mir-opt: Postpone the MatchBranchSimplification to after the final DSE

dianqk · dianqk · commit dabec6602ad5 · 2025-07-21T23:06:37.000+08:00
This may not be the best phase, but we do need a CFG simplification.
A potential improvement would be to implement a trivial simplification.
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
@@ -703,7 +703,6 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             &dead_store_elimination::DeadStoreElimination::Initial,
             &gvn::GVN,
             &simplify::SimplifyLocals::AfterGVN,
-            &match_branches::MatchBranchSimplification,
             &dataflow_const_prop::DataflowConstProp,
             &single_use_consts::SingleUseConsts,
             &o1(simplify_branches::SimplifyConstCondition::AfterConstProp),
@@ -718,6 +717,7 @@ pub(crate) fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'
             &strip_debuginfo::StripDebugInfo,
             &copy_prop::CopyProp,
             &dead_store_elimination::DeadStoreElimination::Final,
+            &match_branches::MatchBranchSimplification,
             &nrvo::RenameReturnPlace,
             &simplify::SimplifyLocals::Final,
             &multiple_return_terminators::MultipleReturnTerminators,
diff --git a/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.32bit.panic-abort.mir b/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.32bit.panic-abort.mir
@@ -18,12 +18,12 @@ fn num_to_digit(_1: char) -> u32 {
     }
 
     bb0: {
+        StorageLive(_3);
         StorageLive(_2);
         _2 = char::methods::<impl char>::to_digit(copy _1, const 8_u32) -> [return: bb1, unwind unreachable];
     }
 
     bb1: {
-        StorageLive(_3);
         _3 = discriminant(_2);
         StorageDead(_2);
         switchInt(move _3) -> [1: bb2, otherwise: bb7];
diff --git a/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.32bit.panic-unwind.mir b/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.32bit.panic-unwind.mir
@@ -18,12 +18,12 @@ fn num_to_digit(_1: char) -> u32 {
     }
 
     bb0: {
+        StorageLive(_3);
         StorageLive(_2);
         _2 = char::methods::<impl char>::to_digit(copy _1, const 8_u32) -> [return: bb1, unwind continue];
     }
 
     bb1: {
-        StorageLive(_3);
         _3 = discriminant(_2);
         StorageDead(_2);
         switchInt(move _3) -> [1: bb2, otherwise: bb7];
diff --git a/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.64bit.panic-abort.mir b/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.64bit.panic-abort.mir
@@ -18,12 +18,12 @@ fn num_to_digit(_1: char) -> u32 {
     }
 
     bb0: {
+        StorageLive(_3);
         StorageLive(_2);
         _2 = char::methods::<impl char>::to_digit(copy _1, const 8_u32) -> [return: bb1, unwind unreachable];
     }
 
     bb1: {
-        StorageLive(_3);
         _3 = discriminant(_2);
         StorageDead(_2);
         switchInt(move _3) -> [1: bb2, otherwise: bb7];
diff --git a/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.64bit.panic-unwind.mir b/tests/mir-opt/issues/issue_59352.num_to_digit.PreCodegen.after.64bit.panic-unwind.mir
@@ -18,12 +18,12 @@ fn num_to_digit(_1: char) -> u32 {
     }
 
     bb0: {
+        StorageLive(_3);
         StorageLive(_2);
         _2 = char::methods::<impl char>::to_digit(copy _1, const 8_u32) -> [return: bb1, unwind continue];
     }
 
     bb1: {
-        StorageLive(_3);
         _3 = discriminant(_2);
         StorageDead(_2);
         switchInt(move _3) -> [1: bb2, otherwise: bb7];
diff --git a/tests/mir-opt/issues/issue_75439.foo.MatchBranchSimplification.diff b/tests/mir-opt/issues/issue_75439.foo.MatchBranchSimplification.diff
@@ -6,6 +6,7 @@
       let mut _0: std::option::Option<[u8; 4]>;
       let _2: [u32; 4];
       let mut _4: [u8; 4];
+      let mut _5: bool;
       scope 1 {
           debug dwords => _2;
           scope 2 {
@@ -17,34 +18,35 @@
       bb0: {
           StorageLive(_2);
           _2 = copy _1 as [u32; 4] (Transmute);
-          switchInt(copy _2[0 of 4]) -> [0: bb1, otherwise: bb4];
+          _5 = Ne(copy _2[0 of 4], copy _2[1 of 4]);
+          switchInt(move _5) -> [0: bb5, otherwise: bb3];
       }
   
       bb1: {
-          switchInt(copy _2[1 of 4]) -> [0: bb2, otherwise: bb4];
+          switchInt(copy _2[2 of 4]) -> [0: bb2, 4294901760: bb2, otherwise: bb3];
       }
   
       bb2: {
-          switchInt(copy _2[2 of 4]) -> [0: bb3, 4294901760: bb3, otherwise: bb4];
-      }
-  
-      bb3: {
           _3 = copy _2[3 of 4];
           StorageLive(_4);
           _4 = copy _3 as [u8; 4] (Transmute);
           _0 = Option::<[u8; 4]>::Some(move _4);
           StorageDead(_4);
-          goto -> bb5;
+          goto -> bb4;
       }
   
