Auto merge of #140115 - dianqk:gvn-matchbr, r=<try>

bors · bors · commit 1aadc7fccd4d · 2025-04-21T13:54:36.000Z
mir-opt: execute MatchBranchSimplification after GVN This can provide more opportunities for MatchBranchSimplification. Currently, rustc does not optimize the following code into a single statement at mir-opt, and this PR fixes the first case. ```rust pub fn match1(c: bool, v1: i32, v2: i32) -> i32 { if c { v1 - v2 } else { v1 - v2 } } pub fn match2(c: bool, v1: i32) -> i32 { if c { v1 - 1 } else { v1 - 1 } } ``` https://rust.godbolt.org/z/Y8xPMjrfM r? mir-opt
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
@@ -699,8 +699,6 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             // Now, we need to shrink the generated MIR.
             &ref_prop::ReferencePropagation,
             &sroa::ScalarReplacementOfAggregates,
-            &match_branches::MatchBranchSimplification,
-            // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
             &multiple_return_terminators::MultipleReturnTerminators,
             // After simplifycfg, it allows us to discover new opportunities for peephole
             // optimizations.
@@ -709,6 +707,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             &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),
diff --git a/tests/mir-opt/inline/unchecked_shifts.unchecked_shl_unsigned_smaller.Inline.panic-abort.diff b/tests/mir-opt/inline/unchecked_shifts.unchecked_shl_unsigned_smaller.Inline.panic-abort.diff
@@ -8,9 +8,9 @@
       let mut _3: u16;
       let mut _4: u32;
 +     scope 1 (inlined core::num::<impl u16>::unchecked_shl) {
-+         let mut _5: bool;
-+         let _6: ();
++         let _5: ();
 +         scope 2 (inlined core::ub_checks::check_language_ub) {
++             let mut _6: bool;
 +             scope 3 (inlined core::ub_checks::check_language_ub::runtime) {
 +             }
 +         }
@@ -22,20 +22,20 @@
           StorageLive(_4);
           _4 = copy _2;
 -         _0 = core::num::<impl u16>::unchecked_shl(move _3, move _4) -> [return: bb1, unwind unreachable];
-+         StorageLive(_6);
 +         StorageLive(_5);
-+         _5 = UbChecks();
-+         switchInt(move _5) -> [0: bb2, otherwise: bb1];
++         StorageLive(_6);
++         _6 = UbChecks();
++         switchInt(copy _6) -> [0: bb2, otherwise: bb1];
       }
   
       bb1: {
-+         _6 = core::num::<impl u16>::unchecked_shl::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
++         _5 = core::num::<impl u16>::unchecked_shl::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
 +     }
 + 
 +     bb2: {
-+         StorageDead(_5);
 +         _0 = ShlUnchecked(copy _3, copy _4);
 +         StorageDead(_6);
++         StorageDead(_5);
           StorageDead(_4);
           StorageDead(_3);
           return;
diff --git a/tests/mir-opt/inline/unchecked_shifts.unchecked_shl_unsigned_smaller.Inline.panic-unwind.diff b/tests/mir-opt/inline/unchecked_shifts.unchecked_shl_unsigned_smaller.Inline.panic-unwind.diff
@@ -8,9 +8,9 @@
       let mut _3: u16;
       let mut _4: u32;
 +     scope 1 (inlined core::num::<impl u16>::unchecked_shl) {
-+         let mut _5: bool;
-+         let _6: ();
++         let _5: ();
 +         scope 2 (inlined core::ub_checks::check_language_ub) {
++             let mut _6: bool;
 +             scope 3 (inlined core::ub_checks::check_language_ub::runtime) {
 +             }
 +         }
@@ -22,20 +22,20 @@
           StorageLive(_4);
           _4 = copy _2;
 -         _0 = core::num::<impl u16>::unchecked_shl(move _3, move _4) -> [return: bb1, unwind continue];
-+         StorageLive(_6);
 +         StorageLive(_5);
-+         _5 = UbChecks();
-+         switchInt(move _5) -> [0: bb2, otherwise: bb1];
++         StorageLive(_6);
++         _6 = UbChecks();
++         switchInt(copy _6) -> [0: bb2, otherwise: bb1];
       }
   
