Support naked functions

Fixes rust-lang/rustc_codegen_cranelift#1203

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 @@ struct ExternTypeWrapper {
     ))]
     unsafe {
         global_asm_test();
+        naked_test();
     }
 
     // Both statics have a reference that points to the same anonymous allocation.
@@ -395,6 +397,14 @@ fn stack_val_align() {
     "
 }
 
+#[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_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 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(_) => {}
             }

src/inline_asm.rs

@@ -169,6 +169,7 @@ pub(crate) fn codegen_inline_asm_inner<'tcx>(
         stack_slots_input: Vec::new(),
         stack_slots_output: Vec::new(),
         stack_slot_size: Size::from_bytes(0),
+        is_naked: false,
     };
     asm_gen.allocate_registers();
     asm_gen.allocate_stack_slots();
@@ -209,6 +210,121 @@ pub(crate) fn codegen_inline_asm_inner<'tcx>(
     call_inline_asm(fx, &asm_name, asm_gen.stack_slot_size, inputs, outputs);
 }
 
+pub(crate) fn codegen_naked_asm<'tcx>(
+    tcx: TyCtxt<'tcx>,
+    cx: &mut crate::CodegenCx,
+    module: &mut dyn Module,
+    instance: Instance<'tcx>,
+    span: Span,
+    symbol_name: &str,
+    template: &[InlineAsmTemplatePiece],
+    operands: &[InlineAsmOperand<'tcx>],
+    options: InlineAsmOptions,
+) {
+    // FIXME add .eh_frame unwind info directives
+
+    let operands = operands
+        .iter()
+        .map(|operand| match *operand {
+            InlineAsmOperand::In { .. }
+            | InlineAsmOperand::Out { .. }
+            | InlineAsmOperand::InOut { .. } => {
+                span_bug!(span, "invalid operand type for naked asm")
+            }
+            InlineAsmOperand::Const { ref value } => {
+                let cv = instance.instantiate_mir_and_normalize_erasing_regions(
+                    tcx,
+                    ty::ParamEnv::reveal_all(),
+                    ty::EarlyBinder::bind(value.const_),
+                );
+                let const_value = cv
+                    .eval(tcx, ty::ParamEnv::reveal_all(), value.span)
+                    .expect("erroneous constant missed by mono item collection");
+
+                let value = rustc_codegen_ssa::common::asm_const_to_str(
+                    tcx,
+                    span,
+                    const_value,
+                    RevealAllLayoutCx(tcx).layout_of(cv.ty()),
+                );
+                CInlineAsmOperand::Const { value }
+            }
+            InlineAsmOperand::SymFn { ref value } => {
+                if cfg!(not(feature = "inline_asm_sym")) {
+                    tcx.dcx()
+                        .span_err(span, "asm! and global_asm! sym operands are not yet supported");
+                }
+
+                let const_ = instance.instantiate_mir_and_normalize_erasing_regions(
+                    tcx,
+                    ty::ParamEnv::reveal_all(),
+                    ty::EarlyBinder::bind(value.const_),
+                );
+                if let ty::FnDef(def_id, args) = *const_.ty().kind() {
+                    let instance = ty::Instance::resolve_for_fn_ptr(
+                        tcx,
+                        ty::ParamEnv::reveal_all(),
+                        def_id,
+                        args,
+                    )
+                    .unwrap();
+                    let symbol = tcx.symbol_name(instance);
+
+                    // Pass a wrapper rather than the function itself as the function itself may not
+                    // be exported from the main codegen unit and may thus be unreachable from the
+                    // object file created by an external assembler.
+                    let inline_asm_index = cx.inline_asm_index.get();
+                    cx.inline_asm_index.set(inline_asm_index + 1);
+                    let wrapper_name = format!(
+                        "__inline_asm_{}_wrapper_n{}",
+                        cx.cgu_name.as_str().replace('.', "__").replace('-', "_"),
+                        inline_asm_index
+                    );
+                    let sig =
+                        get_function_sig(tcx, module.target_config().default_call_conv, instance);
+                    create_wrapper_function(
+                        module,
+                        &mut cx.unwind_context,
+                        sig,
+                        &wrapper_name,
+                        symbol.name,
+                    );
+
+                    CInlineAsmOperand::Symbol { symbol: wrapper_name }
+                } else {
+                    span_bug!(span, "invalid type for asm sym (fn)");
+                }
+            }
+            InlineAsmOperand::SymStatic { def_id } => {
+                assert!(tcx.is_static(def_id));
+                let instance = Instance::mono(tcx, def_id);
+                CInlineAsmOperand::Symbol { symbol: tcx.symbol_name(instance).name.to_owned() }
+            }
+            InlineAsmOperand::Label { .. } => {
+                span_bug!(span, "asm! label operands are not yet supported");
+            }
+        })
+        .collect::<Vec<_>>();
+
+    let asm_gen = InlineAssemblyGenerator {
+        tcx,
+        arch: tcx.sess.asm_arch.unwrap(),
+        enclosing_def_id: instance.def_id(),
+        template,
+        operands: &operands,
+        options,
+        registers: Vec::new(),
+        stack_slots_clobber: Vec::new(),
+        stack_slots_input: Vec::new(),
+        stack_slots_output: Vec::new(),
+        stack_slot_size: Size::from_bytes(0),
+        is_naked: true,
+    };
+
+    let generated_asm = asm_gen.generate_asm_wrapper(symbol_name);
+    cx.global_asm.push_str(&generated_asm);
+}
+
 struct InlineAssemblyGenerator<'a, 'tcx> {
     tcx: TyCtxt<'tcx>,
     arch: InlineAsmArch,
@@ -221,10 +337,13 @@ struct InlineAssemblyGenerator<'a, 'tcx> {
     stack_slots_input: Vec<Option<Size>>,
     stack_slots_output: Vec<Option<Size>>,
     stack_slot_size: Size,
+    is_naked: bool,
 }
 
