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naked functions: respect function-sections #147811

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2375a88
naked functions: respect `function_sections` on windows
folkertdev Oct 17, 2025
f3e3824
naked functions: respect `function_sections` on linux/macos
folkertdev Oct 17, 2025
6580a5f
naked functions: add run-make test for DCE
folkertdev Oct 17, 2025
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35 changes: 25 additions & 10 deletions compiler/rustc_codegen_ssa/src/mir/naked_asm.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -127,6 +127,8 @@ fn prefix_and_suffix<'tcx>(

let is_arm = tcx.sess.target.arch == "arm";
let is_thumb = tcx.sess.unstable_target_features.contains(&sym::thumb_mode);
let function_sections =
tcx.sess.opts.unstable_opts.function_sections.unwrap_or(tcx.sess.target.function_sections);

let attrs = tcx.codegen_instance_attrs(instance.def);
let link_section = attrs.link_section.map(|symbol| symbol.as_str().to_string());
Expand Down Expand Up @@ -201,8 +203,6 @@ fn prefix_and_suffix<'tcx>(
let mut end = String::new();
match asm_binary_format {
BinaryFormat::Elf => {
let section = link_section.unwrap_or_else(|| format!(".text.{asm_name}"));

let progbits = match is_arm {
true => "%progbits",
false => "@progbits",
Expand All @@ -213,7 +213,11 @@ fn prefix_and_suffix<'tcx>(
false => "@function",
};

writeln!(begin, ".pushsection {section},\"ax\", {progbits}").unwrap();
if let Some(section) = &link_section {
writeln!(begin, ".pushsection {section},\"ax\", {progbits}").unwrap();
} else if function_sections {
writeln!(begin, ".pushsection .text.{asm_name},\"ax\", {progbits}").unwrap();
}
writeln!(begin, ".balign {align_bytes}").unwrap();
write_linkage(&mut begin).unwrap();
match item_data.visibility {
Expand All @@ -232,14 +236,18 @@ fn prefix_and_suffix<'tcx>(
// pattern match on assembly generated by LLVM.
writeln!(end, ".Lfunc_end_{asm_name}:").unwrap();
writeln!(end, ".size {asm_name}, . - {asm_name}").unwrap();
writeln!(end, ".popsection").unwrap();
if link_section.is_some() || function_sections {
writeln!(end, ".popsection").unwrap();
}
if !arch_suffix.is_empty() {
writeln!(end, "{}", arch_suffix).unwrap();
}
}
BinaryFormat::MachO => {
let section = link_section.unwrap_or_else(|| "__TEXT,__text".to_string());
writeln!(begin, ".pushsection {},regular,pure_instructions", section).unwrap();
// NOTE: LLVM ignores `-Zfunction-sections` on macos.
if let Some(section) = &link_section {
writeln!(begin, ".pushsection {section},regular,pure_instructions").unwrap();
}
writeln!(begin, ".balign {align_bytes}").unwrap();
write_linkage(&mut begin).unwrap();
match item_data.visibility {
Expand All @@ -250,14 +258,19 @@ fn prefix_and_suffix<'tcx>(

writeln!(end).unwrap();
writeln!(end, ".Lfunc_end_{asm_name}:").unwrap();
writeln!(end, ".popsection").unwrap();
if link_section.is_some() {
writeln!(end, ".popsection").unwrap();
}
if !arch_suffix.is_empty() {
writeln!(end, "{}", arch_suffix).unwrap();
}
}
BinaryFormat::Coff => {
let section = link_section.unwrap_or_else(|| format!(".text.{asm_name}"));
writeln!(begin, ".pushsection {},\"xr\"", section).unwrap();
if let Some(section) = &link_section {
writeln!(begin, ".pushsection {section},\"xr\"").unwrap()
} else if function_sections {
writeln!(begin, ".pushsection .text${asm_name},\"xr\"").unwrap()
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On -msvc targets, function_sections is ignored. .text$sym is only used on -gnu targets.

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Also LLVM currently generates the following line on COFF targets:

.section        {section},"xr",one_only,{sym},unique,0`

Should we be doing the same here?

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On -msvc targets, function_sections is ignored.

Meaning that it's just always assumed to be on? https://godbolt.org/z/a6ofW49YK

.text$sym is only used on -gnu targets.

It should still work for msvc, no?

Should we be doing the same here?

I don't think we can reliably emit the unique,id bit of that line because how do we make that ID?

