Remove assignments to ZST places instead of marking ZST return place as unused

compiler/rustc_mir/src/transform/mod.rs

@@ -44,6 +44,7 @@ pub mod promote_consts;
 pub mod remove_noop_landing_pads;
 pub mod remove_storage_markers;
 pub mod remove_unneeded_drops;
+pub mod remove_zsts;
 pub mod required_consts;
 pub mod rustc_peek;
 pub mod simplify;
@@ -494,6 +495,7 @@ fn run_optimization_passes<'tcx>(tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
     // The main optimizations that we do on MIR.
     let optimizations: &[&dyn MirPass<'tcx>] = &[
         &remove_storage_markers::RemoveStorageMarkers,
+        &remove_zsts::RemoveZsts,
         &const_goto::ConstGoto,
         &remove_unneeded_drops::RemoveUnneededDrops,
         &match_branches::MatchBranchSimplification,

compiler/rustc_mir/src/transform/remove_zsts.rs

@@ -0,0 +1,57 @@
+//! Removes assignments to ZST places.
+
+use crate::transform::MirPass;
+use rustc_middle::mir::{Body, StatementKind};
+use rustc_middle::ty::{self, Ty, TyCtxt};
+
+pub struct RemoveZsts;
+
+impl<'tcx> MirPass<'tcx> for RemoveZsts {
+    fn run_pass(&self, tcx: TyCtxt<'tcx>, body: &mut Body<'tcx>) {
+        if tcx.sess.mir_opt_level() < 3 {
+            return;
+        }
+        let param_env = tcx.param_env(body.source.def_id());
+        let (basic_blocks, local_decls) = body.basic_blocks_and_local_decls_mut();
+        for block in basic_blocks.iter_mut() {
+            for statement in block.statements.iter_mut() {
+                match statement.kind {
+                    StatementKind::Assign(box (place, _)) => {
+                        let place_ty = place.ty(local_decls, tcx).ty;
+                        if !maybe_zst(place_ty) {
+                            continue;
+                        }
+                        let layout = match tcx.layout_of(param_env.and(place_ty)) {
+                            Ok(layout) => layout,
+                            Err(_) => continue,
+                        };
+                        if !layout.is_zst() {
+                            continue;
+                        }
+                        if tcx.consider_optimizing(|| {
+                            format!(
+                                "RemoveZsts - Place: {:?} SourceInfo: {:?}",
+                                place, statement.source_info
+                            )
+                        }) {
+                            statement.make_nop();
+                        }
+                    }
+                    _ => {}
+                }
+            }
+        }
+    }
+}
+
+/// A cheap, approximate check to avoid unnecessary `layout_of` calls.
+fn maybe_zst(ty: Ty<'_>) -> bool {
+    match ty.kind() {
+        // maybe ZST (could be more precise)
+        ty::Adt(..) | ty::Array(..) | ty::Closure(..) | ty::Tuple(..) | ty::Opaque(..) => true,
+        // definitely ZST
+        ty::FnDef(..) | ty::Never => true,
+        // unreachable or can't be ZST
+        _ => false,
+    }
+}

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, convert::TryInto};
+use std::borrow::Cow;
+use std::convert::TryInto;
 
 pub struct SimplifyCfg {
     label: String,
@@ -326,7 +326,7 @@ impl<'tcx> MirPass<'tcx> for SimplifyLocals {
 
 pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
     // First, we're going to get a count of *actual* uses for every `Local`.
-    let mut used_locals = UsedLocals::new(body, tcx);
+    let mut used_locals = UsedLocals::new(body);
 
     // 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`
@@ -336,8 +336,7 @@ pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
     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 arg_count = body.arg_count.try_into().unwrap();
-    let map = make_local_map(&mut body.local_decls, &used_locals, arg_count);
+    let map = make_local_map(&mut body.local_decls, &used_locals);
 
     // Only bother running the `LocalUpdater` if we actually found locals to remove.
     if map.iter().any(Option::is_none) {
@@ -350,61 +349,54 @@ pub fn simplify_locals<'tcx>(body: &mut Body<'tcx>, tcx: TyCtxt<'tcx>) {
 }
 
 /// Construct the mapping while swapping out unused stuff out from the `vec`.
-fn make_local_map<'tcx, V>(
+fn make_local_map<V>(
     local_decls: &mut IndexVec<Local, V>,
-    used_locals: &UsedLocals<'tcx>,
-    arg_count: u32,
+    used_locals: &UsedLocals,
 ) -> 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() {
-        // 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);
+        // `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);
+        }
+        used.increment_by(1);
     }
     local_decls.truncate(used.index());
     map
 }
 
