Remove MIR assignments to ZST types

compiler/rustc_codegen_ssa/src/mir/analyze.rs

@@ -281,7 +281,18 @@ impl<'mir, 'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> Visitor<'tcx>
                     Some(assignment_location) => {
                         assignment_location.dominates(location, &self.dominators)
                     }
-                    None => false,
+                    None => {
+                        debug!("No first assignment found for {:?}", local);
+                        // We have not seen any assignment to the local yet,
+                        // but before marking not_ssa, check if it is a ZST,
+                        // in which case we don't need to initialize the local.
+                        let ty = self.fx.mir.local_decls[local].ty;
+                        let ty = self.fx.monomorphize(ty);
+
+                        let is_zst = self.fx.cx.layout_of(ty).is_zst();
+                        debug!("is_zst: {}", is_zst);
+                        is_zst
+                    }
                 };
                 if !ssa_read {
                     self.not_ssa(local);

compiler/rustc_mir/src/transform/simplify.rs

@@ -31,10 +31,10 @@ use crate::transform::MirPass;
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_middle::mir::visit::{MutVisitor, MutatingUseContext, PlaceContext, Visitor};
 use rustc_middle::mir::*;
+use rustc_middle::ty::ParamEnv;
 use rustc_middle::ty::TyCtxt;
 use smallvec::SmallVec;
-use std::borrow::Cow;
-use std::convert::TryInto;
+use std::{borrow::Cow, convert::TryInto};
 
 pub struct SimplifyCfg {
     label: String,
@@ -322,7 +322,7 @@ impl<'tcx> MirPass<'tcx> for SimplifyLocals {
         trace!("running SimplifyLocals on {:?}", body.source);
 
         // First, we're going to get a count of *actual* uses for every `Local`.
-        let mut used_locals = UsedLocals::new(body);
+        let mut used_locals = UsedLocals::new(body, tcx);
 
         // Next, we're going to remove any `Local` with zero actual uses. When we remove those
         // `Locals`, we're also going to subtract any uses of other `Locals` from the `used_locals`
@@ -332,7 +332,8 @@ impl<'tcx> MirPass<'tcx> for SimplifyLocals {
         remove_unused_definitions(&mut used_locals, body);
 
         // Finally, we'll actually do the work of shrinking `body.local_decls` and remapping the `Local`s.
-        let map = make_local_map(&mut body.local_decls, &used_locals);
+        let arg_count = body.arg_count.try_into().unwrap();
+        let map = make_local_map(&mut body.local_decls, &used_locals, arg_count);
 
         // Only bother running the `LocalUpdater` if we actually found locals to remove.
         if map.iter().any(Option::is_none) {
@@ -346,54 +347,61 @@ impl<'tcx> MirPass<'tcx> for SimplifyLocals {
 }
 
 /// Construct the mapping while swapping out unused stuff out from the `vec`.
-fn make_local_map<V>(
+fn make_local_map<'tcx, V>(
     local_decls: &mut IndexVec<Local, V>,
-    used_locals: &UsedLocals,
+    used_locals: &UsedLocals<'tcx>,
+    arg_count: u32,
 ) -> IndexVec<Local, Option<Local>> {
-    let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, &*local_decls);
+    let mut map: IndexVec<Local, Option<Local>> = IndexVec::from_elem(None, local_decls);
     let mut used = Local::new(0);
 
     for alive_index in local_decls.indices() {
-        // `is_used` treats the `RETURN_PLACE` and arguments as used.
-        if !used_locals.is_used(alive_index) {
-            continue;
-        }
-
-        map[alive_index] = Some(used);
-        if alive_index != used {
-            local_decls.swap(alive_index, used);
+        // When creating the local map treat the `RETURN_PLACE` and arguments as used.
+        if alive_index.as_u32() <= arg_count || used_locals.is_used(alive_index) {
+            map[alive_index] = Some(used);
+            if alive_index != used {
+                local_decls.swap(alive_index, used);
+            }
+            used.increment_by(1);
         }
-        used.increment_by(1);
     }
     local_decls.truncate(used.index());
     map
 }
 
 /// Keeps track of used & unused locals.
-struct UsedLocals {
+struct UsedLocals<'tcx> {
     increment: bool,
-    arg_count: u32,
     use_count: IndexVec<Local, u32>,
+    is_static: bool,
+    local_decls: IndexVec<Local, LocalDecl<'tcx>>,
+    param_env: ParamEnv<'tcx>,
+    tcx: TyCtxt<'tcx>,
 }
 
