naked functions: respect function-sections

[folkertdev]

fixes #147789

r? @Amanieu

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[Amanieu]

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

[Amanieu]

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?

[folkertdev]

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?

[folkertdev]

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?

[Amanieu]

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

[folkertdev]

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

[bors]

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

[rustbot]

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.

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spellchecking files
building external tool typos from package typos-cli@1.38.1
finished building tool typos
npm WARN deprecated puppeteer@22.15.0: < 24.10.2 is no longer supported
npm ERR! code 127
npm ERR! git dep preparation failed
npm ERR! command /node/bin/node /node/lib/node_modules/npm/bin/npm-cli.js install --force --cache=/home/user/.npm --prefer-offline=false --prefer-online=false --offline=false --no-progress --no-save --no-audit --include=dev --include=peer --include=optional --no-package-lock-only --no-dry-run
npm ERR! npm WARN using --force Recommended protections disabled.
npm ERR! npm WARN deprecated inflight@1.0.6: This module is not supported, and leaks memory. Do not use it. Check out lru-cache if you want a good and tested way to coalesce async requests by a key value, which is much more comprehensive and powerful.
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated glob@7.2.3: Glob versions prior to v9 are no longer supported
npm ERR! npm WARN deprecated readdir-scoped-modules@1.1.0: This functionality has been moved to @npmcli/fs
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npm ERR! npm ERR! code 127
npm ERR! npm ERR! path /home/user/.npm/_cacache/tmp/git-cloneXXXXXXFFqMuu/node_modules/rollup
npm ERR! npm ERR! command failed
npm ERR! npm ERR! command sh -c patch-package
npm ERR! npm ERR! sh: 1: patch-package: not found
npm ERR! 
npm ERR! npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T17_06_48_002Z-debug-0.log

npm ERR! A complete log of this run can be found in: /home/user/.npm/_logs/2025-11-07T17_06_40_599Z-debug-0.log
npm install did not exit successfully
tidy [extra_checks]: IO error: npm install returned exit code exit status: 127
tidy [extra_checks]: FAIL
tidy: The following check failed: extra_checks
Command `/checkout/obj/build/x86_64-unknown-linux-gnu/stage1-tools-bin/rust-tidy /checkout /checkout/obj/build/x86_64-unknown-linux-gnu/stage0/bin/cargo /checkout/obj/build 4 /node/bin/npm --extra-checks=py,cpp,js,spellcheck` failed with exit code 1
Created at: src/bootstrap/src/core/build_steps/tool.rs:1549:23
Executed at: src/bootstrap/src/core/build_steps/test.rs:1279:29

Command has failed. Rerun with -v to see more details.
Bootstrap failed while executing `test src/tools/tidy tidyselftest --extra-checks=py,cpp,js,spellcheck`
Build completed unsuccessfully in 0:02:59
  local time: Fri Nov  7 17:07:06 UTC 2025
  network time: Fri, 07 Nov 2025 17:07:06 GMT
##[error]Process completed with exit code 1.

[folkertdev]

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