Allow to disable thinLTO buffer to support lto-embed-bitcode lld feature by darkness-ai · Pull Request #98162 · rust-lang/rust

darkness-ai · 2022-06-16T08:49:57Z

Hello
This change is to fix issue (#84395) in which passing "-lto-embed-bitcode=optimized" to lld when linking rust code via linker-plugin-lto doesn't produce the expected result.

Instead of emitting a single unified module into a llvmbc section of the linked elf, it emits multiple submodules.
This is caused because rustc emits the BC modules after running llvm createWriteThinLTOBitcodePass pass.
Which in turn triggers a thinLTO linkage and causes the said issue.

This patch allows via compiler flag (-Cemit-thin-lto=) to select between running createWriteThinLTOBitcodePass and createBitcodeWriterPass.
Note this pattern of selecting between those 2 passes is common inside of LLVM code.
The default is to match the old behavior.

rust-highfive · 2022-06-16T08:50:00Z

Thanks for the pull request, and welcome! The Rust team is excited to review your changes, and you should hear from @lcnr (or someone else) soon.

Please see the contribution instructions for more information.

lcnr · 2022-06-28T10:33:14Z

seems like i missed the notification for this PR 😅 sorry

r? rust-lang/compiler

darkness-ai · 2022-07-03T08:36:14Z

@rustbot label -S-waiting-on-author +S-waiting-on-review

darkness-ai · 2022-07-03T09:50:31Z

@rustbot label -S-waiting-on-review +S-waiting-on-author

darkness-ai · 2022-07-03T13:38:24Z

@rustbot label -S-waiting-on-author +S-waiting-on-review

davidtwco

Do you think you could add a test case that uses this and -lto-embed-bitcode=optimized to confirm that it fixes the issue?

darkness-ai · 2022-07-09T12:55:43Z

@rustbot label -S-waiting-on-author +S-waiting-on-review

bors · 2022-07-10T13:26:31Z

@darkness-ai: 🔑 Insufficient privileges: Not in reviewers

davidtwco · 2022-07-11T07:44:20Z

@bors r+

bors · 2022-07-11T07:44:22Z

📌 Commit 3858cddaec9a7480b94bfa8432c9ed4df13b0478 has been approved by davidtwco

It is now in the queue for this repository.

Adding the option to control from rustc CLI if the resulted ".o" bitcode module files are with thinLTO info or regular LTO info. Allows using "-lto-embed-bitcode=optimized" during linkage correctly. Signed-off-by: Ziv Dunkelman <ziv.dunkelman@nextsilicon.com>

darkness-ai · 2022-07-14T19:25:12Z

@davidtwco saw PRs conflicting with this one got merged into master.
Pushed fixes for the mismatch.

davidtwco · 2022-07-15T08:10:06Z

@bors r+

bors · 2022-07-15T08:10:08Z

📌 Commit 724c912 has been approved by davidtwco

It is now in the queue for this repository.

bors · 2022-07-21T10:14:02Z

⌛ Testing commit 724c912 with merge 74f600b...

bors · 2022-07-21T13:05:16Z

☀️ Test successful - checks-actions
Approved by: davidtwco
Pushing 74f600b to master...

rust-timer · 2022-07-21T14:21:06Z

Finished benchmarking commit (74f600b): comparison url.

Instruction count

Primary benchmarks: 🎉 relevant improvements found
Secondary benchmarks: no relevant changes found

	mean¹	max	count²
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	-0.4%	-0.4%	2
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	-0.4%	-0.4%	2

Max RSS (memory usage)

Results

Primary benchmarks: 🎉 relevant improvement found
Secondary benchmarks: 😿 relevant regression found

	mean¹	max	count²
Regressions 😿 (primary)	N/A	N/A	0
Regressions 😿 (secondary)	5.7%	5.7%	1
Improvements 🎉 (primary)	-2.2%	-2.2%	1
Improvements 🎉 (secondary)	N/A	N/A	0
All 😿🎉 (primary)	-2.2%	-2.2%	1

Cycles

Results

Primary benchmarks: 😿 relevant regression found
Secondary benchmarks: 🎉 relevant improvement found

	mean¹	max	count²
Regressions 😿 (primary)	1.8%	1.8%	1
Regressions 😿 (secondary)	N/A	N/A	0
Improvements 🎉 (primary)	N/A	N/A	0
Improvements 🎉 (secondary)	-2.5%	-2.5%	1
All 😿🎉 (primary)	1.8%	1.8%	1

If you disagree with this performance assessment, please file an issue in rust-lang/rustc-perf.

@rustbot label: -perf-regression

the arithmetic mean of the percent change↩↩² ↩³
number of relevant changes↩↩² ↩³

…uviper Remove -Zemit-thin-lto flag As far as I can tell it was introduced in rust-lang#98162 to allow fat LTO with `-Clinker-plugin-lto`. In rust-lang#136840 a change was made to automatically disable ThinLTO summary generation when `-Clinker-plugin-lto -Clto=fat` is used, so we can safely remove it. Fixesrust-lang#152490

Rollup merge of #152527 - bjorn3:remove_z_emit_thin_lto, r=cuviper Remove -Zemit-thin-lto flag As far as I can tell it was introduced in #98162 to allow fat LTO with `-Clinker-plugin-lto`. In #136840 a change was made to automatically disable ThinLTO summary generation when `-Clinker-plugin-lto -Clto=fat` is used, so we can safely remove it. Fixes#152490

Remove -Zemit-thin-lto flag As far as I can tell it was introduced in rust-lang/rust#98162 to allow fat LTO with `-Clinker-plugin-lto`. In rust-lang/rust#136840 a change was made to automatically disable ThinLTO summary generation when `-Clinker-plugin-lto -Clto=fat` is used, so we can safely remove it. Fixesrust-lang/rust#152490

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bors added the merged-by-bors This PR was explicitly merged by bors. label Jul 21, 2022

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bors mentioned this pull request Jul 21, 2022
RFC-2841: add codegen flag export symbols from executable #85673
Merged

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This was referenced Feb 11, 2026
Remove -Zemit-thin-lto flag #152490
Closed
Remove -Zemit-thin-lto flag #152527
Merged

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darkness-ai commented Jun 16, 2022•
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