Don't alloca for unused locals

Author: saethlin

compiler/rustc_codegen_cranelift/src/base.rs

@@ -299,7 +299,8 @@ fn codegen_fn_body(fx: &mut FunctionCx<'_, '_, '_>, start_block: Block) {
         .generic_activity("codegen prelude")
         .run(|| crate::abi::codegen_fn_prelude(fx, start_block));
 
-    let reachable_blocks = traversal::mono_reachable_as_bitset(fx.mir, fx.tcx, fx.instance);
+    let (reachable_blocks, _reachable_locals) =
+        traversal::mono_reachable_as_bitset(fx.mir, fx.tcx, fx.instance);
 
     for (bb, bb_data) in fx.mir.basic_blocks.iter_enumerated() {
         let block = fx.get_block(bb);

compiler/rustc_codegen_ssa/src/mir/mod.rs

@@ -192,6 +192,9 @@ pub fn codegen_mir<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>>(
             })
             .collect();
 
+    let (reachable_blocks, reachable_locals) =
+        traversal::mono_reachable_as_bitset(mir, cx.tcx(), instance);
+
     let mut fx = FunctionCx {
         instance,
         mir,
@@ -218,7 +221,8 @@ 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 mut memory_locals = analyze::non_ssa_locals(&fx);
+    memory_locals.intersect(&reachable_locals);
 
     // Allocate variable and temp allocas
     let local_values = {
@@ -277,8 +281,6 @@ 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) {

compiler/rustc_middle/src/mir/traversal.rs

@@ -303,10 +303,10 @@ pub fn mono_reachable_as_bitset<'a, 'tcx>(
     body: &'a Body<'tcx>,
     tcx: TyCtxt<'tcx>,
     instance: Instance<'tcx>,
-) -> BitSet<BasicBlock> {
+) -> (BitSet<BasicBlock>, BitSet<Local>) {
     let mut iter = mono_reachable(body, tcx, instance);
     while let Some(_) = iter.next() {}
-    iter.visited
+    (iter.visited, iter.locals)
 }
 
 pub struct MonoReachable<'a, 'tcx> {
@@ -318,6 +318,23 @@ pub struct MonoReachable<'a, 'tcx> {
     // store ours in a BitSet and thus save allocations because BitSet has a small size
     // optimization.
     worklist: BitSet<BasicBlock>,
+    locals: BitSet<Local>,
+}
+
+struct UsedLocals<'a> {
+    locals: &'a mut BitSet<Local>,
+}
+
+use crate::mir::visit::Visitor;
+impl<'a, 'tcx> Visitor<'tcx> for UsedLocals<'a> {
+    fn visit_local(
+        &mut self,
+        local: Local,
+        _ctx: crate::mir::visit::PlaceContext,
+        _location: Location,
+    ) {
+        self.locals.insert(local);
+    }
 }
 
 impl<'a, 'tcx> MonoReachable<'a, 'tcx> {
@@ -334,6 +351,7 @@ impl<'a, 'tcx> MonoReachable<'a, 'tcx> {
             instance,
             visited: BitSet::new_empty(body.basic_blocks.len()),
             worklist,
+            locals: BitSet::new_empty(body.local_decls.len()),
         }
     }
 
@@ -357,6 +375,7 @@ impl<'a, 'tcx> Iterator for MonoReachable<'a, 'tcx> {
             }
 
             let data = &self.body[idx];
+            UsedLocals { locals: &mut self.locals }.visit_basic_block_data(idx, data);
 
             if let Some((bits, targets)) =
                 Body::try_const_mono_switchint(self.tcx, self.instance, data)