Move ZST check inside UsedLocals

Author: simonvandel

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,
@@ -322,17 +322,18 @@ 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`
         // count. For example, if we removed `_2 = discriminant(_1)`, then we'll subtract one from
         // `use_counts[_1]`. That in turn might make `_1` unused, so we loop until we hit a
         // fixedpoint where there are no more unused locals.
-        remove_unused_definitions(&mut used_locals, body, 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 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 {
@@ -460,21 +484,14 @@ 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,
+    used_locals: &'a mut UsedLocals<'tcx>,
     body: &mut Body<'tcx>,
-    tcx: TyCtxt<'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
     // computation reaches a fixed point.
 
-    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 modified = true;
     while modified {
         modified = false;
@@ -486,21 +503,7 @@ fn remove_unused_definitions<'a, 'tcx>(
                     StatementKind::StorageLive(local) | StatementKind::StorageDead(local) => {
                         used_locals.is_used(*local)
                     }
-                    StatementKind::Assign(box (place, _)) => {
-                        let used = used_locals.is_used(place.local);
-                        let mut is_zst = false;
-
-                        // ZST locals can be removed
-                        if used && !is_static {
-                            let ty = local_decls[place.local].ty;
-                            let param_env_and = param_env.and(ty);
-                            if let Ok(layout) = tcx.layout_of(param_env_and) {
-                                is_zst = layout.is_zst();
-                            }
-                        }
-
-                        used && !is_zst
-                    }
+                    StatementKind::Assign(box (place, _)) => used_locals.is_used(place.local),
 
                     StatementKind::SetDiscriminant { ref place, .. } => {
                         used_locals.is_used(place.local)

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

@@ -28,6 +28,7 @@ fn main() -> () {
         StorageLive(_5);                 // scope 1 at $DIR/issue-76997-inline-scopes-parenting.rs:6:5: 6:10
         _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
         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