 /// Keeps track of used & unused locals.
-struct UsedLocals<'tcx> {
+struct UsedLocals {
     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<'tcx> {
+impl UsedLocals {
     /// Determines which locals are used & unused in the given body.
-    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();
+    fn new(body: &Body<'_>) -> Self {
         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]);
-        self.use_count[local] != 0
+        local.as_u32() <= self.arg_count || self.use_count[local] != 0
     }
 
     /// Updates the use counts to reflect the removal of given statement.
@@ -432,7 +424,7 @@ impl UsedLocals<'tcx> {
     }
 }
 
-impl Visitor<'tcx> for UsedLocals<'tcx> {
+impl Visitor<'_> for UsedLocals {
     fn visit_statement(&mut self, statement: &Statement<'tcx>, location: Location) {
         match statement.kind {
             StatementKind::LlvmInlineAsm(..)
@@ -459,21 +451,7 @@ impl Visitor<'tcx> for UsedLocals<'tcx> {
         }
     }
 
-    fn visit_local(&mut self, local: &Local, ctx: PlaceContext, _location: Location) {
-        debug!("local: {:?} is_static: {:?}, ctx: {:?}", local, self.is_static, ctx);
-        // 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 && 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;
-                }
-            }
-        }
+    fn visit_local(&mut self, local: &Local, _ctx: PlaceContext, _location: Location) {
         if self.increment {
             self.use_count[*local] += 1;
         } else {
@@ -484,10 +462,7 @@ impl Visitor<'tcx> for UsedLocals<'tcx> {
 }
 
 /// Removes unused definitions. Updates the used locals to reflect the changes made.
-fn remove_unused_definitions<'a, 'tcx>(
-    used_locals: &'a mut UsedLocals<'tcx>,
-    body: &mut Body<'tcx>,
-) {
+fn remove_unused_definitions<'a, 'tcx>(used_locals: &'a mut UsedLocals, 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_allocation.main.ConstProp.after.32bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation.rs:8:5: 8:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation.rs:8:8: 8:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation.rs:8:8: 8:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation.rs:7:11: 9:2
+        nop;                             // scope 0 at $DIR/const_allocation.rs:7:11: 9:2
         return;                          // scope 0 at $DIR/const_allocation.rs:9:2: 9:2
     }
 }

src/test/mir-opt/const_allocation.main.ConstProp.after.64bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation.rs:8:5: 8:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation.rs:8:8: 8:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation.rs:8:8: 8:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation.rs:7:11: 9:2
+        nop;                             // scope 0 at $DIR/const_allocation.rs:7:11: 9:2
         return;                          // scope 0 at $DIR/const_allocation.rs:9:2: 9:2
     }
 }

src/test/mir-opt/const_allocation2.main.ConstProp.after.32bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation2.rs:5:5: 5:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation2.rs:5:8: 5:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation2.rs:5:8: 5:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation2.rs:4:11: 6:2
+        nop;                             // scope 0 at $DIR/const_allocation2.rs:4:11: 6:2
         return;                          // scope 0 at $DIR/const_allocation2.rs:6:2: 6:2
     }
 }

src/test/mir-opt/const_allocation2.main.ConstProp.after.64bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation2.rs:5:5: 5:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation2.rs:5:8: 5:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation2.rs:5:8: 5:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation2.rs:4:11: 6:2
+        nop;                             // scope 0 at $DIR/const_allocation2.rs:4:11: 6:2
         return;                          // scope 0 at $DIR/const_allocation2.rs:6:2: 6:2
     }
 }

src/test/mir-opt/const_allocation3.main.ConstProp.after.32bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation3.rs:5:5: 5:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation3.rs:5:8: 5:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation3.rs:5:8: 5:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation3.rs:4:11: 6:2
+        nop;                             // scope 0 at $DIR/const_allocation3.rs:4:11: 6:2
         return;                          // scope 0 at $DIR/const_allocation3.rs:6:2: 6:2
     }
 }

src/test/mir-opt/const_allocation3.main.ConstProp.after.64bit.mir

@@ -18,7 +18,7 @@ fn main() -> () {
         _1 = (*_2);                      // scope 0 at $DIR/const_allocation3.rs:5:5: 5:8
         StorageDead(_2);                 // scope 0 at $DIR/const_allocation3.rs:5:8: 5:9
         StorageDead(_1);                 // scope 0 at $DIR/const_allocation3.rs:5:8: 5:9
-        _0 = const ();                   // scope 0 at $DIR/const_allocation3.rs:4:11: 6:2
+        nop;                             // scope 0 at $DIR/const_allocation3.rs:4:11: 6:2
         return;                          // scope 0 at $DIR/const_allocation3.rs:6:2: 6:2
     }
 }

