Don't spill operands onto the stack in naked functions

src/librustc_codegen_ssa/mir/debuginfo.rs

@@ -1,6 +1,7 @@
 use crate::traits::*;
 use rustc_hir::def_id::CrateNum;
 use rustc_index::vec::IndexVec;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir;
 use rustc_middle::ty;
 use rustc_session::config::DebugInfo;
@@ -216,6 +217,13 @@ impl<'a, 'tcx, Bx: BuilderMethods<'a, 'tcx>> FunctionCx<'a, 'tcx, Bx> {
             LocalRef::Operand(None) => return,
 
             LocalRef::Operand(Some(operand)) => {
+                // Don't spill operands onto the stack in naked functions.
+                // See: https://github.com/rust-lang/rust/issues/42779
+                let attrs = bx.tcx().codegen_fn_attrs(self.instance.def_id());
+                if attrs.flags.contains(CodegenFnAttrFlags::NAKED) {
+                    return;
+                }
+
                 // "Spill" the value onto the stack, for debuginfo,
                 // without forcing non-debuginfo uses of the local
                 // to also load from the stack every single time.

src/librustc_mir_build/build/mod.rs

@@ -10,7 +10,6 @@ use rustc_hir::lang_items;
 use rustc_hir::{GeneratorKind, HirIdMap, Node};
 use rustc_index::vec::{Idx, IndexVec};
 use rustc_infer::infer::TyCtxtInferExt;
-use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::middle::region;
 use rustc_middle::mir::*;
 use rustc_middle::ty::subst::Subst;
@@ -798,22 +797,12 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
         argument_scope: region::Scope,
         ast_body: &'tcx hir::Expr<'tcx>,
     ) -> BlockAnd<()> {
-        let tcx = self.hir.tcx();
-        let attrs = tcx.codegen_fn_attrs(fn_def_id);
-        let naked = attrs.flags.contains(CodegenFnAttrFlags::NAKED);
-
         // Allocate locals for the function arguments
         for &ArgInfo(ty, _, arg_opt, _) in arguments.iter() {
             let source_info =
                 SourceInfo::outermost(arg_opt.map_or(self.fn_span, |arg| arg.pat.span));
             let arg_local = self.local_decls.push(LocalDecl::with_source_info(ty, source_info));
 
-            // Emit function argument debuginfo only for non-naked functions.
-            // See: https://github.com/rust-lang/rust/issues/42779
-            if naked {
-                continue;
-            }
-
             // If this is a simple binding pattern, give debuginfo a nice name.
             if let Some(arg) = arg_opt {
                 if let Some(ident) = arg.pat.simple_ident() {
@@ -826,6 +815,7 @@ impl<'a, 'tcx> Builder<'a, 'tcx> {
             }
         }
 
+        let tcx = self.hir.tcx();
         let tcx_hir = tcx.hir();
         let hir_typeck_results = self.hir.typeck_results();
 

src/test/codegen/naked-functions.rs

@@ -18,7 +18,7 @@ pub fn naked_empty() {
 // CHECK-NEXT: define void @naked_with_args(i{{[0-9]+( %0)?}})
 pub fn naked_with_args(a: isize) {
     // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: %_1 = alloca i{{[0-9]+}}
+    // CHECK-NEXT: %a = alloca i{{[0-9]+}}
     &a; // keep variable in an alloca
     // CHECK: ret void
 }
@@ -39,7 +39,7 @@ pub fn naked_with_return() -> isize {
 #[naked]
 pub fn naked_with_args_and_return(a: isize) -> isize {
     // CHECK-NEXT: {{.+}}:
-    // CHECK-NEXT: %_1 = alloca i{{[0-9]+}}
+    // CHECK-NEXT: %a = alloca i{{[0-9]+}}
     &a; // keep variable in an alloca
     // CHECK: ret i{{[0-9]+}} %{{[0-9]+}}
     a