Split LTO out of the main codegen coordinator event loop into a separate event loop on the same thread

[bjorn3]

This will make it easier to in the future move all this code to link_binary.

Follow up to #146209
Part of rust-lang/compiler-team#908

[rustbot]

Some changes occurred in compiler/rustc_codegen_ssa

cc @WaffleLapkin

[rustbot]

r? @jackh726

rustbot has assigned @jackh726.
They will have a look at your PR within the next two weeks and either review your PR or reassign to another reviewer.

Use r? to explicitly pick a reviewer

[bjorn3]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: c8901ae (c8901aea8472c485ddb8304c562f06f8b77a55ca, parent: a41214f9bd7ce869d21898f31107f92c90a9112f)

[rust-timer]

Finished benchmarking commit (c8901ae): comparison URL.

Overall result: ✅ improvements - no action needed

Benchmarking this pull request means it may be perf-sensitive – we'll automatically label it not fit for rolling up. You can override this, but we strongly advise not to, due to possible changes in compiler perf.

@bors rollup=never
@rustbot label: -S-waiting-on-perf -perf-regression

Instruction count

Our most reliable metric. Used to determine the overall result above. However, even this metric can be noisy.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-1.0%, -0.4%]	7
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -1.2%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-3.1%	[-3.1%, -3.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.2%	[-3.1%, 0.8%]	2

Cycles

Results (secondary 2.5%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	2.5%	[2.5%, 2.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 476.516s -> 476.021s (-0.10%)
Artifact size: 390.37 MiB -> 390.39 MiB (0.01%)

[wesleywiser]

@bors r+

[bors]

📌 Commit a077dbd has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit a077dbd with merge 6244eff...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing 6244eff to master...

[github-actions]

What is this?

This is an experimental post-merge analysis report that shows differences in test outcomes between the merged PR and its parent PR.

Comparing 1d23d06 (parent) -> 6244eff (this PR)

Test differences

Show 2 test diffs

2 doctest diffs were found. These are ignored, as they are noisy.

Test dashboard

Run

cargo run --manifest-path src/ci/citool/Cargo.toml -- \
    test-dashboard 6244effd0372d5d88fc859d3bf17ce1efcc2c9ec --output-dir test-dashboard

And then open test-dashboard/index.html in your browser to see an overview of all executed tests.

Job duration changes

1. dist-aarch64-linux: 8776.6s -> 5970.3s (-32.0%)
2. dist-x86_64-apple: 7479.0s -> 6082.3s (-18.7%)
3. i686-gnu-nopt-1: 8244.7s -> 7114.9s (-13.7%)
4. aarch64-gnu-llvm-20-1: 3871.3s -> 3372.3s (-12.9%)
5. dist-s390x-linux: 5655.5s -> 4965.3s (-12.2%)
6. dist-aarch64-msvc: 6150.3s -> 5409.1s (-12.1%)
7. x86_64-gnu-miri: 4929.9s -> 4347.6s (-11.8%)
8. x86_64-rust-for-linux: 2954.3s -> 2609.3s (-11.7%)
9. i686-gnu-2: 6070.9s -> 5364.1s (-11.6%)
10. aarch64-apple: 8390.6s -> 7508.3s (-10.5%)

How to interpret the job duration changes?

Job durations can vary a lot, based on the actual runner instance
that executed the job, system noise, invalidated caches, etc. The table above is provided
mostly for t-infra members, for simpler debugging of potential CI slow-downs.

[rust-timer]

Finished benchmarking commit (6244eff): comparison URL.

Overall result: no relevant changes - no action needed

@rustbot label: -perf-regression

Instruction count

This benchmark run did not return any relevant results for this metric.

Max RSS (memory usage)

Results (secondary 0.7%)

A less reliable metric. May be of interest, but not used to determine the overall result above.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	0.7%	[0.7%, 0.7%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Cycles

This benchmark run did not return any relevant results for this metric.

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 472.957s -> 471.983s (-0.21%)
Artifact size: 390.69 MiB -> 390.70 MiB (0.00%)