Fix variable debuginfo being optimized away at mir-opt-level=2

compiler/rustc_mir_transform/src/const_debuginfo.rs

@@ -16,7 +16,7 @@ pub struct ConstDebugInfo;
 
 impl<'tcx> MirPass<'tcx> for ConstDebugInfo {
     fn is_enabled(&self, sess: &rustc_session::Session) -> bool {
-        sess.opts.unstable_opts.unsound_mir_opts && sess.mir_opt_level() > 0
+        sess.mir_opt_level() > 0
     }
 
     fn run_pass(&self, _tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {

compiler/rustc_mir_transform/src/dead_store_elimination.rs

@@ -20,10 +20,12 @@ use rustc_mir_dataflow::Analysis;
 
 /// Performs the optimization on the body
 ///
-/// The `borrowed` set must be a `BitSet` of all the locals that are ever borrowed in this body. It
+/// The `always_live` set must be a `BitSet` of all the locals that are considered always alive and
+/// never eliminated. This should be, at least, the set of locals which are ever borrowed in this
+/// body. It may include other locals as well if necessary. The minimum set of always alive locals
 /// can be generated via the [`borrowed_locals`] function.
-pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitSet<Local>) {
-    let mut live = MaybeTransitiveLiveLocals::new(borrowed)
+pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, always_live: &BitSet<Local>) {
+    let mut live = MaybeTransitiveLiveLocals::new(always_live)
         .into_engine(tcx, body)
         .iterate_to_fixpoint()
         .into_results_cursor(body);
@@ -41,7 +43,7 @@ pub fn eliminate<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>, borrowed: &BitS
                 StatementKind::Assign(box (place, _))
                 | StatementKind::SetDiscriminant { place: box place, .. }
                 | StatementKind::Deinit(box place) => {
-                    if !place.is_indirect() && !borrowed.contains(place.local) {
+                    if !place.is_indirect() && !always_live.contains(place.local) {
                         live.seek_before_primary_effect(loc);
                         if !live.get().contains(place.local) {
                             patch.push(loc);
@@ -80,7 +82,21 @@ impl<'tcx> MirPass<'tcx> for DeadStoreElimination {
     }
 
     fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
-        let borrowed = borrowed_locals(body);
-        eliminate(tcx, body, &borrowed);
+        let mut always_live = borrowed_locals(body);
+
+        // Include any locals which are used by debuginfo unless we're at a high enough MIR opt
+        // level that degrading debuginfo is acceptable.
+        if tcx.sess.mir_opt_level() < 3 {
+            for x in &body.var_debug_info {
+                match x.value {
+                    VarDebugInfoContents::Place(p) => {
+                        always_live.insert(p.local);
+                    }
+                    VarDebugInfoContents::Const(..) => {}
+                }
+            }
+        }
+
+        eliminate(tcx, body, &always_live);
     }
 }

src/test/codegen/debuginfo-constant-locals.rs

@@ -0,0 +1,28 @@
+// compile-flags: -g -O
+
+// Check that simple constant values are preserved in debuginfo across both MIR opts and LLVM opts
+
+#![crate_type = "lib"]
+
+#[no_mangle]
+pub fn check_it() {
+    let a = 1;
+    let b = 42;
+
+    foo(a + b);
+}
+
+#[inline(never)]
+fn foo(x: i32) {
+    std::process::exit(x);
+}
+
+// CHECK-LABEL: @check_it
+// CHECK: call void @llvm.dbg.value(metadata i32 1, metadata ![[a_metadata:[0-9]+]], metadata !DIExpression())
+// CHECK: call void @llvm.dbg.value(metadata i32 42, metadata ![[b_metadata:[0-9]+]], metadata !DIExpression())
+
+// CHECK: ![[a_metadata]] = !DILocalVariable(name: "a"
+// CHECK-SAME: line: 9
+
+// CHECK: ![[b_metadata]] = !DILocalVariable(name: "b"
+// CHECK-SAME: line: 10

src/test/mir-opt/const_prop/optimizes_into_variable.main.SimplifyLocals.after.64bit.mir