       bb1: {
-+         _6 = core::num::<impl u16>::unchecked_shl::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
++         _5 = core::num::<impl u16>::unchecked_shl::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
 +     }
 + 
 +     bb2: {
-+         StorageDead(_5);
 +         _0 = ShlUnchecked(copy _3, copy _4);
 +         StorageDead(_6);
++         StorageDead(_5);
           StorageDead(_4);
           StorageDead(_3);
           return;
diff --git a/tests/mir-opt/inline/unchecked_shifts.unchecked_shr_signed_bigger.Inline.panic-abort.diff b/tests/mir-opt/inline/unchecked_shifts.unchecked_shr_signed_bigger.Inline.panic-abort.diff
@@ -8,9 +8,9 @@
       let mut _3: i64;
       let mut _4: u32;
 +     scope 1 (inlined core::num::<impl i64>::unchecked_shr) {
-+         let mut _5: bool;
-+         let _6: ();
++         let _5: ();
 +         scope 2 (inlined core::ub_checks::check_language_ub) {
++             let mut _6: bool;
 +             scope 3 (inlined core::ub_checks::check_language_ub::runtime) {
 +             }
 +         }
@@ -22,20 +22,20 @@
           StorageLive(_4);
           _4 = copy _2;
 -         _0 = core::num::<impl i64>::unchecked_shr(move _3, move _4) -> [return: bb1, unwind unreachable];
-+         StorageLive(_6);
 +         StorageLive(_5);
-+         _5 = UbChecks();
-+         switchInt(move _5) -> [0: bb2, otherwise: bb1];
++         StorageLive(_6);
++         _6 = UbChecks();
++         switchInt(copy _6) -> [0: bb2, otherwise: bb1];
       }
   
       bb1: {
-+         _6 = core::num::<impl i64>::unchecked_shr::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
++         _5 = core::num::<impl i64>::unchecked_shr::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
 +     }
 + 
 +     bb2: {
-+         StorageDead(_5);
 +         _0 = ShrUnchecked(copy _3, copy _4);
 +         StorageDead(_6);
++         StorageDead(_5);
           StorageDead(_4);
           StorageDead(_3);
           return;
diff --git a/tests/mir-opt/inline/unchecked_shifts.unchecked_shr_signed_bigger.Inline.panic-unwind.diff b/tests/mir-opt/inline/unchecked_shifts.unchecked_shr_signed_bigger.Inline.panic-unwind.diff
@@ -8,9 +8,9 @@
       let mut _3: i64;
       let mut _4: u32;
 +     scope 1 (inlined core::num::<impl i64>::unchecked_shr) {
-+         let mut _5: bool;
-+         let _6: ();
++         let _5: ();
 +         scope 2 (inlined core::ub_checks::check_language_ub) {
++             let mut _6: bool;
 +             scope 3 (inlined core::ub_checks::check_language_ub::runtime) {
 +             }
 +         }
@@ -22,20 +22,20 @@
           StorageLive(_4);
           _4 = copy _2;
 -         _0 = core::num::<impl i64>::unchecked_shr(move _3, move _4) -> [return: bb1, unwind continue];
-+         StorageLive(_6);
 +         StorageLive(_5);
-+         _5 = UbChecks();
-+         switchInt(move _5) -> [0: bb2, otherwise: bb1];
++         StorageLive(_6);
++         _6 = UbChecks();
++         switchInt(copy _6) -> [0: bb2, otherwise: bb1];
       }
   
