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

folkertdev · 2025-10-17T14:02:32Z

rustbot · 2025-10-17T14:02:35Z

Some changes occurred in compiler/rustc_codegen_ssa

Amanieu · 2025-10-21T09:39:21Z

+            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

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.

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

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

JonathanBrouwer · 2026-04-16T16:34:29Z

@bors r-

Probably this pr?
#155398 (comment)

rust-bors · 2026-04-16T16:34:38Z

This pull request was unapproved.

This PR was contained in a rollup (#155398), which was unapproved.

folkertdev · 2026-04-16T19:15:51Z

@bors try jobs=dist-various-1,dist-various-2,aarch64-apple,x86_64-mingw-1

naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

rust-bors · 2026-04-16T22:09:16Z

💔 Test for 1a49225 failed: CI. Failed job:

try - aarch64-apple (web logs, enhanced plaintext logs)

folkertdev · 2026-04-16T22:56:49Z

hmm

@bors try jobs=dist-various-1,dist-various-2,aarch64-apple,x86_64-mingw-1

naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

rust-bors · 2026-04-17T01:55:53Z

💔 Test for 74be459 failed: CI. Failed job:

try - aarch64-apple (web logs, enhanced plaintext logs)

folkertdev · 2026-04-17T10:40:29Z

ok whatever aarch64

@bors try jobs=dist-various-1,dist-various-2,aarch64-apple,x86_64-mingw-1

naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

rust-bors · 2026-04-17T14:02:37Z

☀️ Try build successful (CI)
Build commit: 30ba749 (30ba749b739d451baf017c104599a074a054f48f, parent: 1b8f2e46e14b08208a53585570edd9206374aae8)

folkertdev · 2026-04-17T14:04:55Z

@bors iffy
@bors r=Amanieu

rust-bors · 2026-04-17T14:04:59Z

Unknown command "iffy". Run @bors help to see available commands.

rust-bors · 2026-04-17T14:05:00Z

📌 Commit 2db9de3 has been approved by Amanieu

It is now in the queue for this repository.

folkertdev · 2026-04-17T14:06:34Z

@bors rollup=iffy

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Comment thread tests/codegen-llvm/naked-fn/dead-code-elimination.rs Outdated

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

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Comment thread compiler/rustc_codegen_ssa/src/mir/naked_asm.rs Outdated

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Comment thread compiler/rustc_codegen_ssa/src/mir/naked_asm.rs Outdated

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Comment thread compiler/rustc_codegen_ssa/src/mir/naked_asm.rs Outdated

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

5aa60e9

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

JonathanBrouwer mentioned this pull request Apr 16, 2026

Rollup of 15 pull requests #155398

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Auto merge of #147811 - folkertdev:naked-function-sections, r=<try>

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naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

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Auto merge of #147811 - folkertdev:naked-function-sections, r=<try>

74be459

naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

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add ignore-cross-compile to run-make test

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folkertdev force-pushed the naked-function-sections branch from c186ad4 to 2db9de3 Compare April 17, 2026 09:26

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Auto merge of #147811 - folkertdev:naked-function-sections, r=<try>

30ba749

naked functions: respect `function-sections` try-job: dist-various-1 try-job: dist-various-2 try-job: aarch64-apple try-job: x86_64-mingw-1

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