-impl UsedLocals {
+impl UsedLocals<'tcx> {
     /// Determines which locals are used & unused in the given body.
-    fn new(body: &Body<'_>) -> Self {
+    fn new(body: &Body<'tcx>, tcx: TyCtxt<'tcx>) -> Self {
+        let def_id = body.source.def_id();
+        let is_static = tcx.is_static(def_id);
+        let param_env = tcx.param_env(def_id);
+        let local_decls = body.local_decls.clone();
         let mut this = Self {
             increment: true,
-            arg_count: body.arg_count.try_into().unwrap(),
             use_count: IndexVec::from_elem(0, &body.local_decls),
+            is_static,
+            local_decls,
+            param_env,
+            tcx,
         };
         this.visit_body(body);
         this
     }
 
     /// Checks if local is used.
-    ///
-    /// Return place and arguments are always considered used.
     fn is_used(&self, local: Local) -> bool {
         trace!("is_used({:?}): use_count: {:?}", local, self.use_count[local]);
-        local.as_u32() <= self.arg_count || self.use_count[local] != 0
+        self.use_count[local] != 0
     }
 
     /// Updates the use counts to reflect the removal of given statement.
@@ -422,7 +430,7 @@ impl UsedLocals {
     }
 }
 
-impl Visitor<'_> for UsedLocals {
+impl Visitor<'tcx> for UsedLocals<'tcx> {
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
         match statement.kind {
             StatementKind::LlvmInlineAsm(..)
@@ -448,7 +456,23 @@ impl Visitor<'_> for UsedLocals {
         }
     }
 
-    fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) {
+    fn visit_local(&mut self, local: &Local, ctx: PlaceContext, _location: Location) {
+        debug!("local: {:?} is_static: {:?}, ctx: {:?}", local, self.is_static, ctx);
+        // Do not count a local as used in `_local = <rhs>` if RHS is a ZST.
+        let store = matches!(ctx, PlaceContext::MutatingUse(MutatingUseContext::Store));
+        // Do not count _0 as a used in `return;` if it is a ZST.
+        let return_place = *local == RETURN_PLACE
+            && matches!(ctx, PlaceContext::NonMutatingUse(visit::NonMutatingUseContext::Move));
+        if !self.is_static && (store || return_place) {
+            let ty = self.local_decls[*local].ty;
+            let param_env_and = self.param_env.and(ty);
+            if let Ok(layout) = self.tcx.layout_of(param_env_and) {
+                debug!("layout.is_zst: {:?}", layout.is_zst());
+                if layout.is_zst() {
+                    return;
+                }
+            }
+        }
         if self.increment {
             self.use_count[*local] += 1;
         } else {
@@ -459,7 +483,10 @@ impl Visitor<'_> for UsedLocals {
 }
 
 /// Removes unused definitions. Updates the used locals to reflect the changes made.
-fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, body: &mut Body<'tcx>) {
+fn remove_unused_definitions<'a, 'tcx>(
+    used_locals: &'a mut UsedLocals<'tcx>,
+    body: &mut Body<'tcx>,
+) {
     // The use counts are updated as we remove the statements. A local might become unused
     // during the retain operation, leading to a temporary inconsistency (storage statements or
     // definitions referencing the local might remain). For correctness it is crucial that this

src/test/mir-opt/const_prop/control_flow_simplification.hello.PreCodegen.before.mir

@@ -4,7 +4,6 @@ fn hello() -> () {
     let mut _0: ();                      // return place in scope 0 at $DIR/control-flow-simplification.rs:11:14: 11:14
 
     bb0: {
-        _0 = const ();                   // scope 0 at $DIR/control-flow-simplification.rs:14:6: 14:6
         return;                          // scope 0 at $DIR/control-flow-simplification.rs:15:2: 15:2
     }
 }

src/test/mir-opt/const_prop/optimizes_into_variable.main.SimplifyLocals.after.32bit.mir

@@ -22,7 +22,6 @@ fn main() -> () {
         _2 = const 3_i32;                // scope 1 at $DIR/optimizes_into_variable.rs:13:13: 13:34
         StorageLive(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:14:9: 14:10
         _3 = const 42_u32;               // scope 2 at $DIR/optimizes_into_variable.rs:14:13: 14:38
-        _0 = const ();                   // scope 0 at $DIR/optimizes_into_variable.rs:11:11: 15:2
         StorageDead(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:15:1: 15:2
         StorageDead(_2);                 // scope 1 at $DIR/optimizes_into_variable.rs:15:1: 15:2
         StorageDead(_1);                 // scope 0 at $DIR/optimizes_into_variable.rs:15:1: 15:2