       bb1: {
-+         _6 = core::num::<impl i64>::unchecked_shr::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
++         _5 = core::num::<impl i64>::unchecked_shr::precondition_check(copy _4) -> [return: bb2, unwind unreachable];
 +     }
 + 
 +     bb2: {
-+         StorageDead(_5);
 +         _0 = ShrUnchecked(copy _3, copy _4);
 +         StorageDead(_6);
++         StorageDead(_5);
           StorageDead(_4);
           StorageDead(_3);
           return;
diff --git a/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.panic-abort.diff b/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.panic-abort.diff
@@ -10,9 +10,9 @@
 +         scope 2 {
 +         }
 +         scope 3 (inlined unreachable_unchecked) {
-+             let mut _4: bool;
-+             let _5: ();
++             let _4: ();
 +             scope 4 (inlined core::ub_checks::check_language_ub) {
++                 let mut _5: bool;
 +                 scope 5 (inlined core::ub_checks::check_language_ub::runtime) {
 +                 }
 +             }
@@ -24,7 +24,7 @@
           _2 = move _1;
 -         _0 = Option::<T>::unwrap_unchecked(move _2) -> [return: bb1, unwind unreachable];
 +         StorageLive(_3);
-+         StorageLive(_5);
++         StorageLive(_4);
 +         _3 = discriminant(_2);
 +         switchInt(move _3) -> [0: bb2, 1: bb3, otherwise: bb1];
       }
@@ -34,15 +34,15 @@
 +     }
 + 
 +     bb2: {
-+         StorageLive(_4);
-+         _4 = UbChecks();
-+         assume(copy _4);
-+         _5 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
++         StorageLive(_5);
++         _5 = UbChecks();
++         assume(copy _5);
++         _4 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
 +     }
 + 
 +     bb3: {
 +         _0 = move ((_2 as Some).0: T);
-+         StorageDead(_5);
++         StorageDead(_4);
 +         StorageDead(_3);
           StorageDead(_2);
           return;
diff --git a/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.panic-unwind.diff b/tests/mir-opt/inline/unwrap_unchecked.unwrap_unchecked.Inline.panic-unwind.diff
@@ -10,9 +10,9 @@
 +         scope 2 {
 +         }
 +         scope 3 (inlined unreachable_unchecked) {
-+             let mut _4: bool;
-+             let _5: ();
++             let _4: ();
 +             scope 4 (inlined core::ub_checks::check_language_ub) {
++                 let mut _5: bool;
 +                 scope 5 (inlined core::ub_checks::check_language_ub::runtime) {
 +                 }
 +             }
@@ -24,7 +24,7 @@
           _2 = move _1;
 -         _0 = Option::<T>::unwrap_unchecked(move _2) -> [return: bb1, unwind: bb2];
 +         StorageLive(_3);
-+         StorageLive(_5);
++         StorageLive(_4);
 +         _3 = discriminant(_2);
 +         switchInt(move _3) -> [0: bb2, 1: bb3, otherwise: bb1];
       }
@@ -38,15 +38,15 @@
 -     bb2 (cleanup): {
 -         resume;
 +     bb2: {
-+         StorageLive(_4);
-+         _4 = UbChecks();
-+         assume(copy _4);
-+         _5 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
++         StorageLive(_5);
++         _5 = UbChecks();
++         assume(copy _5);
++         _4 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
 +     }
 + 
 +     bb3: {
 +         _0 = move ((_2 as Some).0: T);
-+         StorageDead(_5);
++         StorageDead(_4);
 +         StorageDead(_3);
 +         StorageDead(_2);
 +         return;
diff --git a/tests/mir-opt/instsimplify/ub_check.unwrap_unchecked.InstSimplify-after-simplifycfg.diff b/tests/mir-opt/instsimplify/ub_check.unwrap_unchecked.InstSimplify-after-simplifycfg.diff
@@ -10,9 +10,9 @@
           scope 2 {
           }
           scope 3 (inlined unreachable_unchecked) {
-              let mut _4: bool;
-              let _5: ();
+              let _4: ();
               scope 4 (inlined core::ub_checks::check_language_ub) {
+                  let mut _5: bool;
                   scope 5 (inlined core::ub_checks::check_language_ub::runtime) {
                   }
               }
@@ -23,7 +23,7 @@
           StorageLive(_2);
           _2 = copy _1;
           StorageLive(_3);
-          StorageLive(_5);
+          StorageLive(_4);
           _3 = discriminant(_2);
           switchInt(move _3) -> [0: bb2, 1: bb3, otherwise: bb1];
       }
@@ -33,16 +33,16 @@
       }
   