src/test/mir-opt/const_debuginfo.main.ConstDebugInfo.diff

@@ -99,7 +99,6 @@
           _13 = const 64_u32;              // scope 8 at $DIR/const_debuginfo.rs:21:13: 21:22
           StorageDead(_15);                // scope 8 at $DIR/const_debuginfo.rs:21:21: 21:22
           StorageDead(_14);                // scope 8 at $DIR/const_debuginfo.rs:21:21: 21:22
-          _0 = const ();                   // scope 0 at $DIR/const_debuginfo.rs:8:11: 22:2
           StorageDead(_13);                // scope 8 at $DIR/const_debuginfo.rs:22:1: 22:2
           StorageDead(_12);                // scope 7 at $DIR/const_debuginfo.rs:22:1: 22:2
           StorageDead(_11);                // scope 6 at $DIR/const_debuginfo.rs:22:1: 22:2

src/test/mir-opt/const_prop/aggregate.main.ConstProp.diff

@@ -23,7 +23,7 @@
 +         _1 = const 1_i32;                // scope 0 at $DIR/aggregate.rs:5:13: 5:28
           StorageDead(_2);                 // scope 0 at $DIR/aggregate.rs:5:27: 5:28
           StorageDead(_3);                 // scope 0 at $DIR/aggregate.rs:5:28: 5:29
-          _0 = const ();                   // scope 0 at $DIR/aggregate.rs:4:11: 6:2
+          nop;                             // scope 0 at $DIR/aggregate.rs:4:11: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/aggregate.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/aggregate.rs:6:2: 6:2
       }

src/test/mir-opt/const_prop/array_index.main.ConstProp.32bit.diff

@@ -30,7 +30,7 @@
 +         _1 = const 2_u32;                // scope 0 at $DIR/array_index.rs:5:18: 5:33
           StorageDead(_3);                 // scope 0 at $DIR/array_index.rs:5:33: 5:34
           StorageDead(_2);                 // scope 0 at $DIR/array_index.rs:5:33: 5:34
-          _0 = const ();                   // scope 0 at $DIR/array_index.rs:4:11: 6:2
+          nop;                             // scope 0 at $DIR/array_index.rs:4:11: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/array_index.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/array_index.rs:6:2: 6:2
       }

src/test/mir-opt/const_prop/array_index.main.ConstProp.64bit.diff

@@ -30,7 +30,7 @@
 +         _1 = const 2_u32;                // scope 0 at $DIR/array_index.rs:5:18: 5:33
           StorageDead(_3);                 // scope 0 at $DIR/array_index.rs:5:33: 5:34
           StorageDead(_2);                 // scope 0 at $DIR/array_index.rs:5:33: 5:34
-          _0 = const ();                   // scope 0 at $DIR/array_index.rs:4:11: 6:2
+          nop;                             // scope 0 at $DIR/array_index.rs:4:11: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/array_index.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/array_index.rs:6:2: 6:2
       }

src/test/mir-opt/const_prop/bad_op_div_by_zero.main.ConstProp.diff

@@ -45,7 +45,7 @@
 -         _2 = Div(const 1_i32, move _3);  // scope 1 at $DIR/bad_op_div_by_zero.rs:5:14: 5:19
 +         _2 = Div(const 1_i32, const 0_i32); // scope 1 at $DIR/bad_op_div_by_zero.rs:5:14: 5:19
           StorageDead(_3);                 // scope 1 at $DIR/bad_op_div_by_zero.rs:5:18: 5:19
-          _0 = const ();                   // scope 0 at $DIR/bad_op_div_by_zero.rs:3:11: 6:2
+          nop;                             // scope 0 at $DIR/bad_op_div_by_zero.rs:3:11: 6:2
           StorageDead(_2);                 // scope 1 at $DIR/bad_op_div_by_zero.rs:6:1: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/bad_op_div_by_zero.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/bad_op_div_by_zero.rs:6:2: 6:2