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

@@ -22,7 +22,6 @@ fn main() -> () {
         _2 = const 3_i32;                // scope 1 at $DIR/optimizes_into_variable.rs:13:13: 13:34
         StorageLive(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:14:9: 14:10
         _3 = const 42_u32;               // scope 2 at $DIR/optimizes_into_variable.rs:14:13: 14:38
-        _0 = const ();                   // scope 0 at $DIR/optimizes_into_variable.rs:11:11: 15:2
         StorageDead(_3);                 // scope 2 at $DIR/optimizes_into_variable.rs:15:1: 15:2
         StorageDead(_2);                 // scope 1 at $DIR/optimizes_into_variable.rs:15:1: 15:2
         StorageDead(_1);                 // scope 0 at $DIR/optimizes_into_variable.rs:15:1: 15:2

src/test/mir-opt/dest-prop/cycle.main.DestinationPropagation.diff

@@ -56,7 +56,6 @@
           StorageLive(_6);                 // scope 3 at $DIR/cycle.rs:14:10: 14:11
 -         _6 = _1;                         // scope 3 at $DIR/cycle.rs:14:10: 14:11
 +         _6 = _4;                         // scope 3 at $DIR/cycle.rs:14:10: 14:11
-          _5 = const ();                   // scope 4 at $DIR/cycle.rs:14:5: 14:12
           StorageDead(_6);                 // scope 3 at $DIR/cycle.rs:14:11: 14:12
           StorageDead(_5);                 // scope 3 at $DIR/cycle.rs:14:12: 14:13
           _0 = const ();                   // scope 0 at $DIR/cycle.rs:8:11: 15:2

src/test/mir-opt/dest-prop/union.main.DestinationPropagation.diff

@@ -31,7 +31,6 @@
           StorageLive(_3);                 // scope 1 at $DIR/union.rs:15:5: 15:27
           StorageLive(_4);                 // scope 1 at $DIR/union.rs:15:10: 15:26
           _4 = (_1.0: u32);                // scope 2 at $DIR/union.rs:15:19: 15:24
-          _3 = const ();                   // scope 3 at $DIR/union.rs:15:5: 15:27
           StorageDead(_4);                 // scope 1 at $DIR/union.rs:15:26: 15:27
           StorageDead(_3);                 // scope 1 at $DIR/union.rs:15:27: 15:28
           _0 = const ();                   // scope 0 at $DIR/union.rs:8:11: 16:2

src/test/mir-opt/inline/inline_compatibility.inlined_no_sanitize.Inline.diff

@@ -16,7 +16,6 @@
 -     }
 - 
 -     bb1: {
-+         _1 = const ();                   // scope 1 at $DIR/inline-compatibility.rs:24:5: 24:18
           StorageDead(_1);                 // scope 0 at $DIR/inline-compatibility.rs:24:18: 24:19
           _0 = const ();                   // scope 0 at $DIR/inline-compatibility.rs:23:37: 25:2
           return;                          // scope 0 at $DIR/inline-compatibility.rs:25:2: 25:2

src/test/mir-opt/inline/inline_compatibility.inlined_target_feature.Inline.diff

@@ -16,7 +16,6 @@
 -     }
 - 
 -     bb1: {
-+         _1 = const ();                   // scope 1 at $DIR/inline-compatibility.rs:13:5: 13:21
           StorageDead(_1);                 // scope 0 at $DIR/inline-compatibility.rs:13:21: 13:22
           _0 = const ();                   // scope 0 at $DIR/inline-compatibility.rs:12:40: 14:2
           return;                          // scope 0 at $DIR/inline-compatibility.rs:14:2: 14:2

src/test/mir-opt/inline/inline_cycle.two.Inline.diff

@@ -37,7 +37,6 @@
 +         StorageDead(_5);                 // scope 1 at $DIR/inline-cycle.rs:49:5: 49:12
 +         StorageDead(_4);                 // scope 1 at $DIR/inline-cycle.rs:49:5: 49:12
 +         StorageDead(_3);                 // scope 1 at $DIR/inline-cycle.rs:49:5: 49:12
-+         _1 = const ();                   // scope 1 at $DIR/inline-cycle.rs:49:5: 49:12
 +         StorageDead(_2);                 // scope 0 at $DIR/inline-cycle.rs:49:5: 49:12
           StorageDead(_1);                 // scope 0 at $DIR/inline-cycle.rs:49:12: 49:13
           _0 = const ();                   // scope 0 at $DIR/inline-cycle.rs:48:10: 50:2