@@ -2,26 +2,17 @@
 
 fn main() -> () {
     let mut _0: ();                      // return place in scope 0 at $DIR/optimizes_into_variable.rs:+0:11: +0:11
-    let _1: i32;                         // in scope 0 at $DIR/optimizes_into_variable.rs:+1:9: +1:10
     scope 1 {
-        debug x => _1;                   // in scope 1 at $DIR/optimizes_into_variable.rs:+1:9: +1:10
-        let _2: i32;                     // in scope 1 at $DIR/optimizes_into_variable.rs:+2:9: +2:10
+        debug x => const 4_i32;          // in scope 1 at $DIR/optimizes_into_variable.rs:+1:9: +1:10
         scope 2 {
-            debug y => _2;               // in scope 2 at $DIR/optimizes_into_variable.rs:+2:9: +2:10
-            let _3: u32;                 // in scope 2 at $DIR/optimizes_into_variable.rs:+3:9: +3:10
+            debug y => const 3_i32;      // in scope 2 at $DIR/optimizes_into_variable.rs:+2:9: +2:10
             scope 3 {
-                debug z => _3;           // in scope 3 at $DIR/optimizes_into_variable.rs:+3:9: +3:10
+                debug z => const 42_u32; // in scope 3 at $DIR/optimizes_into_variable.rs:+3:9: +3:10
             }
         }
     }
 
     bb0: {
-        StorageLive(_1);                 // scope 0 at $DIR/optimizes_into_variable.rs:+1:9: +1:10
-        StorageLive(_2);                 // scope 1 at $DIR/optimizes_into_variable.rs:+2:9: +2:10
-        StorageLive(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:+3:9: +3:10
-        StorageDead(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:+4:1: +4:2
-        StorageDead(_2);                 // scope 1 at $DIR/optimizes_into_variable.rs:+4:1: +4:2
-        StorageDead(_1);                 // scope 0 at $DIR/optimizes_into_variable.rs:+4:1: +4:2
         return;                          // scope 0 at $DIR/optimizes_into_variable.rs:+4:2: +4:2
     }
 }

src/test/mir-opt/dead-store-elimination/constant-local-debuginfo.rs

@@ -0,0 +1,14 @@
+// compile-flags: -Zmir-opt-level=2
+// mir-opt-level is fixed at 2 because that's the max level that can be set on stable
+// EMIT_MIR constant_local_debuginfo.main.DeadStoreElimination.diff
+fn main() {
+    let a = 1;
+    let b = 4;
+
+    foo(a + b);
+}
+
+#[inline(never)]
+fn foo(x: i32) {
+    std::process::exit(x);
+}

src/test/mir-opt/dead-store-elimination/constant_local_debuginfo.main.DeadStoreElimination.diff

@@ -0,0 +1,52 @@
+- // MIR for `main` before DeadStoreElimination
++ // MIR for `main` after DeadStoreElimination
+
+  fn main() -> () {
+      let mut _0: ();                      // return place in scope 0 at $DIR/constant-local-debuginfo.rs:+0:11: +0:11
+      let _1: i32;                         // in scope 0 at $DIR/constant-local-debuginfo.rs:+1:9: +1:10
+      let _3: ();                          // in scope 0 at $DIR/constant-local-debuginfo.rs:+4:5: +4:15
+      let mut _4: i32;                     // in scope 0 at $DIR/constant-local-debuginfo.rs:+4:9: +4:14
+      let mut _5: i32;                     // in scope 0 at $DIR/constant-local-debuginfo.rs:+4:9: +4:10
+      let mut _6: i32;                     // in scope 0 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
+      scope 1 {
+          debug a => const 1_i32;          // in scope 1 at $DIR/constant-local-debuginfo.rs:+1:9: +1:10
+          let _2: i32;                     // in scope 1 at $DIR/constant-local-debuginfo.rs:+2:9: +2:10
+          scope 2 {
+              debug b => const 4_i32;      // in scope 2 at $DIR/constant-local-debuginfo.rs:+2:9: +2:10
+          }
+      }
+
+      bb0: {
+          StorageLive(_1);                 // scope 0 at $DIR/constant-local-debuginfo.rs:+1:9: +1:10
+-         _1 = const 1_i32;                // scope 0 at $DIR/constant-local-debuginfo.rs:+1:13: +1:14
++         nop;                             // scope 0 at $DIR/constant-local-debuginfo.rs:+1:13: +1:14
+          StorageLive(_2);                 // scope 1 at $DIR/constant-local-debuginfo.rs:+2:9: +2:10
+-         _2 = const 4_i32;                // scope 1 at $DIR/constant-local-debuginfo.rs:+2:13: +2:14
++         nop;                             // scope 1 at $DIR/constant-local-debuginfo.rs:+2:13: +2:14
+          StorageLive(_3);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:5: +4:15
+          StorageLive(_4);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:9: +4:14
+          StorageLive(_5);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:9: +4:10
+-         _5 = const 1_i32;                // scope 2 at $DIR/constant-local-debuginfo.rs:+4:9: +4:10
++         nop;                             // scope 2 at $DIR/constant-local-debuginfo.rs:+4:9: +4:10
+          StorageLive(_6);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
+-         _6 = const 4_i32;                // scope 2 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
++         nop;                             // scope 2 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
+          _4 = const 5_i32;                // scope 2 at $DIR/constant-local-debuginfo.rs:+4:9: +4:14
+          StorageDead(_6);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
+          StorageDead(_5);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:13: +4:14
+          _3 = foo(move _4) -> bb1;        // scope 2 at $DIR/constant-local-debuginfo.rs:+4:5: +4:15
+                                           // mir::Constant
+                                           // + span: $DIR/constant-local-debuginfo.rs:8:5: 8:8
+                                           // + literal: Const { ty: fn(i32) {foo}, val: Value(<ZST>) }
+      }
+
+      bb1: {
+          StorageDead(_4);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:14: +4:15
+          StorageDead(_3);                 // scope 2 at $DIR/constant-local-debuginfo.rs:+4:15: +4:16
+          nop;                             // scope 0 at $DIR/constant-local-debuginfo.rs:+0:11: +5:2
+          StorageDead(_2);                 // scope 1 at $DIR/constant-local-debuginfo.rs:+5:1: +5:2
+          StorageDead(_1);                 // scope 0 at $DIR/constant-local-debuginfo.rs:+5:1: +5:2
+          return;                          // scope 0 at $DIR/constant-local-debuginfo.rs:+5:2: +5:2
+      }
+  }
+

