Auto merge of #32410 - Ticki:master, r=eddyb

Add support for naked functions

See rust-lang/rfcs#1201 (comment)

This PR adds `#[naked]` for marking naked functions.

src/doc/reference.md

@@ -1905,6 +1905,8 @@ type int8_t = i8;
 - `should_panic` - indicates that this test function should panic, inverting the success condition.
 - `cold` - The function is unlikely to be executed, so optimize it (and calls
   to it) differently.
+- `naked` - The function utilizes a custom ABI or custom inline ASM that requires
+  epilogue and prologue to be skipped.
 
 ### Static-only attributes
 

src/librustc_trans/trans/attributes.rs

@@ -81,6 +81,18 @@ pub fn set_optimize_for_size(val: ValueRef, optimize: bool) {
     }
 }
 
+/// Tell LLVM if this function should be 'naked', i.e. skip the epilogue and prologue.
+#[inline]
+pub fn naked(val: ValueRef, is_naked: bool) {
+    if is_naked {
+        llvm::SetFunctionAttribute(val, llvm::Attribute::Naked);
+    } else {
+        unsafe {
+            llvm::LLVMRemoveFunctionAttr(val, llvm::Attribute::Naked.bits() as c_ulonglong);
+        }
+    }
+}
+
 /// Composite function which sets LLVM attributes for function depending on its AST (#[attribute])
 /// attributes.
 pub fn from_fn_attrs(ccx: &CrateContext, attrs: &[ast::Attribute], llfn: ValueRef) {
@@ -105,6 +117,8 @@ pub fn from_fn_attrs(ccx: &CrateContext, attrs: &[ast::Attribute], llfn: ValueRe
         if attr.check_name("cold") {
             llvm::Attributes::default().set(llvm::Attribute::Cold)
                 .apply_llfn(llvm::FunctionIndex as usize, llfn)
+        } else if attr.check_name("naked") {
+            naked(llfn, true);
         } else if attr.check_name("allocator") {
             llvm::Attributes::default().set(llvm::Attribute::NoAlias)
                 .apply_llfn(llvm::ReturnIndex as usize, llfn)

src/libsyntax/feature_gate.rs

@@ -212,6 +212,9 @@ const KNOWN_FEATURES: &'static [(&'static str, &'static str, Option<u32>, Status
     // rust runtime internal
     ("unwind_attributes", "1.4.0", None, Active),
 
+    // allow the use of `#[naked]` on functions.
+    ("naked_functions", "1.9.0", None, Active),
+
     // allow empty structs and enum variants with braces
     ("braced_empty_structs", "1.5.0", Some(29720), Accepted),
 
@@ -376,6 +379,9 @@ pub const KNOWN_ATTRIBUTES: &'static [(&'static str, AttributeType, AttributeGat
     // FIXME: #14406 these are processed in trans, which happens after the
     // lint pass
     ("cold", Whitelisted, Ungated),
+    ("naked", Whitelisted, Gated("naked_functions",
+                                 "the `#[naked]` attribute \
+                                  is an experimental feature")),
     ("export_name", Whitelisted, Ungated),
     ("inline", Whitelisted, Ungated),
     ("link", Whitelisted, Ungated),

src/test/codegen/naked-functions.rs

@@ -0,0 +1,69 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+// ignore-tidy-linelength
+
+// compile-flags: -C no-prepopulate-passes
+
+#![crate_type = "lib"]
+#![feature(naked_functions)]
+
+// CHECK: Function Attrs: naked uwtable
+// CHECK-NEXT: define internal void @naked_empty()
+#[no_mangle]
+#[naked]
+fn naked_empty() {
+    // CHECK: ret void
+}
+
+// CHECK: Function Attrs: naked uwtable
+#[no_mangle]
+#[naked]
+// CHECK-NEXT: define internal void @naked_with_args(i{{[0-9]+}})
+fn naked_with_args(a: isize) {
+    // CHECK: %a = alloca i{{[0-9]+}}
+    // CHECK: ret void
+}
+
+// CHECK: Function Attrs: naked uwtable
+// CHECK-NEXT: define internal i{{[0-9]+}} @naked_with_return()
+#[no_mangle]
+#[naked]
+fn naked_with_return() -> isize {
+    // CHECK: ret i{{[0-9]+}} 0
+    0
+}
+
+// CHECK: Function Attrs: naked uwtable
+// CHECK-NEXT: define internal i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}})
+#[no_mangle]
+#[naked]
+fn naked_with_args_and_return(a: isize) -> isize {
+    // CHECK: %a = alloca i{{[0-9]+}}
+    // CHECK: ret i{{[0-9]+}} %{{[0-9]+}}
+    a
+}
+
+// CHECK: Function Attrs: naked uwtable
+// CHECK-NEXT: define internal void @naked_recursive()
+#[no_mangle]
+#[naked]
+fn naked_recursive() {
+    // CHECK: call void @naked_empty()
+    naked_empty();
+    // CHECK: %{{[0-9]+}} = call i{{[0-9]+}} @naked_with_return()
+    naked_with_args(
+        // CHECK: %{{[0-9]+}} = call i{{[0-9]+}} @naked_with_args_and_return(i{{[0-9]+}} %{{[0-9]+}})
+        naked_with_args_and_return(
+            // CHECK: call void @naked_with_args(i{{[0-9]+}} %{{[0-9]+}})
+            naked_with_return()
+        )
+    );
+}

src/test/compile-fail/gated-naked_functions.rs

@@ -0,0 +1,19 @@
+// Copyright 2015 The Rust Project Developers. See the COPYRIGHT
+// file at the top-level directory of this distribution and at
+// http://rust-lang.org/COPYRIGHT.
+//
+// Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
+// http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
+// <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
+// option. This file may not be copied, modified, or distributed
+// except according to those terms.
+
+#[naked]
+//~^ the `#[naked]` attribute is an experimental feature
+fn naked() {}
+
+#[naked]
+//~^ the `#[naked]` attribute is an experimental feature
+fn naked_2() -> isize {
+    0
+}