_u32;
 -         goto -> bb6;
 +         goto -> bb4;
       }
@@ -79,11 +79,16 @@
 +     bb4: {
           StorageDead(_3);
           return;
+      }
+  
+-     bb7: {
++     bb5: {
+          unreachable;
 +     }
 + 
-+     bb5: {
++     bb6: {
 +         StorageDead(_12);
-+         switchInt(_8) -> [0: bb3, 1: bb2, otherwise: bb1];
++         switchInt(_8) -> [0: bb3, 1: bb2, otherwise: bb5];
       }
   }
 

tests/mir-opt/early_otherwise_branch.opt2.EarlyOtherwiseBranch.diff

@@ -10,13 +10,14 @@
       let mut _5: std::option::Option<bool>;
       let mut _6: isize;
       let mut _7: isize;
-      let _8: u32;
-      let _9: bool;
-+     let mut _10: isize;
-+     let mut _11: bool;
+      let mut _8: isize;
+      let _9: u32;
+      let _10: bool;
++     let mut _11: isize;
++     let mut _12: bool;
       scope 1 {
-          debug a => _8;
-          debug b => _9;
+          debug a => _9;
+          debug b => _10;
       }
 
       bb0: {
@@ -28,49 +29,66 @@
           _3 = (move _4, move _5);
           StorageDead(_5);
           StorageDead(_4);
-          _7 = discriminant((_3.0: std::option::Option<u32>));
--         switchInt(move _7) -> [1: bb2, otherwise: bb1];
-+         StorageLive(_10);
-+         _10 = discriminant((_3.1: std::option::Option<bool>));
+          _8 = discriminant((_3.0: std::option::Option<u32>));
+-         switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb7];
 +         StorageLive(_11);
-+         _11 = Ne(_7, move _10);
-+         StorageDead(_10);
-+         switchInt(move _11) -> [0: bb4, otherwise: bb1];
++         _11 = discriminant((_3.1: std::option::Option<bool>));
++         StorageLive(_12);
++         _12 = Ne(_8, move _11);
++         StorageDead(_11);
++         switchInt(move _12) -> [0: bb6, otherwise: bb1];
       }
 
       bb1: {
-+         StorageDead(_11);
++         StorageDead(_12);
           _0 = const 1_u32;
--         goto -> bb4;
-+         goto -> bb3;
+-         goto -> bb6;
++         goto -> bb4;
       }
 
       bb2: {
 -         _6 = discriminant((_3.1: std::option::Option<bool>));
--         switchInt(move _6) -> [1: bb3, otherwise: bb1];
+-         switchInt(move _6) -> [0: bb5, otherwise: bb1];
 -     }
 - 
 -     bb3: {
-          StorageLive(_8);
-          _8 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+-         _7 = discriminant((_3.1: std::option::Option<bool>));
+-         switchInt(move _7) -> [1: bb4, otherwise: bb1];
+-     }
+- 
+-     bb4: {
           StorageLive(_9);
-          _9 = (((_3.1: std::option::Option<bool>) as Some).0: bool);
+          _9 = (((_3.0: std::option::Option<u32>) as Some).0: u32);
+          StorageLive(_10);
+          _10 = (((_3.1: std::option::Option<bool>) as Some).0: bool);
           _0 = const 0_u32;
+          StorageDead(_10);
           StorageDead(_9);
-          StorageDead(_8);
--         goto -> bb4;
-+         goto -> bb3;
+-         goto -> bb6;
++         goto -> bb4;
       }
 
--     bb4: {
+-     bb5: {
 +     bb3: {
+          _0 = const 2_u32;
+-         goto -> bb6;
++         goto -> bb4;
+      }
+  
+-     bb6: {
++     bb4: {
           StorageDead(_3);
           return;
+      }
+  
+-     bb7: {
++     bb5: {
+          unreachable;
 +     }
 + 
-+     bb4: {
-+         StorageDead(_11);
-+         switchInt(_7) -> [1: bb2, otherwise: bb1];
++     bb6: {
++         StorageDead(_12);
++         switchInt(_8) -> [0: bb3, 1: bb2, otherwise: bb5];
       }
   }