src/test/mir-opt/dead-store-elimination/cycle.cycle.DeadStoreElimination.diff

@@ -37,25 +37,18 @@
 
       bb3: {
           StorageLive(_6);                 // scope 0 at $DIR/cycle.rs:+4:13: +4:17
--         _6 = _3;                         // scope 0 at $DIR/cycle.rs:+4:20: +4:21
-+         nop;                             // scope 0 at $DIR/cycle.rs:+4:20: +4:21
+          _6 = _3;                         // scope 0 at $DIR/cycle.rs:+4:20: +4:21
           StorageLive(_7);                 // scope 1 at $DIR/cycle.rs:+5:13: +5:14
--         _7 = _2;                         // scope 1 at $DIR/cycle.rs:+5:13: +5:14
--         _3 = move _7;                    // scope 1 at $DIR/cycle.rs:+5:9: +5:14
-+         nop;                             // scope 1 at $DIR/cycle.rs:+5:13: +5:14
-+         nop;                             // scope 1 at $DIR/cycle.rs:+5:9: +5:14
+          _7 = _2;                         // scope 1 at $DIR/cycle.rs:+5:13: +5:14
+          _3 = move _7;                    // scope 1 at $DIR/cycle.rs:+5:9: +5:14
           StorageDead(_7);                 // scope 1 at $DIR/cycle.rs:+5:13: +5:14
           StorageLive(_8);                 // scope 1 at $DIR/cycle.rs:+6:13: +6:14
--         _8 = _1;                         // scope 1 at $DIR/cycle.rs:+6:13: +6:14
--         _2 = move _8;                    // scope 1 at $DIR/cycle.rs:+6:9: +6:14
-+         nop;                             // scope 1 at $DIR/cycle.rs:+6:13: +6:14
-+         nop;                             // scope 1 at $DIR/cycle.rs:+6:9: +6:14
+          _8 = _1;                         // scope 1 at $DIR/cycle.rs:+6:13: +6:14
+          _2 = move _8;                    // scope 1 at $DIR/cycle.rs:+6:9: +6:14
           StorageDead(_8);                 // scope 1 at $DIR/cycle.rs:+6:13: +6:14
           StorageLive(_9);                 // scope 1 at $DIR/cycle.rs:+7:13: +7:17
--         _9 = _6;                         // scope 1 at $DIR/cycle.rs:+7:13: +7:17
--         _1 = move _9;                    // scope 1 at $DIR/cycle.rs:+7:9: +7:17
-+         nop;                             // scope 1 at $DIR/cycle.rs:+7:13: +7:17
-+         nop;                             // scope 1 at $DIR/cycle.rs:+7:9: +7:17
+          _9 = _6;                         // scope 1 at $DIR/cycle.rs:+7:13: +7:17
+          _1 = move _9;                    // scope 1 at $DIR/cycle.rs:+7:9: +7:17
           StorageDead(_9);                 // scope 1 at $DIR/cycle.rs:+7:16: +7:17
 -         _4 = const ();                   // scope 0 at $DIR/cycle.rs:+3:18: +8:6
 +         nop;                             // scope 0 at $DIR/cycle.rs:+3:18: +8:6