Auto merge of #112040 - cjgillot:separate-const-switch, r=oli-obk

Enable ConstGoto and SeparateConstSwitch passes by default

These 2 passes implement a limited form of jump-threading.
Filing this PR to see if enabling them would be lighter than https://github.com/rust-lang/rust/pull/107009.

6 files changed, 62 insertions, 84 deletions

diff --git a/compiler/rustc_mir_transform/src/const_goto.rs b/compiler/rustc_mir_transform/src/const_goto.rs
index da101ca7ad2..024bea62098 100644
--- a/compiler/rustc_mir_transform/src/const_goto.rs
+++ b/compiler/rustc_mir_transform/src/const_goto.rs
@@ -28,7 +28,7 @@ pub struct ConstGoto;
 
 impl<'tcx> MirPass<'tcx> for ConstGoto {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 4
+        sess.mir_opt_level() >= 2
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
diff --git a/compiler/rustc_mir_transform/src/const_prop.rs b/compiler/rustc_mir_transform/src/const_prop.rs
index 1ba1951afde..f66a7a42ff8 100644
--- a/compiler/rustc_mir_transform/src/const_prop.rs
+++ b/compiler/rustc_mir_transform/src/const_prop.rs
@@ -103,7 +103,14 @@ impl<'tcx> MirPass<'tcx> for ConstProp {
         // That would require a uniform one-def no-mutation analysis
         // and RPO (or recursing when needing the value of a local).
         let mut optimization_finder = ConstPropagator::new(body, dummy_body, tcx);
-        optimization_finder.visit_body(body);
+
+        // Traverse the body in reverse post-order, to ensure that `FullConstProp` locals are
+        // assigned before being read.
+        let postorder = body.basic_blocks.postorder().to_vec();
+        for bb in postorder.into_iter().rev() {
+            let data = &mut body.basic_blocks.as_mut_preserves_cfg()[bb];
+            optimization_finder.visit_basic_block_data(bb, data);
+        }
 
         trace!("ConstProp done for {:?}", def_id);
     }
@@ -789,12 +796,6 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
         self.tcx
     }
 
-    fn visit_body(&mut self, body: &mut Body<'tcx>) {
-        for (bb, data) in body.basic_blocks.as_mut_preserves_cfg().iter_enumerated_mut() {
-            self.visit_basic_block_data(bb, data);
-        }
-    }
-
     fn visit_operand(&mut self, operand: &mut Operand<'tcx>, location: Location) {
         self.super_operand(operand, location);
 
@@ -885,14 +886,23 @@ impl<'tcx> MutVisitor<'tcx> for ConstPropagator<'_, 'tcx> {
                 }
             }
             StatementKind::StorageLive(local) => {
-                let frame = self.ecx.frame_mut();
-                frame.locals[local].value =
-                    LocalValue::Live(interpret::Operand::Immediate(interpret::Immediate::Uninit));
-            }
-            StatementKind::StorageDead(local) => {
-                let frame = self.ecx.frame_mut();
-                frame.locals[local].value = LocalValue::Dead;
+                Self::remove_const(&mut self.ecx, local);
             }
+            // We do not need to mark dead locals as such. For `FullConstProp` locals,
+            // this allows to propagate the single assigned value in this case:
+            // ```
+            // let x = SOME_CONST;
+            // if a {
+            //   f(copy x);
+            //   StorageDead(x);
+            // } else {
+            //   g(copy x);
+            //   StorageDead(x);
+            // }
+            // ```
+            //
+            // This may propagate a constant where the local would be uninit or dead.
+            // In both cases, this does not matter, as those reads would be UB anyway.
             _ => {}
         }
     }
diff --git a/compiler/rustc_mir_transform/src/lib.rs b/compiler/rustc_mir_transform/src/lib.rs
index 54c138b6fbd..7d9f6c38e36 100644
--- a/compiler/rustc_mir_transform/src/lib.rs
+++ b/compiler/rustc_mir_transform/src/lib.rs
@@ -559,10 +559,13 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
             // inst combine is after MatchBranchSimplification to clean up Ne(_1, false)
             &multiple_return_terminators::MultipleReturnTerminators,
             &instsimplify::InstSimplify,
-            &separate_const_switch::SeparateConstSwitch,
             &simplify::SimplifyLocals::BeforeConstProp,
             &copy_prop::CopyProp,
             &ref_prop::ReferencePropagation,
+            // Perform `SeparateConstSwitch` after SSA-based analyses, as cloning blocks may
+            // destroy the SSA property. It should still happen before const-propagation, so the
+            // latter pass will leverage the created opportunities.
+            &separate_const_switch::SeparateConstSwitch,
             &const_prop::ConstProp,
             &dataflow_const_prop::DataflowConstProp,
             //
diff --git a/compiler/rustc_mir_transform/src/separate_const_switch.rs b/compiler/rustc_mir_transform/src/separate_const_switch.rs
index 2479856b727..f35a5fb4276 100644
--- a/compiler/rustc_mir_transform/src/separate_const_switch.rs
+++ b/compiler/rustc_mir_transform/src/separate_const_switch.rs
@@ -46,7 +46,7 @@ pub struct SeparateConstSwitch;
 