-      bb4: {
+      bb3: {
           _0 = Option::<[u8; 4]>::None;
-          goto -> bb5;
+          goto -> bb4;
       }
   
-      bb5: {
+      bb4: {
           StorageDead(_2);
           return;
       }
+  
+      bb5: {
+          switchInt(copy _2[0 of 4]) -> [0: bb1, otherwise: bb3];
+      }
   }
   
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.clone_as_copy.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/clone_as_copy.clone_as_copy.PreCodegen.after.mir
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.enum_clone_as_copy.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/clone_as_copy.enum_clone_as_copy.PreCodegen.after.mir
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.rs b/tests/mir-opt/pre-codegen/clone_as_copy.rs
@@ -22,22 +22,23 @@ enum Enum1 {
     B(NestCopy),
 }
 
-// EMIT_MIR clone_as_copy.clone_as_copy.PreCodegen.after.mir
-fn clone_as_copy(v: &NestCopy) -> NestCopy {
-    // CHECK-LABEL: fn clone_as_copy(
-    // CHECK: let [[DEAD_VAR:_.*]]: &AllCopy;
-    // CHECK: bb0: {
-    // CHECK-NEXT: DBG: [[DEAD_VAR]] = &((*_1).1: AllCopy)
-    // CHECK-NEXT: _0 = copy (*_1);
-    // CHECK-NEXT: return;
-    v.clone()
-}
+// EMIT_MIR clone_as_copy.{impl#0}-clone.PreCodegen.after.mir
+// CHECK-LABEL: fn <impl at {{.*}}>::clone(_1: &AllCopy) -> AllCopy {
+// CHECK: bb0: {
+// CHECK-NEXT: _0 = copy (*_1);
+// CHECK-NEXT: return;
 
-// EMIT_MIR clone_as_copy.enum_clone_as_copy.PreCodegen.after.mir
-fn enum_clone_as_copy(v: &Enum1) -> Enum1 {
-    // CHECK-LABEL: fn enum_clone_as_copy(
-    // CHECK: bb0: {
-    // CHECK-NEXT: _0 = copy (*_1);
-    // CHECK-NEXT: return;
-    v.clone()
-}
+// EMIT_MIR clone_as_copy.{impl#1}-clone.PreCodegen.after.mir
+// CHECK-LABEL: fn <impl at {{.*}}>::clone(_1: &NestCopy) -> NestCopy {
+// CHECK: scope 1 (inlined <AllCopy as Clone>::clone) {
+// CHECK-NEXT: debug self => [[inlined_AllCopy_self:_[0-9]+]];
+// CHECK: bb0: {
+// CHECK-NEXT: DBG: [[inlined_AllCopy_self]] = &((*_1).1: AllCopy)
+// CHECK-NEXT: _0 = copy (*_1);
+// CHECK-NEXT: return;
+
+// EMIT_MIR clone_as_copy.{impl#2}-clone.PreCodegen.after.mir
+// CHECK-LABEL: fn <impl at {{.*}}>::clone(_1: &Enum1) -> Enum1 {
+// CHECK: bb0: {
+// CHECK-NEXT: _0 = copy (*_1);
+// CHECK-NEXT: return;
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.{impl#0}-clone.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/clone_as_copy.{impl#0}-clone.PreCodegen.after.mir
@@ -0,0 +1,11 @@
+// MIR for `<impl at $DIR/clone_as_copy.rs:5:10: 5:15>::clone` after PreCodegen
+
+fn <impl at $DIR/clone_as_copy.rs:5:10: 5:15>::clone(_1: &AllCopy) -> AllCopy {
+    debug self => _1;
+    let mut _0: AllCopy;
+
+    bb0: {
+        _0 = copy (*_1);
+        return;
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.{impl#1}-clone.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/clone_as_copy.{impl#1}-clone.PreCodegen.after.mir
@@ -0,0 +1,16 @@
+// MIR for `<impl at $DIR/clone_as_copy.rs:12:10: 12:15>::clone` after PreCodegen
+
+fn <impl at $DIR/clone_as_copy.rs:12:10: 12:15>::clone(_1: &NestCopy) -> NestCopy {
+    debug self => _1;
+    let mut _0: NestCopy;
+    let _2: &AllCopy;
+    scope 1 (inlined <AllCopy as Clone>::clone) {
+        debug self => _2;
+    }
+
+    bb0: {
+        // DBG: _2 = &((*_1).1: AllCopy)
+        _0 = copy (*_1);
+        return;
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/clone_as_copy.{impl#2}-clone.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/clone_as_copy.{impl#2}-clone.PreCodegen.after.mir
@@ -0,0 +1,29 @@
+// MIR for `<impl at $DIR/clone_as_copy.rs:19:10: 19:15>::clone` after PreCodegen
+
+fn <impl at $DIR/clone_as_copy.rs:19:10: 19:15>::clone(_1: &Enum1) -> Enum1 {
+    debug self => _1;
+    let mut _0: Enum1;
+    let mut _2: &AllCopy;
+    let mut _3: &NestCopy;
+    scope 1 {
+        debug __self_0 => _2;
+        scope 5 (inlined <AllCopy as Clone>::clone) {
+            debug self => _2;
+        }
+    }
+    scope 2 {
+        debug __self_0 => _3;
+        scope 3 (inlined <NestCopy as Clone>::clone) {
+            debug self => _3;
+            let _4: &AllCopy;
+            scope 4 (inlined <AllCopy as Clone>::clone) {
+                debug self => _4;
+            }
+        }
+    }
+
+    bb0: {
+        _0 = copy (*_1);
+        return;
+    }
+}
diff --git a/tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-abort.diff b/tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-abort.diff
@@ -36,6 +36,12 @@
   
