Support naked functions

Fixes rust-lang/rustc_codegen_cranelift#1203

Author: bjorn3

build_system/build_sysroot.rs

@@ -276,7 +276,7 @@ fn build_clif_sysroot_for_triple(
     if channel == "release" {
         build_cmd.arg("--release");
     }
-    build_cmd.arg("--features").arg("compiler-builtins-no-asm backtrace panic-unwind");
+    build_cmd.arg("--features").arg("backtrace panic-unwind");
     build_cmd.env("CARGO_PROFILE_RELEASE_DEBUG", "true");
     build_cmd.env("__CARGO_DEFAULT_LIB_METADATA", "cg_clif");
     if compiler.triple.contains("apple") {

example/mini_core_hello_world.rs

@@ -4,6 +4,7 @@
     never_type,
     linkage,
     extern_types,
+    naked_functions,
     thread_local,
     repr_simd,
     raw_ref_op
@@ -340,6 +341,7 @@ fn main() {
     ))]
     unsafe {
         global_asm_test();
+        naked_test();
     }
 
     // Both statics have a reference that points to the same anonymous allocation.
@@ -395,6 +397,14 @@ global_asm! {
     "
 }
 
+#[cfg(all(not(jit), not(no_unstable_features), target_arch = "x86_64"))]
+#[naked]
+extern "C" fn naked_test() {
+    unsafe {
+        asm!("ret", options(noreturn));
+    }
+}
+
 #[repr(C)]
 enum c_void {
     _1,

src/base.rs

@@ -6,6 +6,7 @@ use cranelift_frontend::{FunctionBuilder, FunctionBuilderContext};
 use cranelift_module::ModuleError;
 use rustc_ast::InlineAsmOptions;
 use rustc_index::IndexVec;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::ty::adjustment::PointerCoercion;
 use rustc_middle::ty::layout::FnAbiOf;
 use rustc_middle::ty::print::with_no_trimmed_paths;
@@ -14,6 +15,7 @@ use rustc_monomorphize::is_call_from_compiler_builtins_to_upstream_monomorphizat
 
 use crate::constant::ConstantCx;
 use crate::debuginfo::{FunctionDebugContext, TypeDebugContext};
+use crate::inline_asm::codegen_naked_asm;
 use crate::prelude::*;
 use crate::pretty_clif::CommentWriter;
 
@@ -32,7 +34,7 @@ pub(crate) fn codegen_fn<'tcx>(
     cached_func: Function,
     module: &mut dyn Module,
     instance: Instance<'tcx>,
-) -> CodegenedFunction {
+) -> Option<CodegenedFunction> {
     debug_assert!(!instance.args.has_infer());
 
     let symbol_name = tcx.symbol_name(instance).name.to_string();
@@ -48,6 +50,37 @@ pub(crate) fn codegen_fn<'tcx>(
         String::from_utf8_lossy(&buf).into_owned()
     });
 
+    if tcx.codegen_fn_attrs(instance.def_id()).flags.contains(CodegenFnAttrFlags::NAKED) {
+        assert_eq!(mir.basic_blocks.len(), 1);
+        assert!(mir.basic_blocks[START_BLOCK].statements.is_empty());
+
+        match &mir.basic_blocks[START_BLOCK].terminator().kind {
+            TerminatorKind::InlineAsm {
+                template,
+                operands,
+                options,
+                line_spans: _,
+                targets: _,
+                unwind: _,
+            } => {
+                codegen_naked_asm(
+                    tcx,
+                    cx,
+                    module,
+                    instance,
+                    mir.basic_blocks[START_BLOCK].terminator().source_info.span,
+                    &symbol_name,
+                    template,
+                    operands,
+                    *options,
+                );
+            }
+            _ => unreachable!(),
+        }
+
+        return None;
+    }
+
     // Declare function
     let sig = get_function_sig(tcx, module.target_config().default_call_conv, instance);
     let func_id = module.declare_function(&symbol_name, Linkage::Local, &sig).unwrap();
@@ -128,7 +161,7 @@ pub(crate) fn codegen_fn<'tcx>(
     // Verify function
     verify_func(tcx, &clif_comments, &func);
 
-    CodegenedFunction { symbol_name, func_id, func, clif_comments, func_debug_cx }
+    Some(CodegenedFunction { symbol_name, func_id, func, clif_comments, func_debug_cx })
 }
 
 pub(crate) fn compile_fn(

src/driver/aot.rs

@@ -482,15 +482,16 @@ fn module_codegen(
             for (mono_item, _) in mono_items {
                 match mono_item {
                     MonoItem::Fn(inst) => {
-                        let codegened_function = crate::base::codegen_fn(
+                        if let Some(codegened_function) = crate::base::codegen_fn(
                             tcx,
                             &mut cx,
                             &mut type_dbg,
                             Function::new(),
                             &mut module,
                             inst,
-                        );
-                        codegened_functions.push(codegened_function);
+                        ) {
+                            codegened_functions.push(codegened_function);
+                        }
                     }
                     MonoItem::Static(def_id) => {
                         let data_id = crate::constant::codegen_static(tcx, &mut module, def_id);

src/driver/jit.rs

@@ -232,16 +232,16 @@ pub(crate) fn codegen_and_compile_fn<'tcx>(
             crate::PrintOnPanic(|| format!("{:?} {}", instance, tcx.symbol_name(instance).name));
 
         let cached_func = std::mem::replace(&mut cached_context.func, Function::new());
-        let codegened_func = crate::base::codegen_fn(
+        if let Some(codegened_func) = crate::base::codegen_fn(
             tcx,
             cx,
             &mut TypeDebugContext::default(),
             cached_func,
             module,
             instance,
-        );
-
-        crate::base::compile_fn(cx, cached_context, module, codegened_func);
+        ) {
+            crate::base::compile_fn(cx, cached_context, module, codegened_func);
+        }
     });
 }
 

src/driver/mod.rs

@@ -5,6 +5,7 @@
 //! [`codegen_static`]: crate::constant::codegen_static
 
 use rustc_data_structures::profiling::SelfProfilerRef;
+use rustc_middle::middle::codegen_fn_attrs::CodegenFnAttrFlags;
 use rustc_middle::mir::mono::{MonoItem, MonoItemData};
 
 use crate::prelude::*;
@@ -33,7 +34,20 @@ fn predefine_mono_items<'tcx>(
                         data.visibility,
                         is_compiler_builtins,
                     );
-                    module.declare_function(name, linkage, &sig).unwrap();
+                    let is_naked = tcx
+                        .codegen_fn_attrs(instance.def_id())
+                        .flags
+                        .contains(CodegenFnAttrFlags::NAKED);
+                    module
+                        .declare_function(
+                            name,
+                            // Naked functions are defined in a separate object
+                            // file from the codegen unit rustc expects them to
+                            // be defined in.
+                            if is_naked { Linkage::Import } else { linkage },
+                            &sig,
+                        )
+                        .unwrap();
                 }
                 MonoItem::Static(_) | MonoItem::GlobalAsm(_) => {}
             }