naked functions: respect `function-sections` by folkertdev · Pull Request #147811 · rust-lang/rust

folkertdev · 2025-10-17T14:02:32Z

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fixes #147789

r? @Amanieu

rustbot · 2025-10-17T14:02:35Z

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

tests/codegen-llvm/naked-fn/dead-code-elimination.rs

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

Amanieu · 2025-10-21T09:39:21Z

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

+            if let Some(section) = &link_section {
+                writeln!(begin, ".pushsection {section},\"xr\"").unwrap()
+            } else if function_sections {
+                writeln!(begin, ".pushsection .text${asm_name},\"xr\"").unwrap()


On -msvc targets, function_sections is ignored. .text$sym is only used on -gnu targets.

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?

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?

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?

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

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

The unique,0 bit is apparently an LLVM extension https://llvm.org/docs/Extensions.html#id2. It is documented as elf-specific, but clearly it's used for COFF as well...

I just don't see how we can generate the unique ID in a reliable way. Also because we would generate a function per section, the function name (which should be unique?) should be sufficient to disambiguate the sections.

In short, I think the implementation in this PR is the best we can reliably do.

bors · 2025-11-03T00:35:35Z

☔ The latest upstream changes (presumably #148305) made this pull request unmergeable. Please resolve the merge conflicts.

folkertdev · 2025-11-08T22:29:08Z

Looks like windows will also be using function sections by default soon #148669

For `gnu` function_sections is off by default.

rustbot · 2025-11-09T10:42:34Z

This PR was rebased onto a different master commit. Here's a range-diff highlighting what actually changed.

Rebasing is a normal part of keeping PRs up to date, so no action is needed—this note is just to help reviewers.

mati865 · 2025-11-09T14:50:15Z

Looks like windows will also be using function sections by default soon #148669

This is unlikely to happen, as you can see crashes in failed tests in that PR. That is what I was talking about in #147789 (comment)

wesleywiser · 2026-01-15T15:25:19Z

Hi @folkertdev, are you waiting for another review or is the discussion thread here still active?

folkertdev · 2026-01-15T15:31:00Z

In my mind this was blocked on review, but now that I read it again I should give this a once-over and probably test both with and without -Zfunction-sections=no

@rustbot author

rustbot · 2026-01-15T15:31:05Z

Reminder, once the PR becomes ready for a review, use @rustbot ready.

compiler/rustc_codegen_ssa/src/mir/naked_asm.rs

folkertdev · 2026-01-17T14:27:24Z

@rustbot ready

wesleywiser · 2026-03-05T15:15:11Z

Hi @Amanieu, it looks like this is ready for another round of review when you have a chance 🙂

Amanieu · 2026-03-28T10:37:12Z

@bors r+

rust-bors · 2026-03-28T10:37:15Z

📌 Commit 77de006 has been approved by Amanieu

It is now in the queue for this repository.

@Amanieu

…, r=Amanieu naked functions: respect `function-sections` fixes rust-lang#147789 r? @Amanieu

…uwer Rollup of 7 pull requests Successful merges: - #147811 (naked functions: respect `function-sections`) - #153834 (Merge `fabsf16/32/64/128` into `fabs::<F>`) - #154043 (simd_fmin/fmax: make semantics and name consistent with scalar intrinsics) - #154494 (triagebot: add reminder for bumping CI LLVM stamp) - #153374 (Fix LegacyKeyValueFormat report from docker build: dist-x86_64) - #154320 (`trim_prefix` for paths) - #154453 (Fix ice in rustdoc of private reexport)

JonathanBrouwer · 2026-03-28T21:12:25Z

@bors r-
#154514 (comment)

rust-bors · 2026-03-28T21:12:32Z

This pull request was unapproved.

This PR was contained in a rollup (#154514), which was unapproved.

folkertdev · 2026-03-29T18:04:05Z

tests/run-make/naked-dead-code-elimination/main.rs

+#[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")
+}


@bjorn3 does it make sense to you that these functions are not removed at link time on windows? They are not present on linux at least, but with windows they are. So either we're missing something or the windows linker just keeps these functions despite them being unreachable.

I bet they'd get removed before the changes in this PR. Since function sections are disabled for windows-gnu, beginning with this PR they are no longer output in separate sections, so ld.bfd doesn't GC them.

In that case I'd expect -Zfunction-sections=yes to cause them to be removed again, but that is not what I see. In fact even unused_link_section is not removed, and it always has its own section.

I'm using cross-compilation here so maybe the issue is there? But it looks like unused sections just aren't removed. The same is true when I use normal (i.e. non-naked) functions.

rustbot added S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Oct 17, 2025

rustbot assigned Amanieu Oct 17, 2025

folkertdev commented Oct 17, 2025

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tests/codegen-llvm/naked-fn/dead-code-elimination.rs Outdated Show resolved Hide resolved

folkertdev changed the title ~~Naked function sections~~ naked functions: respect function-sections Oct 17, 2025

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folkertdev force-pushed the naked-function-sections branch from 2ebd197 to ac4b179 Compare October 17, 2025 14:39

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folkertdev force-pushed the naked-function-sections branch from ac4b179 to 6580a5f Compare October 17, 2025 14:51

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folkertdev force-pushed the naked-function-sections branch from 6580a5f to b12830c Compare November 7, 2025 17:00

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folkertdev added 3 commits November 9, 2025 11:41

naked functions: respect function_sections on windows

180984c

For `gnu` function_sections is off by default.

naked functions: respect function_sections on linux/macos

446da17

naked functions: add run-make test for DCE

77de006

folkertdev force-pushed the naked-function-sections branch from b12830c to 77de006 Compare November 9, 2025 10:42

rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. labels Jan 15, 2026

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JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Mar 28, 2026

Rollup merge of rust-lang#147811 - folkertdev:naked-function-sections…

57d9098

…, r=Amanieu naked functions: respect `function-sections` fixes rust-lang#147789 r? @Amanieu

JonathanBrouwer mentioned this pull request Mar 28, 2026

Rollup of 7 pull requests #154514

Closed

rust-bors bot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. and removed S-waiting-on-bors Status: Waiting on bors to run and complete tests. Bors will change the label on completion. labels Mar 28, 2026

folkertdev commented Mar 29, 2026

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