 impl<'tcx> InlineAssemblyGenerator<'_, 'tcx> {
     fn allocate_registers(&mut self) {
+        assert!(!self.is_naked);
+
         let sess = self.tcx.sess;
         let map = allocatable_registers(
             self.arch,
@@ -348,6 +467,8 @@ fn allocate_registers(&mut self) {
     }
 
     fn allocate_stack_slots(&mut self) {
+        assert!(!self.is_naked);
+
         let mut slot_size = Size::from_bytes(0);
         let mut slots_clobber = vec![None; self.operands.len()];
         let mut slots_input = vec![None; self.operands.len()];
@@ -468,30 +589,32 @@ fn generate_asm_wrapper(&self, asm_name: &str) -> String {
         if is_x86 {
             generated_asm.push_str(".intel_syntax noprefix\n");
         }
-        Self::prologue(&mut generated_asm, self.arch);
+        if !self.is_naked {
+            Self::prologue(&mut generated_asm, self.arch);
 
-        // Save clobbered registers
+            // Save clobbered registers
-        if !self.options.contains(InlineAsmOptions::NORETURN) {
+            if !self.options.contains(InlineAsmOptions::NORETURN) {
+                for (reg, slot) in self
+                    .registers
+                    .iter()
+                    .zip(self.stack_slots_clobber.iter().copied())
+                    .filter_map(|(r, s)| r.zip(s))
+                {
+                    Self::save_register(&mut generated_asm, self.arch, reg, slot);
+                }
+            }
+
+            // Write input registers
             for (reg, slot) in self
                 .registers
                 .iter()
-                .zip(self.stack_slots_clobber.iter().copied())
+                .zip(self.stack_slots_input.iter().copied())
                 .filter_map(|(r, s)| r.zip(s))
             {
-                Self::save_register(&mut generated_asm, self.arch, reg, slot);
+                Self::restore_register(&mut generated_asm, self.arch, reg, slot);
             }
         }
 
-        // Write input registers
-        for (reg, slot) in self
-            .registers
-            .iter()
-            .zip(self.stack_slots_input.iter().copied())
-            .filter_map(|(r, s)| r.zip(s))
-        {
-            Self::restore_register(&mut generated_asm, self.arch, reg, slot);
-        }
-
         if is_x86 && self.options.contains(InlineAsmOptions::ATT_SYNTAX) {
             generated_asm.push_str(".att_syntax\n");
         }
@@ -553,30 +676,32 @@ fn generate_asm_wrapper(&self, asm_name: &str) -> String {
             generated_asm.push_str(".intel_syntax noprefix\n");
         }
 
-        if !self.options.contains(InlineAsmOptions::NORETURN) {
+        if !self.is_naked {
-            // Read output registers
+            if !self.options.contains(InlineAsmOptions::NORETURN) {
-            for (reg, slot) in self
+                // Read output registers
-                .registers
+                for (reg, slot) in self
-                .iter()
+                    .registers
-                .zip(self.stack_slots_output.iter().copied())
+                    .iter()
-                .filter_map(|(r, s)| r.zip(s))
+                    .zip(self.stack_slots_output.iter().copied())
-            {
+                    .filter_map(|(r, s)| r.zip(s))
-                Self::save_register(&mut generated_asm, self.arch, reg, slot);
+                {
-            }
+                    Self::save_register(&mut generated_asm, self.arch, reg, slot);
+                }
 
-            // Restore clobbered registers
+                // Restore clobbered registers
-            for (reg, slot) in self
+                for (reg, slot) in self
-                .registers
+                    .registers
-                .iter()
+                    .iter()
-                .zip(self.stack_slots_clobber.iter().copied())
+                    .zip(self.stack_slots_clobber.iter().copied())
-                .filter_map(|(r, s)| r.zip(s))
+                    .filter_map(|(r, s)| r.zip(s))
-            {
+                {
-                Self::restore_register(&mut generated_asm, self.arch, reg, slot);
+                    Self::restore_register(&mut generated_asm, self.arch, reg, slot);
-            }
+                }
 
-            Self::epilogue(&mut generated_asm, self.arch);
+                Self::epilogue(&mut generated_asm, self.arch);
-        } else {
+            } else {
-            Self::epilogue_noreturn(&mut generated_asm, self.arch);
+                Self::epilogue_noreturn(&mut generated_asm, self.arch);
+            }
         }
 
         if is_x86 {