       bb2: {
-          StorageLive(_4);
--         _4 = UbChecks();
-+         _4 = const false;
-          assume(copy _4);
-          _5 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
+          StorageLive(_5);
+-         _5 = UbChecks();
++         _5 = const false;
+          assume(copy _5);
+          _4 = unreachable_unchecked::precondition_check() -> [return: bb1, unwind unreachable];
       }
   
       bb3: {
           _0 = move ((_2 as Some).0: i32);
-          StorageDead(_5);
+          StorageDead(_4);
           StorageDead(_3);
           StorageDead(_2);
           return;
diff --git a/tests/mir-opt/issues/issue_75439.foo.MatchBranchSimplification.diff b/tests/mir-opt/issues/issue_75439.foo.MatchBranchSimplification.diff
@@ -5,23 +5,18 @@
       debug bytes => _1;
       let mut _0: std::option::Option<[u8; 4]>;
       let _2: [u32; 4];
-      let mut _3: [u8; 16];
-      let mut _5: [u8; 4];
-      let mut _6: u32;
+      let mut _4: [u8; 4];
       scope 1 {
           debug dwords => _2;
           scope 2 {
-              debug ip => _4;
-              let _4: u32;
+              debug ip => _3;
+              let _3: u32;
           }
       }
   
       bb0: {
           StorageLive(_2);
-          StorageLive(_3);
-          _3 = copy _1;
-          _2 = move _3 as [u32; 4] (Transmute);
-          StorageDead(_3);
+          _2 = copy _1 as [u32; 4] (Transmute);
           switchInt(copy _2[0 of 4]) -> [0: bb1, otherwise: bb4];
       }
   
@@ -34,15 +29,10 @@
       }
   
       bb3: {
+          _3 = copy _2[3 of 4];
           StorageLive(_4);
-          _4 = copy _2[3 of 4];
-          StorageLive(_5);
-          StorageLive(_6);
-          _6 = copy _4;
-          _5 = move _6 as [u8; 4] (Transmute);
-          StorageDead(_6);
-          _0 = Option::<[u8; 4]>::Some(move _5);
-          StorageDead(_5);
+          _4 = copy _3 as [u8; 4] (Transmute);
+          _0 = Option::<[u8; 4]>::Some(move _4);
           StorageDead(_4);
           goto -> bb5;
       }
diff --git a/tests/mir-opt/pre-codegen/matchbr.match1.PreCodegen.after.mir b/tests/mir-opt/pre-codegen/matchbr.match1.PreCodegen.after.mir
@@ -0,0 +1,13 @@
+// MIR for `match1` after PreCodegen
+
+fn match1(_1: bool, _2: i32, _3: i32) -> i32 {
+    debug c => _1;
+    debug v1 => _2;
+    debug v2 => _3;
+    let mut _0: i32;
+
+    bb0: {
+        _0 = Sub(copy _2, copy _3);
+        return;
+    }
+}
diff --git a/tests/mir-opt/pre-codegen/matchbr.rs b/tests/mir-opt/pre-codegen/matchbr.rs
@@ -0,0 +1,10 @@
+#![crate_type = "lib"]
+
+// EMIT_MIR matchbr.match1.PreCodegen.after.mir
+pub fn match1(c: bool, v1: i32, v2: i32) -> i32 {
+    // CHECK-LABEL: fn match1(
+    // CHECK: bb0:
+    // CHECK-NEXT: _0 = Sub
+    // CHECK-NEXT: return;
+    if c { v1 - v2 } else { v1 - v2 }
+}
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
@@ -8,6 +8,9 @@
       let mut _3: std::option::Option<T>;
       let mut _4: isize;
       let mut _5: isize;
+-     let mut _7: bool;
+-     let mut _8: u8;
+-     let mut _9: bool;
       scope 1 {
           debug a => _6;
           let _6: u8;
@@ -32,9 +35,7 @@
       }
   
       bb2: {
-          StorageLive(_6);
           _6 = copy (((_1.0: std::option::Option<u8>) as Some).0: u8);
-          StorageDead(_6);
           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
