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Mono item collection microoptimizations - #162480

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Mono item collection microoptimizations#162480
Kobzol wants to merge 2 commits into
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Kobzol:mono-collect-3

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@Kobzol

@Kobzol Kobzol commented Sep 8, 2026

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Found this while staring at the mono item collection code. I kinda doubt it will even show up on our benchmarks, but it seems like a reasonable improvement nonetheless.

@rustbot rustbot added S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. T-compiler Relevant to the compiler team, which will review and decide on the PR/issue. labels Sep 8, 2026
@Kobzol

Kobzol commented Sep 8, 2026

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@bors try @rust-timer queue

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@rustbot rustbot added the S-waiting-on-perf Status: Waiting on a perf run to be completed. label Sep 8, 2026
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rust-bors Bot pushed a commit that referenced this pull request Sep 8, 2026
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Auto merge of #162480 - Kobzol:mono-collect-3, r=<try>
bfad934 
Mono item collection microoptimizations
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☀️ Try build successful (CI)
Build commit: bfad934 (bfad9347ddd9cacf4405e7aeb492433d97128891)
Base parent: 745de6e (745de6eca673de5329ec68f2689629a5ca45ab35)

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rust-timer commented Sep 8, 2026

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Finished benchmarking commit (bfad934): comparison URL.

Overall result: ❌✅ regressions and improvements - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@bors rollup=never rustc-perf
@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)		 0.3% [0.2%, 0.4%] 6
Improvements ✅
(primary)		 -0.3% [-0.4%, -0.2%] 3
Improvements ✅
(secondary)		 -0.3% [-0.3%, -0.3%] 1
All ❌✅ (primary) -0.3% [-0.4%, -0.2%] 3

Max RSS (memory usage)

Results (primary 0.5%)

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

mean range count
Regressions ❌
(primary)		 0.5% [0.4%, 0.7%] 7
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 - - 0
Improvements ✅
(secondary)		 - - 0
All ❌✅ (primary) 0.5% [0.4%, 0.7%] 7

Cycles

Results (primary 0.2%, secondary -3.4%)

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

mean range count
Regressions ❌
(primary)		 1.2% [0.4%, 2.6%] 6
Regressions ❌
(secondary)		 - - 0
Improvements ✅
(primary)		 -0.8% [-1.3%, -0.5%] 7
Improvements ✅
(secondary)		 -3.4% [-3.5%, -3.2%] 3
All ❌✅ (primary) 0.2% [-1.3%, 2.6%] 13

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 478.441s -> 477.072s (-0.29%)
Artifact size: 403.53 MiB -> 403.64 MiB (0.03%)

@rustbot rustbot added perf-regression Performance regression. and removed S-waiting-on-perf Status: Waiting on a perf run to be completed. labels Sep 8, 2026
@Kobzol

Kobzol commented Sep 8, 2026

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If you open the non-relevant results, it is very slightly green across the board, but the effect is very small. Anyway, I'll let the reviewer judge whether this is worth it or not :)

@Kobzol
Kobzol marked this pull request as ready for review September 8, 2026 17:28
@rustbot rustbot added the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 8, 2026
@rustbot rustbot removed the S-waiting-on-author Status: This is awaiting some action (such as code changes or more information) from the author. label Sep 8, 2026
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rustbot commented Sep 8, 2026

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r? @petrochenkov

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JonathanBrouwer
JonathanBrouwer reviewed Sep 8, 2026
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Comment thread compiler/rustc_monomorphize/src/collector.rs
def_path_str,
});
};
used_items.reserve(used.len());

@JonathanBrouwer JonathanBrouwer Sep 8, 2026 •
edited by rustbot
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Instead of reserving outside of the extend call which is easily forgotten, how about this implementation:
#162495

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JonathanBrouwer
JonathanBrouwer reviewed Sep 8, 2026
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Comment thread compiler/rustc_monomorphize/src/partitioning.rs
// from multiple root items within a CGU, which is fine, it just means
// the `insert` will be a no-op.
for inlined_item in reachable_inlined_items {
for inlined_item in &reachable_inlined_items {

@JonathanBrouwer JonathanBrouwer Sep 8, 2026 •
edited by rustbot
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An alternative which might be slightly nicer as it doesn't need a copy:
reachable_inlined_items.drain(..)
and remove the .clear() above

The current implementation is also fine by me tho, if you prefer it. I think both of our options should be identical for performance

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@Kobzol Kobzol Sep 8, 2026

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I think that iter + clear is generally faster than drain, because the implementation of drain tends to be complicated.

@JonathanBrouwer JonathanBrouwer mentioned this pull request Sep 8, 2026
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@Kobzol

Kobzol commented Sep 8, 2026

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Closing in favor of #162495.

@Kobzol Kobzol closed this Sep 8, 2026
@rustbot rustbot removed the S-waiting-on-review Status: Awaiting review from the assignee but also interested parties. label Sep 8, 2026
@Kobzol
Kobzol deleted the mono-collect-3 branch September 8, 2026 21:19
@JonathanBrouwer

JonathanBrouwer commented Sep 8, 2026 •
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That pr only replaces one of the two commits from this PR tho

JonathanBrouwer added a commit to JonathanBrouwer/rust that referenced this pull request Sep 8, 2026
@JonathanBrouwer
Rollup merge of rust-lang#162495 - JonathanBrouwer:reserve-mono-exten…
31971f0 
…d, r=Kobzol

Reserve items in `Extend` implementations

This might be a perf win, inspired by @Kobzol's approach in this PR: rust-lang#162480
@Kobzol

Kobzol commented Sep 8, 2026

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Yeah, but the perf. effect was essentially zero, so I didn't consider it to be worth it.

rust-bors Bot pushed a commit that referenced this pull request Sep 9, 2026
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Unrolled build for #162495 in rollup 162503
a2e42af 
Rollup merge of #162495 - JonathanBrouwer:reserve-mono-extend, r=Kobzol

Reserve items in `Extend` implementations

This might be a perf win, inspired by @Kobzol's approach in this PR: #162480
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