Auto merge of #66329 - ktrianta:mir-opt-unreachable-propagation, r=oli-obk

Add unreachable propagation mir optimization pass

@oli-obk suggested we create a MIR pass that optimizes away basic blocks that lead only to basic blocks with terminator kind **unreachable**. This is a first take on this, which we started with @gilescope at RustFest Impl Days.

The test currently fails when the compiled program runs (undefined behaviour). Is there a way to avoid running the compiled program?

src/test/mir-opt/simplify_try.rs

@@ -47,25 +47,22 @@ fn main() {
 //     }
 //     bb0: {
 //         _5 = discriminant(_1);
-//         switchInt(move _5) -> [0isize: bb4, 1isize: bb2, otherwise: bb1];
+//         switchInt(move _5) -> [0isize: bb3, otherwise: bb1];
 //     }
 //     bb1: {
-//         unreachable;
-//     }
-//     bb2: {
 //         _6 = ((_1 as Err).0: i32);
 //         ((_0 as Err).0: i32) = move _6;
 //         discriminant(_0) = 1;
-//         goto -> bb3;
+//         goto -> bb2;
 //     }
-//     bb3: {
+//     bb2: {
 //         return;
 //     }
-//     bb4: {
+//     bb3: {
 //         _10 = ((_1 as Ok).0: u32);
 //         ((_0 as Ok).0: u32) = move _10;
 //         discriminant(_0) = 0;
-//         goto -> bb3;
+//         goto -> bb2;
 //     }
 // }
 // END rustc.try_identity.SimplifyArmIdentity.before.mir
@@ -109,25 +106,22 @@ fn main() {
 //     }
 //     bb0: {
 //         _5 = discriminant(_1);
-//         switchInt(move _5) -> [0isize: bb4, 1isize: bb2, otherwise: bb1];
+//         switchInt(move _5) -> [0isize: bb3, otherwise: bb1];
 //     }
 //     bb1: {
-//         unreachable;
+//         _0 = move _1;
+//         nop;
+//         nop;
+//         goto -> bb2;
 //     }
 //     bb2: {
-//         _0 = move _1;
-//         nop;
-//         nop;
-//         goto -> bb3;
-//     }
-//     bb3: {
 //         return;
 //     }
-//     bb4: {
+//     bb3: {
 //         _0 = move _1;
 //         nop;
 //         nop;
-//         goto -> bb3;
+//         goto -> bb2;
 //     }
 // }
 // END rustc.try_identity.SimplifyArmIdentity.after.mir