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Actually using -Zfunction-sections=no on msvc does have an effect. So I'm not really sure what it being ignored means then. Maybe this is a more recent LLVM change?

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No, it's always assumed to be off on msvc. You can see it always uses .text.

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It does always use .text but normally it emits the line you quoted:

  .section .text,"xr",one_only,foo,unique,0

i.e. it uses a subsection which, as far as I understand, does allow DCE, with -Zfunction-sections=no it emits just

  .text

So then DCE is impossible

}
writeln!(begin, ".balign {align_bytes}").unwrap();
write_linkage(&mut begin).unwrap();
writeln!(begin, ".def {asm_name}").unwrap();
Expand All @@ -268,7 +281,9 @@ fn prefix_and_suffix<'tcx>(

writeln!(end).unwrap();
writeln!(end, ".Lfunc_end_{asm_name}:").unwrap();
writeln!(end, ".popsection").unwrap();
if link_section.is_some() || function_sections {
writeln!(end, ".popsection").unwrap();
}
if !arch_suffix.is_empty() {
writeln!(end, "{}", arch_suffix).unwrap();
}
Expand Down
70 changes: 38 additions & 32 deletions tests/codegen-llvm/naked-fn/naked-functions.rs
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Original file line number Diff line number Diff line change
@@ -1,12 +1,14 @@
//@ add-core-stubs
//@ revisions: linux win_x86 win_i686 macos thumb
//@ revisions: linux win_x86_msvc win_x86_gnu win_i686_gnu macos thumb
//
//@[linux] compile-flags: --target x86_64-unknown-linux-gnu
//@[linux] needs-llvm-components: x86
//@[win_x86] compile-flags: --target x86_64-pc-windows-gnu
//@[win_x86] needs-llvm-components: x86
//@[win_i686] compile-flags: --target i686-pc-windows-gnu
//@[win_i686] needs-llvm-components: x86
//@[win_x86_gnu] compile-flags: --target x86_64-pc-windows-gnu
//@[win_x86_gnu] needs-llvm-components: x86
//@[win_x86_msvc] compile-flags: --target x86_64-pc-windows-msvc
//@[win_x86_msvc] needs-llvm-components: x86
//@[win_i686_gnu] compile-flags: --target i686-pc-windows-gnu
//@[win_i686_gnu] needs-llvm-components: x86
//@[macos] compile-flags: --target aarch64-apple-darwin
//@[macos] needs-llvm-components: aarch64
//@[thumb] compile-flags: --target thumbv7em-none-eabi
Expand All @@ -22,9 +24,10 @@ use minicore::*;
// linux,win: .intel_syntax
//
// linux: .pushsection .text.naked_empty,\22ax\22, @progbits
// macos: .pushsection __TEXT,__text,regular,pure_instructions
// win_x86: .pushsection .text.naked_empty,\22xr\22
// win_i686: .pushsection .text._naked_empty,\22xr\22
// macos-NOT: .pushsection
// win_x86_msvc: .pushsection .text$naked_empty,\22xr\22
// win_x86_gnu-NOT: .pushsection
// win_i686_gnu-NOT: .pushsection
// thumb: .pushsection .text.naked_empty,\22ax\22, %progbits
//
// CHECK: .balign 4
Expand All @@ -37,12 +40,12 @@ use minicore::*;
//
// linux: .type naked_empty, @function
//
// win_x86: .def naked_empty
// win_i686: .def _naked_empty
// win_x86_msvc,win_x86_gnu: .def naked_empty
// win_i686_gnu: .def _naked_empty
//
// win_x86,win_i686: .scl 2
// win_x86,win_i686: .type 32
// win_x86,win_i686: .endef
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .scl 2
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .type 32
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .endef
//
// thumb: .type naked_empty, %function
// thumb: .thumb
Expand All @@ -53,7 +56,8 @@ use minicore::*;
// linux,macos,win: ret
// thumb: bx lr
//
// CHECK: .popsection
// linux,windows,win_x86_msvc,thumb: .popsection
// win_x86_gnu-NOT,win_i686_gnu-NOT: .popsection
//
// thumb: .thumb
//
Expand All @@ -72,9 +76,10 @@ pub extern "C" fn naked_empty() {
// linux,win: .intel_syntax
//
// linux: .pushsection .text.naked_with_args_and_return,\22ax\22, @progbits
// macos: .pushsection __TEXT,__text,regular,pure_instructions
// win_x86: .pushsection .text.naked_with_args_and_return,\22xr\22
// win_i686: .pushsection .text._naked_with_args_and_return,\22xr\22
// macos-NOT: .pushsection
// win_x86_msvc: .pushsection .text$naked_with_args_and_return,\22xr\22
// win_x86_gnu-NOT: .pushsection
// win_i686_gnu-NOT: .pushsection
// thumb: .pushsection .text.naked_with_args_and_return,\22ax\22, %progbits
//
// CHECK: .balign 4
Expand All @@ -87,27 +92,28 @@ pub extern "C" fn naked_empty() {
//
// linux: .type naked_with_args_and_return, @function
//
// win_x86: .def naked_with_args_and_return
// win_i686: .def _naked_with_args_and_return
// win_x86_msvc,win_x86_gnu: .def naked_with_args_and_return
// win_i686_gnu: .def _naked_with_args_and_return
//
// win_x86,win_i686: .scl 2
// win_x86,win_i686: .type 32
// win_x86,win_i686: .endef
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .scl 2
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .type 32
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .endef
//
// thumb: .type naked_with_args_and_return, %function
// thumb: .thumb
// thumb: .thumb_func
//
// CHECK-LABEL: naked_with_args_and_return:
//
// linux, win_x86,win_i686: lea rax, [rdi + rsi]
// linux,win_x86_msvc,win_x86_gnu,win_i686_gnu: lea rax, [rdi + rsi]
// macos: add x0, x0, x1
// thumb: adds r0, r0, r1
//
// linux,macos,win: ret
// thumb: bx lr
//
// CHECK: .popsection
// linux,windows,win_x86_msvc,thumb: .popsection
// win_x86_gnu-NOT,win_i686_gnu-NOT: .popsection
//
// thumb: .thumb
//
Expand All @@ -134,7 +140,7 @@ pub extern "C" fn naked_with_args_and_return(a: isize, b: isize) -> isize {

