[WIP] rustc_metadata: Switch each_child_of_item from callback to iterator

[petrochenkov]

to avoid allocations.

[rust-highfive]

r? @jackh726

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[petrochenkov]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit b738128c6398f4751333ed9acd5e79d11763e1ad with merge c11423e489f70294eb5ece1e027fc8e9b0c376ec...

[bors]

☀️ Try build successful - checks-actions
Build commit: c11423e489f70294eb5ece1e027fc8e9b0c376ec (c11423e489f70294eb5ece1e027fc8e9b0c376ec)

[rust-timer]

Queued c11423e489f70294eb5ece1e027fc8e9b0c376ec with parent 59337cd, future comparison URL.

[rust-timer]

Finished benchmarking commit (c11423e489f70294eb5ece1e027fc8e9b0c376ec): comparison url.

Summary: This change led to very large relevant regressions 😿 in compiler performance.

• Very large regression in instruction counts (up to 13.7% on incr-unchanged builds of issue-88862)

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

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

[petrochenkov]

WTF 😂
Is rustc+LLVM really that bad at inlining a few layers of iterators? Or is it chain to blame?

Anyway, this is waiting on #92245, #92153, #92086, #92034, and #91795.
I'll re-measure once those PRs are merged or closed.

[mati865]

Is rustc+LLVM really that bad at inlining a few layers of iterators? Or is it chain to blame?

I've seen edge cases when chain(..) performs terrible but usually it's more about the iterators or their content. One of extreme examples: https://rust.godbolt.org/z/Paf5ofnrq

[petrochenkov]

@bors try @rust-timer queue

[rust-timer]

Awaiting bors try build completion.

@rustbot label: +S-waiting-on-perf

[bors]

⌛ Trying commit 7bd38c9 with merge de48e65ece49a289d7202477e406fd727029f15a...

[bors]

☀️ Try build successful - checks-actions
Build commit: de48e65ece49a289d7202477e406fd727029f15a (de48e65ece49a289d7202477e406fd727029f15a)

[rust-timer]

Queued de48e65ece49a289d7202477e406fd727029f15a with parent d981633, future comparison URL.

[rust-timer]

Finished benchmarking commit (de48e65ece49a289d7202477e406fd727029f15a): comparison url.

Summary: This benchmark run shows 47 relevant regressions 😿 to instruction counts.

• Arithmetic mean of relevant regressions: 0.6%
• Largest regression in instruction counts: 2.8% on incr-full builds of coercions debug

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

Benchmarking this pull request likely means that it is perf-sensitive, so we're automatically marking it as not fit for rolling up. While you can manually mark this PR as fit for rollup, we strongly recommend not doing so since this PR led to changes in compiler perf.

Next Steps: If you can justify the regressions found in this try perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please fix the regressions and do another perf run. If the next run shows neutral or positive results, the label will be automatically removed.

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

[jackh726]

Let me know when this is ready for review.

[petrochenkov]

Looks like it's still not an improvement.