src/test/mir-opt/uninhabited_enum_branching.rs

@@ -45,53 +45,47 @@ fn main() {
 //   StorageLive(_2);
 //   _2 = Test1::C;
 //   _3 = discriminant(_2);
-//   switchInt(move _3) -> [0isize: bb3, 1isize: bb4, 2isize: bb1, otherwise: bb2];
+//   switchInt(move _3) -> [0isize: bb2, 1isize: bb3, otherwise: bb1];
 // }
 // bb1: {
 //   StorageLive(_5);
 //   _5 = const "C";
 //   _1 = &(*_5);
 //   StorageDead(_5);
-//   goto -> bb5;
+//   goto -> bb4;
 // }
 // bb2: {
-//   unreachable;
+//   _1 = const "A(Empty)";
+//   goto -> bb4;
 // }
 // bb3: {
-//   _1 = const "A(Empty)";
-//   goto -> bb5;
-// }
-// bb4: {
 //   StorageLive(_4);
 //   _4 = const "B(Empty)";
 //   _1 = &(*_4);
 //   StorageDead(_4);
-//   goto -> bb5;
+//   goto -> bb4;
 // }
-// bb5: {
+// bb4: {
 //   StorageDead(_2);
 //   StorageDead(_1);
 //   StorageLive(_6);
 //   StorageLive(_7);
 //   _7 = Test2::D;
 //   _8 = discriminant(_7);
-//   switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
+//   switchInt(move _8) -> [4isize: bb6, otherwise: bb5];
 // }
-// bb6: {
+// bb5: {
 //   StorageLive(_9);
 //   _9 = const "E";
 //   _6 = &(*_9);
 //   StorageDead(_9);
-//   goto -> bb9;
+//   goto -> bb7;
+// }
+// bb6: {
+//   _6 = const "D";
+//   goto -> bb7;
 // }
 // bb7: {
-//   unreachable;
-// }
-// bb8: {
-//   _6 = const "D";
-//   goto -> bb9;
-// }
-// bb9: {
 //   StorageDead(_7);
 //   StorageDead(_6);
 //   _0 = ();
@@ -114,53 +108,47 @@ fn main() {
 //   StorageLive(_2);
 //   _2 = Test1::C;
 //   _3 = discriminant(_2);
-//   switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
+//   switchInt(move _3) -> bb1;
 // }
 // bb1: {
 //   StorageLive(_5);
 //   _5 = const "C";
 //   _1 = &(*_5);
 //   StorageDead(_5);
-//   goto -> bb5;
+//   goto -> bb4;
 // }
 // bb2: {
-//   unreachable;
+//   _1 = const "A(Empty)";
+//   goto -> bb4;
 // }
 // bb3: {
-//   _1 = const "A(Empty)";
-//   goto -> bb5;
-// }
-// bb4: {
 //   StorageLive(_4);
 //   _4 = const "B(Empty)";
 //   _1 = &(*_4);
 //   StorageDead(_4);
-//   goto -> bb5;
+//   goto -> bb4;
 // }
-// bb5: {
+// bb4: {
 //   StorageDead(_2);
 //   StorageDead(_1);
 //   StorageLive(_6);
 //   StorageLive(_7);
 //   _7 = Test2::D;
 //   _8 = discriminant(_7);
-//   switchInt(move _8) -> [4isize: bb8, 5isize: bb6, otherwise: bb7];
+//   switchInt(move _8) -> [4isize: bb6, otherwise: bb5];
 // }
-// bb6: {
+// bb5: {
 //   StorageLive(_9);
 //   _9 = const "E";
 //   _6 = &(*_9);
 //   StorageDead(_9);
-//   goto -> bb9;
+//   goto -> bb7;
+// }
+// bb6: {
+//   _6 = const "D";
+//   goto -> bb7;
 // }
 // bb7: {
-//   unreachable;
-// }
-// bb8: {
-//   _6 = const "D";
-//   goto -> bb9;
-// }
-// bb9: {
 //   StorageDead(_7);
 //   StorageDead(_6);
 //   _0 = ();
@@ -183,9 +171,6 @@ fn main() {
 //   StorageLive(_2);
 //   _2 = Test1::C;
 //   _3 = discriminant(_2);
-//   switchInt(move _3) -> [2isize: bb1, otherwise: bb2];
-// }
-// bb1: {
 //   StorageLive(_5);
 //   _5 = const "C";
 //   _1 = &(*_5);
@@ -196,26 +181,20 @@ fn main() {
 //   StorageLive(_7);
 //   _7 = Test2::D;
 //   _8 = discriminant(_7);
-//   switchInt(move _8) -> [4isize: bb5, 5isize: bb3, otherwise: bb4];
+//   switchInt(move _8) -> [4isize: bb2, otherwise: bb1];
 // }
-// bb2: {
-//   unreachable;
-// }
-// bb3: {
+// bb1: {
 //   StorageLive(_9);
 //   _9 = const "E";
 //   _6 = &(*_9);
 //   StorageDead(_9);
-//   goto -> bb6;
+//   goto -> bb3;
 // }
-// bb4: {
-//   unreachable;
-// }
-// bb5: {
+// bb2: {
 //   _6 = const "D";
-//   goto -> bb6;
+//   goto -> bb3;
 // }
-// bb6: {
+// bb3: {
 //   StorageDead(_7);
 //   StorageDead(_6);
 //   _0 = ();

src/test/mir-opt/unreachable.rs

@@ -0,0 +1,78 @@
+enum Empty {}
+
+fn empty() -> Option<Empty> {
+    None
+}
+
+fn main() {
+    if let Some(_x) = empty() {
+        let mut _y;
+
+        if true {
+            _y = 21;
+        } else {
+            _y = 42;
+        }
+
+        match _x { }
+    }
+}
+
+// END RUST SOURCE
+// START rustc.main.UnreachablePropagation.before.mir
+//      bb0: {
+//          StorageLive(_1);
+//          _1 = const empty() -> bb1;
+//      }
+//      bb1: {
+//          _2 = discriminant(_1);
+//          switchInt(move _2) -> [1isize: bb3, otherwise: bb2];
+//      }
+//      bb2: {
+//          _0 = ();
+//          StorageDead(_1);
+//          return;
+//      }
+//      bb3: {
+//          StorageLive(_3);
+//          _3 = move ((_1 as Some).0: Empty);
+//          StorageLive(_4);
+//          StorageLive(_5);
+//          StorageLive(_6);
+//          _6 = const true;
+//          switchInt(_6) -> [false: bb4, otherwise: bb5];
+//      }
+//      bb4: {
+//          _4 = const 42i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb5: {
+//          _4 = const 21i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb6: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_7);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.before.mir
+// START rustc.main.UnreachablePropagation.after.mir
+//      bb0: {
+//          StorageLive(_1);
+//          _1 = const empty() -> bb1;
+//      }
+//      bb1: {
+//          _2 = discriminant(_1);
+//          goto -> bb2;
+//      }
+//      bb2: {
+//          _0 = ();
+//          StorageDead(_1);
+//          return;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.after.mir