 impl<'tcx> MirPass<'tcx> for SeparateConstSwitch {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.mir_opt_level() >= 4
+        sess.mir_opt_level() >= 2
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
diff --git a/tests/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff b/tests/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff
index bdf1de468b3..7f0e50a23f9 100644
--- a/tests/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff
+++ b/tests/mir-opt/separate_const_switch.identity.SeparateConstSwitch.diff
@@ -9,70 +9,61 @@
       let mut _4: std::result::Result<i32, i32>; // in scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:9
       let mut _5: isize;                   // in scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
       let _6: std::result::Result<std::convert::Infallible, i32>; // in scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-      let mut _7: !;                       // in scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-      let mut _8: std::result::Result<std::convert::Infallible, i32>; // in scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-      let _9: i32;                         // in scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
+      let mut _7: std::result::Result<std::convert::Infallible, i32>; // in scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
+      let _8: i32;                         // in scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
       scope 1 {
           debug residual => _6;            // in scope 1 at $DIR/separate_const_switch.rs:+1:9: +1:10
           scope 2 {
               scope 8 (inlined #[track_caller] <Result<i32, i32> as FromResidual<Result<Infallible, i32>>>::from_residual) { // at $DIR/separate_const_switch.rs:25:8: 25:10
-                  debug residual => _8;    // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
-                  let _14: i32;            // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
-                  let mut _15: i32;        // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
+                  debug residual => _6;    // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
+                  let _13: i32;            // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
+                  let mut _14: i32;        // in scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
                   scope 9 {
-                      debug e => _14;      // in scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
+                      debug e => _13;      // in scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
                       scope 10 (inlined <i32 as From<i32>>::from) { // at $SRC_DIR/core/src/result.rs:LL:COL
-                          debug t => _14;  // in scope 10 at $SRC_DIR/core/src/convert/mod.rs:LL:COL
+                          debug t => _13;  // in scope 10 at $SRC_DIR/core/src/convert/mod.rs:LL:COL
                       }
                   }
               }
           }
       }
       scope 3 {
-          debug val => _9;                 // in scope 3 at $DIR/separate_const_switch.rs:+1:8: +1:10
+          debug val => _8;                 // in scope 3 at $DIR/separate_const_switch.rs:+1:8: +1:10
           scope 4 {
           }
       }
       scope 5 (inlined <Result<i32, i32> as Try>::branch) { // at $DIR/separate_const_switch.rs:25:8: 25:10
-          debug self => _4;                // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          let mut _10: isize;              // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          debug self => _1;                // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          let mut _9: isize;               // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          let _10: i32;                    // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
           let _11: i32;                    // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          let _12: i32;                    // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          let mut _13: std::result::Result<std::convert::Infallible, i32>; // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          let mut _12: std::result::Result<std::convert::Infallible, i32>; // in scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
           scope 6 {
-              debug v => _11;              // in scope 6 at $SRC_DIR/core/src/result.rs:LL:COL
+              debug v => _10;              // in scope 6 at $SRC_DIR/core/src/result.rs:LL:COL
           }
           scope 7 {
-              debug e => _12;              // in scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
+              debug e => _11;              // in scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
           }
       }
   