// linux: .pushsection .text.some_different_name,\22ax\22, @progbits
// macos: .pushsection .text.some_different_name,regular,pure_instructions
// win_x86,win_i686: .pushsection .text.some_different_name,\22xr\22
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .pushsection .text.some_different_name,\22xr\22
// thumb: .pushsection .text.some_different_name,\22ax\22, %progbits
// CHECK-LABEL: test_link_section:
#[no_mangle]
Expand All @@ -148,15 +154,15 @@ pub extern "C" fn test_link_section() {
naked_asm!("bx lr");
}

// win_x86: .def fastcall_cc
// win_i686: .def @fastcall_cc@4
// win_x86_msvc,win_x86_gnu: .def fastcall_cc
// win_i686_gnu: .def @fastcall_cc@4
//
// win_x86,win_i686: .scl 2
// win_x86,win_i686: .type 32
// win_x86,win_i686: .endef
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .scl 2
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .type 32
// win_x86_msvc,win_x86_gnu,win_i686_gnu: .endef
//
// win_x86-LABEL: fastcall_cc:
// win_i686-LABEL: @fastcall_cc@4:
// win_x86_msvc-LABEL,win_x86_gnu-LABEL: fastcall_cc:
// win_i686_gnu-LABEL: @fastcall_cc@4:
#[cfg(target_os = "windows")]
#[no_mangle]
#[unsafe(naked)]
Expand Down
24 changes: 24 additions & 0 deletions tests/run-make/naked-dead-code-elimination/main.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,24 @@
use std::arch::naked_asm;

#[unsafe(naked)]
#[no_mangle]
extern "C" fn used() {
naked_asm!("ret")
}

#[unsafe(naked)]
#[no_mangle]
extern "C" fn unused() {
naked_asm!("ret")
}

#[unsafe(naked)]
#[link_section = "foobar"]
#[no_mangle]
extern "C" fn unused_link_section() {
naked_asm!("ret")
}

fn main() {
used();
}
10 changes: 10 additions & 0 deletions tests/run-make/naked-dead-code-elimination/rmake.rs
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Original file line number Diff line number Diff line change
@@ -0,0 +1,10 @@
//@ needs-asm-support

use run_make_support::symbols::object_contains_any_symbol;
use run_make_support::{bin_name, rustc};

fn main() {
rustc().input("main.rs").opt().run();
let mut unused = vec!["unused", "unused_link_section"];
assert!(!object_contains_any_symbol(bin_name("main"), &unused));
}
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