Don't alloca for unused locals

2 files changed, 21 insertions, 15 deletions

diff --git a/compiler/rustc_codegen_ssa/src/mir/analyze.rs b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
index f13c46e8bef..4e4bffd8b89 100644
--- a/compiler/rustc_codegen_ssa/src/mir/analyze.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/analyze.rs
@@ -15,13 +15,18 @@ use crate::traits::*;
 
 pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     fx: &FunctionCx<'a, 'tcx, Bx>,
+    traversal_order: &[mir::BasicBlock],
+    reachable_locals: &BitSet<mir::Local>,
 ) -> BitSet<mir::Local> {
     let mir = fx.mir;
     let dominators = mir.basic_blocks.dominators();
     let locals = mir
         .local_decls
-        .iter()
-        .map(|decl| {
+        .iter_enumerated()
+        .map(|(local, decl)| {
+            if !reachable_locals.contains(local) {
+                return LocalKind::Unused;
+            }
             let ty = fx.monomorphize(decl.ty);
             let layout = fx.cx.spanned_layout_of(ty, decl.source_info.span);
             if layout.is_zst() {
@@ -44,7 +49,8 @@ pub(crate) fn non_ssa_locals<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // If there exists a local definition that dominates all uses of that local,
     // the definition should be visited first. Traverse blocks in an order that
     // is a topological sort of dominance partial order.
-    for (bb, data) in traversal::reverse_postorder(mir) {
+    for bb in traversal_order.iter().copied() {
+        let data = &mir.basic_blocks[bb];
         analyzer.visit_basic_block_data(bb, data);
     }
 
diff --git a/compiler/rustc_codegen_ssa/src/mir/mod.rs b/compiler/rustc_codegen_ssa/src/mir/mod.rs
index 61e9b9bd774..6b36e730267 100644
--- a/compiler/rustc_codegen_ssa/src/mir/mod.rs
+++ b/compiler/rustc_codegen_ssa/src/mir/mod.rs
@@ -192,6 +192,9 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             })
             .collect();
 
+    let (traversal_order, reachable_locals) =
+        traversal::mono_reachable_reverse_postorder(mir, cx.tcx(), instance);
+
     let mut fx = FunctionCx {
         instance,
         mir,
@@ -218,7 +221,7 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
 
     fx.per_local_var_debug_info = fx.compute_per_local_var_debug_info(&mut start_bx);
 
-    let memory_locals = analyze::non_ssa_locals(&fx);
+    let memory_locals = analyze::non_ssa_locals(&fx, &traversal_order, &reachable_locals);
 
     // Allocate variable and temp allocas
     let local_values = {
@@ -277,17 +280,14 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
     // So drop the builder of `start_llbb` to avoid having two at the same time.
     drop(start_bx);
 
-    let reachable_blocks = traversal::mono_reachable_as_bitset(mir, cx.tcx(), instance);
-
-    // Codegen the body of each block using reverse postorder
-    for (bb, _) in traversal::reverse_postorder(mir) {
-        if reachable_blocks.contains(bb) {
-            fx.codegen_block(bb);
-        } else {
-            // We want to skip this block, because it's not reachable. But we still create
-            // the block so terminators in other blocks can reference it.
-            fx.codegen_block_as_unreachable(bb);
-        }
+    let mut unreached_blocks = BitSet::new_filled(mir.basic_blocks.len());
+    // Codegen the body of each reachable block using our reverse postorder list.
+    for bb in traversal_order {
+        fx.codegen_block(bb);
+        unreached_blocks.remove(bb);
+    }
+    for bb in unreached_blocks.iter() {
+        fx.codegen_block_as_unreachable(bb);
     }
 }