Rollup merge of rust-lang#75417 - npmccallum:naked, r=matthewjasper

Don't spill operands onto the stack in naked functions

Currently, the code spills operands onto the stack for the purpose of
debuginfo. However, naked functions can only contain an asm block. Therefore,
attempting to spill the operands on the stack is undefined behavior.

Fixes rust-lang#42779
cc rust-lang#32408

Note that this PR reverts rust-lang#74105 which ultimately didn't fix the problem.

cc @haraldh @Amanieu @matthewjasper

Author: tmandry

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