       bb0: {
-          StorageLive(_2);                 // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
           StorageLive(_3);                 // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          StorageLive(_4);                 // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:9
-          _4 = _1;                         // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:9
+          StorageLive(_10);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
           StorageLive(_11);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          StorageLive(_12);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          _10 = discriminant(_4);          // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          switchInt(move _10) -> [0: bb7, 1: bb5, otherwise: bb6]; // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          _9 = discriminant(_1);           // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          switchInt(move _9) -> [0: bb7, 1: bb5, otherwise: bb6]; // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
       }
   
       bb1: {
-          StorageDead(_12);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
           StorageDead(_11);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          StorageDead(_4);                 // scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
+          StorageDead(_10);                // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
           _5 = discriminant(_3);           // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
           switchInt(move _5) -> [0: bb2, 1: bb4, otherwise: bb3]; // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
       }
   
       bb2: {
-          StorageLive(_9);                 // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          _9 = ((_3 as Continue).0: i32);  // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          _2 = _9;                         // scope 4 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          StorageDead(_9);                 // scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          _0 = Result::<i32, i32>::Ok(move _2); // scope 0 at $DIR/separate_const_switch.rs:+1:5: +1:11
-          StorageDead(_2);                 // scope 0 at $DIR/separate_const_switch.rs:+1:10: +1:11
+          _8 = ((_3 as Continue).0: i32);  // scope 0 at $DIR/separate_const_switch.rs:+1:8: +1:10
+          _0 = Result::<i32, i32>::Ok(_8); // scope 0 at $DIR/separate_const_switch.rs:+1:5: +1:11
           StorageDead(_3);                 // scope 0 at $DIR/separate_const_switch.rs:+2:1: +2:2
           return;                          // scope 0 at $DIR/separate_const_switch.rs:+2:2: +2:2
       }
@@ -82,30 +73,19 @@
       }
   
       bb4: {
-          StorageLive(_6);                 // scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
           _6 = ((_3 as Break).0: std::result::Result<std::convert::Infallible, i32>); // scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          StorageLive(_8);                 // scope 2 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          _8 = _6;                         // scope 2 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          StorageLive(_14);                // scope 2 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          _14 = move ((_8 as Err).0: i32); // scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
-          StorageLive(_15);                // scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
-          _15 = move _14;                  // scope 10 at $SRC_DIR/core/src/convert/mod.rs:LL:COL
-          _0 = Result::<i32, i32>::Err(move _15); // scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
-          StorageDead(_15);                // scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
-          StorageDead(_14);                // scope 2 at $DIR/separate_const_switch.rs:+1:8: +1:10
-          StorageDead(_8);                 // scope 2 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          StorageDead(_6);                 // scope 0 at $DIR/separate_const_switch.rs:+1:9: +1:10
-          StorageDead(_2);                 // scope 0 at $DIR/separate_const_switch.rs:+1:10: +1:11
+          _13 = ((_6 as Err).0: i32);      // scope 8 at $SRC_DIR/core/src/result.rs:LL:COL
+          _0 = Result::<i32, i32>::Err(move _13); // scope 9 at $SRC_DIR/core/src/result.rs:LL:COL
           StorageDead(_3);                 // scope 0 at $DIR/separate_const_switch.rs:+2:1: +2:2
           return;                          // scope 0 at $DIR/separate_const_switch.rs:+2:2: +2:2
       }
   
       bb5: {
-          _12 = move ((_4 as Err).0: i32); // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          StorageLive(_13);                // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
-          _13 = Result::<Infallible, i32>::Err(move _12); // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
-          _3 = ControlFlow::<Result<Infallible, i32>, i32>::Break(move _13); // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
-          StorageDead(_13);                // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
+          _11 = ((_1 as Err).0: i32);      // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          StorageLive(_12);                // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
+          _12 = Result::<Infallible, i32>::Err(move _11); // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
+          _3 = ControlFlow::<Result<Infallible, i32>, i32>::Break(move _12); // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
+          StorageDead(_12);                // scope 7 at $SRC_DIR/core/src/result.rs:LL:COL
           goto -> bb1;                     // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
       }
   