       bb2: {
           _6 = copy (((_1.0: std::option::Option<u8>) as Some).0: u8);
+-         StorageLive(_7);
+-         _7 = Gt(copy _6, const 42_u8);
+-         StorageLive(_9);
+-         _9 = move _7;
+-         StorageDead(_9);
+-         StorageDead(_7);
           goto -> bb3;
       }
   
diff --git a/tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-unwind.diff b/tests/mir-opt/simplify_locals_fixedpoint.foo.SimplifyLocals-final.panic-unwind.diff
@@ -36,6 +36,12 @@
   
       bb2: {
           _6 = copy (((_1.0: std::option::Option<u8>) as Some).0: u8);
+-         StorageLive(_7);
+-         _7 = Gt(copy _6, const 42_u8);
+-         StorageLive(_9);
+-         _9 = move _7;
+-         StorageDead(_9);
+-         StorageDead(_7);
           goto -> bb3;
       }
   
diff --git a/tests/mir-opt/switch_to_self.test.MatchBranchSimplification.diff b/tests/mir-opt/switch_to_self.test.MatchBranchSimplification.diff
@@ -3,16 +3,27 @@
   
   fn test(_1: bool) -> () {
       let mut _0: ();
+      let mut _2: bool;
++     let mut _3: bool;
   
       bb0: {
           goto -> bb1;
       }
   
       bb1: {
+          _2 = Ne(copy _1, copy _1);
+-         switchInt(move _2) -> [0: bb3, otherwise: bb2];
+-     }
+- 
+-     bb2: {
++         StorageLive(_3);
++         _3 = move _2;
++         StorageDead(_3);
           switchInt(copy _1) -> [0: bb1, otherwise: bb2];
       }
   
-      bb2: {
+-     bb3: {
++     bb2: {
           switchInt(copy _1) -> [0: bb1, otherwise: bb2];
       }
   }

Original file line number	Diff line number	Diff line change
`@@ -18,12 +18,12 @@ fn num_to_digit(_1: char) -> u32 {`
`18`	`18`	`}`
`19`	`19`
`20`	`20`	`bb0: {`
	`21`	`+ StorageLive(_3);`
`21`	`22`	`StorageLive(_2);`
`22`	`23`	`_2 = char::methods::<impl char>::to_digit(copy _1, const 8_u32) -> [return: bb1, unwind unreachable];`
`23`	`24`	`}`
`24`	`25`
`25`	`26`	`bb1: {`
`26`		`- StorageLive(_3);`
`27`	`27`	`_3 = discriminant(_2);`
`28`	`28`	`StorageDead(_2);`
`29`	`29`	`switchInt(move _3) -> [1: bb2, otherwise: bb7];`