src/test/mir-opt/const_prop/bad_op_mod_by_zero.main.ConstProp.diff

@@ -45,7 +45,7 @@
 -         _2 = Rem(const 1_i32, move _3);  // scope 1 at $DIR/bad_op_mod_by_zero.rs:5:14: 5:19
 +         _2 = Rem(const 1_i32, const 0_i32); // scope 1 at $DIR/bad_op_mod_by_zero.rs:5:14: 5:19
           StorageDead(_3);                 // scope 1 at $DIR/bad_op_mod_by_zero.rs:5:18: 5:19
-          _0 = const ();                   // scope 0 at $DIR/bad_op_mod_by_zero.rs:3:11: 6:2
+          nop;                             // scope 0 at $DIR/bad_op_mod_by_zero.rs:3:11: 6:2
           StorageDead(_2);                 // scope 1 at $DIR/bad_op_mod_by_zero.rs:6:1: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/bad_op_mod_by_zero.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/bad_op_mod_by_zero.rs:6:2: 6:2

src/test/mir-opt/const_prop/bad_op_unsafe_oob_for_slices.main.ConstProp.32bit.diff

@@ -50,7 +50,7 @@
       bb1: {
           _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
           StorageDead(_6);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:25: 7:26
-          _0 = const ();                   // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:6:5: 8:6
+          nop;                             // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:6:5: 8:6
           StorageDead(_5);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:8:5: 8:6
           StorageDead(_1);                 // scope 0 at $DIR/bad_op_unsafe_oob_for_slices.rs:9:1: 9:2
           return;                          // scope 0 at $DIR/bad_op_unsafe_oob_for_slices.rs:9:2: 9:2

src/test/mir-opt/const_prop/bad_op_unsafe_oob_for_slices.main.ConstProp.64bit.diff

@@ -50,7 +50,7 @@
       bb1: {
           _5 = (*_1)[_6];                  // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:18: 7:25
           StorageDead(_6);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:7:25: 7:26
-          _0 = const ();                   // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:6:5: 8:6
+          nop;                             // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:6:5: 8:6
           StorageDead(_5);                 // scope 2 at $DIR/bad_op_unsafe_oob_for_slices.rs:8:5: 8:6
           StorageDead(_1);                 // scope 0 at $DIR/bad_op_unsafe_oob_for_slices.rs:9:1: 9:2
           return;                          // scope 0 at $DIR/bad_op_unsafe_oob_for_slices.rs:9:2: 9:2

src/test/mir-opt/const_prop/boxes.main.ConstProp.diff

@@ -28,7 +28,7 @@
 
       bb1: {
           StorageDead(_3);                 // scope 0 at $DIR/boxes.rs:12:26: 12:27
-          _0 = const ();                   // scope 0 at $DIR/boxes.rs:11:11: 13:2
+          nop;                             // scope 0 at $DIR/boxes.rs:11:11: 13:2
           StorageDead(_1);                 // scope 0 at $DIR/boxes.rs:13:1: 13:2
           return;                          // scope 0 at $DIR/boxes.rs:13:2: 13:2
       }

src/test/mir-opt/const_prop/cast.main.ConstProp.diff

@@ -19,7 +19,7 @@
           StorageLive(_2);                 // scope 1 at $DIR/cast.rs:6:9: 6:10
 -         _2 = const 42_u32 as u8 (Misc);  // scope 1 at $DIR/cast.rs:6:13: 6:24
 +         _2 = const 42_u8;                // scope 1 at $DIR/cast.rs:6:13: 6:24
-          _0 = const ();                   // scope 0 at $DIR/cast.rs:3:11: 7:2
+          nop;                             // scope 0 at $DIR/cast.rs:3:11: 7:2
           StorageDead(_2);                 // scope 1 at $DIR/cast.rs:7:1: 7:2
           StorageDead(_1);                 // scope 0 at $DIR/cast.rs:7:1: 7:2
           return;                          // scope 0 at $DIR/cast.rs:7:2: 7:2

src/test/mir-opt/const_prop/checked_add.main.ConstProp.diff

@@ -23,7 +23,7 @@
       bb1: {
 -         _1 = move (_2.0: u32);           // scope 0 at $DIR/checked_add.rs:5:18: 5:23
 +         _1 = const 2_u32;                // scope 0 at $DIR/checked_add.rs:5:18: 5:23
-          _0 = const ();                   // scope 0 at $DIR/checked_add.rs:4:11: 6:2
+          nop;                             // scope 0 at $DIR/checked_add.rs:4:11: 6:2
           StorageDead(_1);                 // scope 0 at $DIR/checked_add.rs:6:1: 6:2
           return;                          // scope 0 at $DIR/checked_add.rs:6:2: 6:2
       }