src/test/mir-opt/inline/inline_options.main.Inline.after.mir

@@ -48,7 +48,6 @@ fn main() -> () {
 
     bb4: {
         StorageDead(_5);                 // scope 1 at $DIR/inline-options.rs:10:5: 10:21
-        _2 = const ();                   // scope 1 at $DIR/inline-options.rs:10:5: 10:21
         StorageDead(_2);                 // scope 0 at $DIR/inline-options.rs:10:21: 10:22
         _0 = const ();                   // scope 0 at $DIR/inline-options.rs:8:11: 11:2
         return;                          // scope 0 at $DIR/inline-options.rs:11:2: 11:2

src/test/mir-opt/inline/issue_76997_inline_scopes_parenting.main.Inline.after.mir

@@ -29,7 +29,6 @@ fn main() -> () {
         _5 = move (_3.0: ());            // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         StorageLive(_6);                 // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         _6 = const ();                   // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
-        _0 = const ();                   // scope 3 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         StorageDead(_6);                 // scope 2 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         StorageDead(_5);                 // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         StorageDead(_4);                 // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:9: 6:10

src/test/mir-opt/issue_73223.main.PreCodegen.32bit.diff

@@ -138,7 +138,6 @@
           StorageDead(_9);                 // scope 4 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           StorageDead(_8);                 // scope 3 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           StorageDead(_7);                 // scope 3 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
-          _0 = const ();                   // scope 0 at $DIR/issue-73223.rs:1:11: 9:2
           StorageDead(_1);                 // scope 0 at $DIR/issue-73223.rs:9:1: 9:2
           return;                          // scope 0 at $DIR/issue-73223.rs:9:2: 9:2
       }

src/test/mir-opt/issue_73223.main.PreCodegen.64bit.diff

@@ -138,7 +138,6 @@
           StorageDead(_9);                 // scope 4 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           StorageDead(_8);                 // scope 3 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
           StorageDead(_7);                 // scope 3 at $SRC_DIR/core/src/macros/mod.rs:LL:COL
-          _0 = const ();                   // scope 0 at $DIR/issue-73223.rs:1:11: 9:2
           StorageDead(_1);                 // scope 0 at $DIR/issue-73223.rs:9:1: 9:2
           return;                          // scope 0 at $DIR/issue-73223.rs:9:2: 9:2
       }

src/test/mir-opt/lower_intrinsics.f_u64.PreCodegen.before.mir

@@ -25,7 +25,6 @@ fn f_u64() -> () {
     bb1: {
         StorageDead(_3);                 // scope 1 at $DIR/lower_intrinsics.rs:35:5: 35:21
         StorageDead(_2);                 // scope 1 at $DIR/lower_intrinsics.rs:35:5: 35:21
-        _0 = const ();                   // scope 0 at $DIR/lower_intrinsics.rs:34:16: 36:2
         return;                          // scope 0 at $DIR/lower_intrinsics.rs:36:2: 36:2
     }
 }

src/test/mir-opt/lower_intrinsics.f_unit.PreCodegen.before.mir

@@ -22,7 +22,6 @@ fn f_unit() -> () {
     bb1: {
         StorageDead(_2);                 // scope 1 at $DIR/lower_intrinsics.rs:29:5: 29:19
         StorageDead(_1);                 // scope 0 at $DIR/lower_intrinsics.rs:29:18: 29:19
-        _0 = const ();                   // scope 0 at $DIR/lower_intrinsics.rs:28:17: 30:2
         return;                          // scope 0 at $DIR/lower_intrinsics.rs:30:2: 30:2
     }
 }

src/test/mir-opt/multiple_return_terminators.test.MultipleReturnTerminators.diff

@@ -13,12 +13,10 @@
       }
 
       bb1: {
-          _0 = const ();                   // scope 0 at $DIR/multiple_return_terminators.rs:5:10: 7:6
           goto -> bb3;                     // scope 0 at $DIR/multiple_return_terminators.rs:5:5: 9:6
       }
 
       bb2: {
-          _0 = const ();                   // scope 0 at $DIR/multiple_return_terminators.rs:7:12: 9:6
           goto -> bb3;                     // scope 0 at $DIR/multiple_return_terminators.rs:5:5: 9:6
       }
 

src/test/mir-opt/remove_unneeded_drops.cannot_opt_generic.RemoveUnneededDrops.diff