@@ -114,8 +94,8 @@
       }
   
       bb7: {
-          _11 = move ((_4 as Ok).0: i32);  // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
-          _3 = ControlFlow::<Result<Infallible, i32>, i32>::Continue(move _11); // scope 6 at $SRC_DIR/core/src/result.rs:LL:COL
+          _10 = ((_1 as Ok).0: i32);       // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
+          _3 = ControlFlow::<Result<Infallible, i32>, i32>::Continue(move _10); // scope 6 at $SRC_DIR/core/src/result.rs:LL:COL
           goto -> bb1;                     // scope 5 at $SRC_DIR/core/src/result.rs:LL:COL
       }
   }
diff --git a/tests/mir-opt/separate_const_switch.too_complex.SeparateConstSwitch.diff b/tests/mir-opt/separate_const_switch.too_complex.SeparateConstSwitch.diff
index b5e0a66d83f..f86a96dec41 100644
--- a/tests/mir-opt/separate_const_switch.too_complex.SeparateConstSwitch.diff
+++ b/tests/mir-opt/separate_const_switch.too_complex.SeparateConstSwitch.diff
@@ -34,13 +34,8 @@
       }
   
       bb1: {
-          StorageLive(_6);                 // scope 0 at $DIR/separate_const_switch.rs:+8:17: +8:18
           _6 = ((_1 as Err).0: usize);     // scope 0 at $DIR/separate_const_switch.rs:+8:17: +8:18
-          StorageLive(_7);                 // scope 2 at $DIR/separate_const_switch.rs:+8:42: +8:43
-          _7 = _6;                         // scope 2 at $DIR/separate_const_switch.rs:+8:42: +8:43
-          _2 = ControlFlow::<usize, i32>::Break(move _7); // scope 2 at $DIR/separate_const_switch.rs:+8:23: +8:44
-          StorageDead(_7);                 // scope 2 at $DIR/separate_const_switch.rs:+8:43: +8:44
-          StorageDead(_6);                 // scope 0 at $DIR/separate_const_switch.rs:+8:43: +8:44
+          _2 = ControlFlow::<usize, i32>::Break(_6); // scope 2 at $DIR/separate_const_switch.rs:+8:23: +8:44
           goto -> bb4;                     // scope 0 at $DIR/separate_const_switch.rs:+8:43: +8:44
       }
   
@@ -49,13 +44,8 @@
       }
   
       bb3: {
-          StorageLive(_4);                 // scope 0 at $DIR/separate_const_switch.rs:+7:16: +7:17
           _4 = ((_1 as Ok).0: i32);        // scope 0 at $DIR/separate_const_switch.rs:+7:16: +7:17
-          StorageLive(_5);                 // scope 1 at $DIR/separate_const_switch.rs:+7:44: +7:45
-          _5 = _4;                         // scope 1 at $DIR/separate_const_switch.rs:+7:44: +7:45
-          _2 = ControlFlow::<usize, i32>::Continue(move _5); // scope 1 at $DIR/separate_const_switch.rs:+7:22: +7:46
-          StorageDead(_5);                 // scope 1 at $DIR/separate_const_switch.rs:+7:45: +7:46
-          StorageDead(_4);                 // scope 0 at $DIR/separate_const_switch.rs:+7:45: +7:46
+          _2 = ControlFlow::<usize, i32>::Continue(_4); // scope 1 at $DIR/separate_const_switch.rs:+7:22: +7:46
           goto -> bb4;                     // scope 0 at $DIR/separate_const_switch.rs:+7:45: +7:46
       }
   
@@ -73,13 +63,8 @@
       }
   
       bb6: {
-          StorageLive(_9);                 // scope 0 at $DIR/separate_const_switch.rs:+11:31: +11:32
           _9 = ((_2 as Continue).0: i32);  // scope 0 at $DIR/separate_const_switch.rs:+11:31: +11:32
-          StorageLive(_10);                // scope 3 at $DIR/separate_const_switch.rs:+11:42: +11:43
-          _10 = _9;                        // scope 3 at $DIR/separate_const_switch.rs:+11:42: +11:43
-          _0 = Option::<i32>::Some(move _10); // scope 3 at $DIR/separate_const_switch.rs:+11:37: +11:44
-          StorageDead(_10);                // scope 3 at $DIR/separate_const_switch.rs:+11:43: +11:44
-          StorageDead(_9);                 // scope 0 at $DIR/separate_const_switch.rs:+11:43: +11:44
+          _0 = Option::<i32>::Some(_9);    // scope 3 at $DIR/separate_const_switch.rs:+11:37: +11:44
           goto -> bb7;                     // scope 0 at $DIR/separate_const_switch.rs:+11:43: +11:44
       }