GVN: Preserve derefs at unreachable

Author: dianqk

compiler/rustc_mir_transform/src/gvn.rs

@@ -1924,7 +1924,9 @@ impl<'tcx> MutVisitor<'tcx> for VnState<'_, '_, 'tcx> {
         // Currently, only preserving derefs for trivial terminators like SwitchInt and Goto.
         let safe_to_preserve_derefs = matches!(
             terminator.kind,
-            TerminatorKind::SwitchInt { .. } | TerminatorKind::Goto { .. }
+            TerminatorKind::SwitchInt { .. }
+                | TerminatorKind::Goto { .. }
+                | TerminatorKind::Unreachable
         );
         if !safe_to_preserve_derefs {
             self.invalidate_derefs();

tests/mir-opt/pre-codegen/two_unwrap_unchecked.rs

@@ -1,11 +1,14 @@
-// skip-filecheck
 //@ compile-flags: -O
 
 #![crate_type = "lib"]
 
 // EMIT_MIR two_unwrap_unchecked.two_unwrap_unchecked.GVN.diff
 // EMIT_MIR two_unwrap_unchecked.two_unwrap_unchecked.PreCodegen.after.mir
 pub fn two_unwrap_unchecked(v: &Option<i32>) -> i32 {
+    // CHECK-LABEL: fn two_unwrap_unchecked(
+    // CHECK: [[DEREF_V:_.*]] = copy (*_1);
+    // CHECK: [[V1V2:_.*]] = copy (([[DEREF_V]] as Some).0: i32);
+    // CHECK: _0 = Add(copy [[V1V2]], copy [[V1V2]]);
     let v1 = unsafe { v.unwrap_unchecked() };
     let v2 = unsafe { v.unwrap_unchecked() };
     v1 + v2

tests/mir-opt/pre-codegen/two_unwrap_unchecked.two_unwrap_unchecked.GVN.diff

@@ -41,12 +41,15 @@
 
       bb0: {
 -         StorageLive(_2);
+-         StorageLive(_3);
++         nop;
 +         nop;
-          StorageLive(_3);
           _3 = copy (*_1);
-          StorageLive(_8);
+-         StorageLive(_8);
++         nop;
           _8 = discriminant(_3);
-          switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb1];
+-         switchInt(move _8) -> [0: bb2, 1: bb3, otherwise: bb1];
++         switchInt(copy _8) -> [0: bb2, 1: bb3, otherwise: bb1];
       }
 
       bb1: {
@@ -58,37 +61,43 @@
       }
 
       bb3: {
-          _2 = move ((_3 as Some).0: i32);
-          StorageDead(_8);
-          StorageDead(_3);
--         StorageLive(_4);
+-         _2 = move ((_3 as Some).0: i32);
+-         StorageDead(_8);
+-         StorageDead(_3);
++         _2 = copy ((_3 as Some).0: i32);
 +         nop;
++         nop;
+          StorageLive(_4);
           StorageLive(_5);
-          _5 = copy (*_1);
+-         _5 = copy (*_1);
++         _5 = copy _3;
           StorageLive(_9);
-          _9 = discriminant(_5);
-          switchInt(move _9) -> [0: bb4, 1: bb5, otherwise: bb1];
+-         _9 = discriminant(_5);
+-         switchInt(move _9) -> [0: bb4, 1: bb5, otherwise: bb1];
++         _9 = copy _8;
++         switchInt(copy _8) -> [0: bb4, 1: bb5, otherwise: bb1];
       }
 
       bb4: {
           unreachable;
       }
 
       bb5: {
-          _4 = move ((_5 as Some).0: i32);
+-         _4 = move ((_5 as Some).0: i32);
++         _4 = copy _2;
           StorageDead(_9);
           StorageDead(_5);
           StorageLive(_6);
           _6 = copy _2;
           StorageLive(_7);
-          _7 = copy _4;
+-         _7 = copy _4;
 -         _0 = Add(move _6, move _7);
-+         _0 = Add(copy _2, copy _4);
++         _7 = copy _2;
++         _0 = Add(copy _2, copy _2);
           StorageDead(_7);
           StorageDead(_6);
--         StorageDead(_4);
+          StorageDead(_4);
 -         StorageDead(_2);
-+         nop;
 +         nop;
           return;
       }

tests/mir-opt/pre-codegen/two_unwrap_unchecked.two_unwrap_unchecked.PreCodegen.after.mir

@@ -5,15 +5,12 @@ fn two_unwrap_unchecked(_1: &Option<i32>) -> i32 {
     let mut _0: i32;
     let mut _2: std::option::Option<i32>;
     let _4: i32;
-    let mut _5: std::option::Option<i32>;
     scope 1 {
         debug v1 => _4;
-        let _7: i32;
         scope 2 {
-            debug v2 => _7;
+            debug v2 => _4;
         }
         scope 8 (inlined #[track_caller] Option::<i32>::unwrap_unchecked) {
-            let mut _6: isize;
             scope 9 {
             }
             scope 10 (inlined #[track_caller] unreachable_unchecked) {
@@ -37,33 +34,18 @@ fn two_unwrap_unchecked(_1: &Option<i32>) -> i32 {
     }
 
     bb0: {
-        StorageLive(_2);
         _2 = copy (*_1);
-        StorageLive(_3);
         _3 = discriminant(_2);
-        switchInt(move _3) -> [0: bb3, 1: bb1, otherwise: bb3];
+        switchInt(copy _3) -> [0: bb2, 1: bb1, otherwise: bb2];
     }
 
     bb1: {
-        _4 = move ((_2 as Some).0: i32);
-        StorageDead(_3);
-        StorageDead(_2);
-        StorageLive(_5);
-        _5 = copy (*_1);
-        StorageLive(_6);
-        _6 = discriminant(_5);
-        switchInt(move _6) -> [0: bb3, 1: bb2, otherwise: bb3];
-    }
-
-    bb2: {
-        _7 = move ((_5 as Some).0: i32);
-        StorageDead(_6);
-        StorageDead(_5);
-        _0 = Add(copy _4, copy _7);
+        _4 = copy ((_2 as Some).0: i32);
+        _0 = Add(copy _4, copy _4);
         return;
     }
 
-    bb3: {
+    bb2: {
         unreachable;
     }
 }