@@ -14,7 +14,6 @@
           StorageLive(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:21:5: 21:12
           StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:21:10: 21:11
           _3 = move _1;                    // scope 0 at $DIR/remove_unneeded_drops.rs:21:10: 21:11
-          _2 = const ();                   // scope 1 at $DIR/remove_unneeded_drops.rs:21:5: 21:12
           drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $DIR/remove_unneeded_drops.rs:21:5: 21:12
       }
 

src/test/mir-opt/remove_unneeded_drops.dont_opt.RemoveUnneededDrops.diff

@@ -14,7 +14,6 @@
           StorageLive(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:9:5: 9:12
           StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:9:10: 9:11
           _3 = move _1;                    // scope 0 at $DIR/remove_unneeded_drops.rs:9:10: 9:11
-          _2 = const ();                   // scope 1 at $DIR/remove_unneeded_drops.rs:9:5: 9:12
           drop(_3) -> [return: bb2, unwind: bb1]; // scope 1 at $DIR/remove_unneeded_drops.rs:9:5: 9:12
       }
 

src/test/mir-opt/remove_unneeded_drops.opt.RemoveUnneededDrops.diff

@@ -14,7 +14,6 @@
           StorageLive(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:4:5: 4:12
           StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:4:10: 4:11
           _3 = _1;                         // scope 0 at $DIR/remove_unneeded_drops.rs:4:10: 4:11
-          _2 = const ();                   // scope 1 at $DIR/remove_unneeded_drops.rs:4:5: 4:12
 -         drop(_3) -> bb1;                 // scope 1 at $DIR/remove_unneeded_drops.rs:4:5: 4:12
 -     }
 - 

src/test/mir-opt/remove_unneeded_drops.opt_generic_copy.RemoveUnneededDrops.diff

@@ -14,7 +14,6 @@
           StorageLive(_2);                 // scope 0 at $DIR/remove_unneeded_drops.rs:14:5: 14:12
           StorageLive(_3);                 // scope 0 at $DIR/remove_unneeded_drops.rs:14:10: 14:11
           _3 = _1;                         // scope 0 at $DIR/remove_unneeded_drops.rs:14:10: 14:11
-          _2 = const ();                   // scope 1 at $DIR/remove_unneeded_drops.rs:14:5: 14:12
 -         drop(_3) -> bb1;                 // scope 1 at $DIR/remove_unneeded_drops.rs:14:5: 14:12
 -     }
 - 

src/test/mir-opt/simplify_locals.c.SimplifyLocals.diff

@@ -20,7 +20,7 @@
 -         _3 = &_1;                        // scope 1 at $DIR/simplify-locals.rs:16:20: 16:26
 -         _2 = move _3 as &[u8] (Pointer(Unsize)); // scope 1 at $DIR/simplify-locals.rs:16:20: 16:26
 -         StorageDead(_2);                 // scope 1 at $DIR/simplify-locals.rs:16:26: 16:27
-          _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:13:8: 17:2
+-         _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:13:8: 17:2
           StorageDead(_1);                 // scope 0 at $DIR/simplify-locals.rs:17:1: 17:2
           return;                          // scope 0 at $DIR/simplify-locals.rs:17:2: 17:2
       }

src/test/mir-opt/simplify_locals.d1.SimplifyLocals.diff

@@ -11,7 +11,7 @@
 -         StorageLive(_1);                 // scope 0 at $DIR/simplify-locals.rs:22:13: 22:17
 -         discriminant(_1) = 0;            // scope 0 at $DIR/simplify-locals.rs:22:13: 22:17
 -         StorageDead(_1);                 // scope 0 at $DIR/simplify-locals.rs:22:17: 22:18
-          _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:20:9: 23:2
+-         _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:20:9: 23:2
           return;                          // scope 0 at $DIR/simplify-locals.rs:23:2: 23:2
       }
   }

src/test/mir-opt/simplify_locals.d2.SimplifyLocals.diff

@@ -31,7 +31,7 @@
 -                                          // + literal: Const { ty: E, val: Value(Scalar(0x01)) }
 -         StorageDead(_1);                 // scope 0 at $DIR/simplify-locals.rs:28:25: 28:26
 -         StorageDead(_2);                 // scope 0 at $DIR/simplify-locals.rs:28:26: 28:27
-          _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:26:9: 29:2
+-         _0 = const ();                   // scope 0 at $DIR/simplify-locals.rs:26:9: 29:2
           return;                          // scope 0 at $DIR/simplify-locals.rs:29:2: 29:2
       }
   }