src/test/mir-opt/unreachable_asm.rs

@@ -0,0 +1,72 @@
+// ignore-tidy-linelength
+#![feature(asm)]
+
+enum Empty {}
+
+fn empty() -> Option<Empty> {
+    None
+}
+
+fn main() {
+    if let Some(_x) = empty() {
+        let mut _y;
+
+        if true {
+            _y = 21;
+        } else {
+            _y = 42;
+        }
+
+        // asm instruction stops unreachable propagation to if else blocks bb4 and bb5.
+        unsafe { asm!("NOP"); }
+        match _x { }
+    }
+}
+
+// END RUST SOURCE
+// START rustc.main.UnreachablePropagation.before.mir
+//      bb4: {
+//          _4 = const 42i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb5: {
+//          _4 = const 21i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb6: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_7);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _7 = ();
+//          StorageDead(_7);
+//          StorageLive(_8);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.before.mir
+// START rustc.main.UnreachablePropagation.after.mir
+//      bb4: {
+//          _4 = const 42i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb5: {
+//          _4 = const 21i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb6: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_7);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _7 = ();
+//          StorageDead(_7);
+//          StorageLive(_8);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.after.mir

src/test/mir-opt/unreachable_asm_2.rs

@@ -0,0 +1,84 @@
+// ignore-tidy-linelength
+#![feature(asm)]
+
+enum Empty {}
+
+fn empty() -> Option<Empty> {
+    None
+}
+
+fn main() {
+    if let Some(_x) = empty() {
+        let mut _y;
+
+        if true {
+            // asm instruction stops unreachable propagation to block bb3.
+            unsafe { asm!("NOP"); }
+            _y = 21;
+        } else {
+            // asm instruction stops unreachable propagation to block bb3.
+            unsafe { asm!("NOP"); }
+            _y = 42;
+        }
+
+        match _x { }
+    }
+}
+
+// END RUST SOURCE
+// START rustc.main.UnreachablePropagation.before.mir
+//      bb3: {
+//          ...
+//          switchInt(_6) -> [false: bb4, otherwise: bb5];
+//      }
+//      bb4: {
+//          StorageLive(_8);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _8 = ();
+//          StorageDead(_8);
+//          _4 = const 42i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//          bb5: {
+//          StorageLive(_7);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _7 = ();
+//          StorageDead(_7);
+//          _4 = const 21i32;
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb6: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_9);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.before.mir
+// START rustc.main.UnreachablePropagation.after.mir
+//      bb3: {
+//          ...
+//          switchInt(_6) -> [false: bb4, otherwise: bb5];
+//      }
+//      bb4: {
+//          StorageLive(_8);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _8 = ();
+//          StorageDead(_8);
+//          _4 = const 42i32;
+//          _5 = ();
+//          unreachable;
+//      }
+//          bb5: {
+//          StorageLive(_7);
+//          asm!(InlineAsmInner { asm: "NOP", asm_str_style: Cooked, outputs: [], inputs: [], clobbers: [], volatile: true, alignstack: false, dialect: Att } : [] : []);
+//          _7 = ();
+//          StorageDead(_7);
+//          _4 = const 21i32;
+//          _5 = ();
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.after.mir

src/test/mir-opt/unreachable_diverging.rs

@@ -0,0 +1,65 @@
+pub enum Empty {}
+
+fn empty() -> Option<Empty> {
+    None
+}
+
+fn loop_forever() {
+    loop {}
+}
+
+fn main() {
+    let x = true;
+    if let Some(bomb) = empty() {
+        if x {
+            loop_forever()
+        }
+        match bomb {}
+    }
+}
+
+// END RUST SOURCE
+// START rustc.main.UnreachablePropagation.before.mir
+//      bb3: {
+//          StorageLive(_4);
+//          _4 = move ((_2 as Some).0: Empty);
+//          StorageLive(_5);
+//          StorageLive(_6);
+//          _6 = _1;
+//          switchInt(_6) -> [false: bb4, otherwise: bb5];
+//      }
+//      bb4: {
+//          _5 = ();
+//          goto -> bb6;
+//      }
+//      bb5: {
+//          _5 = const loop_forever() -> bb6;
+//      }
+//      bb6: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_7);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.before.mir
+// START rustc.main.UnreachablePropagation.after.mir
+//      bb3: {
+//          StorageLive(_4);
+//          _4 = move ((_2 as Some).0: Empty);
+//          StorageLive(_5);
+//          StorageLive(_6);
+//          _6 = _1;
+//          goto -> bb4;
+//      }
+//      bb4: {
+//          _5 = const loop_forever() -> bb5;
+//      }
+//      bb5: {
+//          StorageDead(_6);
+//          StorageDead(_5);
+//          StorageLive(_7);
+//          unreachable;
+//      }
+//  }
+// END rustc.main.UnreachablePropagation.after.mir