[experiment] Try to make index vec smaller when possible by panstromek · Pull Request #157799 · rust-lang/rust

panstromek · 2026-06-12T08:46:21Z

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This is very ugly and unsound, but I want to check CI and perf to see whether there's at least something to gain.

Notes

this roundtrips every mutation through vec, which might mask any gains if it's not optimized out properly. I hope I'm gonna see at least some change to see whether it's worth cleaning up.

TODO

not sure about Dyn[Send|Sync]
Why I had to change the lifetimes in ProjectedMaybeOwner? (Failed in borrowck in rustc_middle)
proper desc
proper commits

…try> [experiment] Try to make index vec smaller when possible

rust-bors · 2026-06-12T18:09:55Z

☀️ Try build successful (CI)
Build commit: 76803df (76803df0625aa385701a24c186e4526355371fd5, parent: a1e52fc1cf67929a7c01ed9c037520e276ec98fd)

rust-timer · 2026-06-12T18:50:58Z

Finished benchmarking commit (76803df): 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
@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)	0.4%	[0.2%, 0.7%]	155
Regressions ❌ (secondary)	0.4%	[0.1%, 1.0%]	107
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.9%, -0.4%]	7
All ❌✅ (primary)	0.4%	[0.2%, 0.7%]	155

Max RSS (memory usage)

Results (primary -0.9%, secondary -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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.9%	[-1.0%, -0.8%]	4
Improvements ✅ (secondary)	-1.2%	[-2.0%, -0.3%]	2
All ❌✅ (primary)	-0.9%	[-1.0%, -0.8%]	4

Cycles

Results (primary 2.2%)

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

	mean	range	count
Regressions ❌ (primary)	2.2%	[2.2%, 2.2%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	2.2%	[2.2%, 2.2%]	1

Binary size

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

Bootstrap: 519.113s -> 519.594s (0.09%)
Artifact size: 400.82 MiB -> 400.82 MiB (-0.00%)

…try> [experiment] Try to make index vec smaller when possible

rust-bors · 2026-06-14T06:43:20Z

☀️ Try build successful (CI)
Build commit: e8c5fee (e8c5feedca5bbedf67ca37e9d86d33007371e1f6, parent: 1f087276ba7a16d74ec04739ecf582ae495b3b79)

rust-timer · 2026-06-14T07:23:28Z

Finished benchmarking commit (e8c5fee): 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
@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)	0.4%	[0.2%, 0.7%]	162
Regressions ❌ (secondary)	0.4%	[0.2%, 0.8%]	118
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.5%	[-0.9%, -0.4%]	7
All ❌✅ (primary)	0.4%	[0.2%, 0.7%]	162

Max RSS (memory usage)

Results (primary -0.9%, secondary -1.9%)

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
Improvements ✅ (primary)	-0.9%	[-1.7%, -0.5%]	8
Improvements ✅ (secondary)	-1.9%	[-2.0%, -1.8%]	5
All ❌✅ (primary)	-0.9%	[-1.7%, -0.5%]	8

Cycles

Results (secondary 3.0%)

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)	3.0%	[2.2%, 4.1%]	7
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 526.872s -> 519.342s (-1.43%)
Artifact size: 401.48 MiB -> 400.98 MiB (-0.12%)

panstromek · 2026-06-15T15:07:25Z

@bors try @rust-timer queue

…try> [experiment] Try to make index vec smaller when possible

rust-bors · 2026-07-06T10:02:33Z

☀️ Try build successful (CI)
Build commit: 24ab8cd (24ab8cdecc3537bcc3f193657d78abe20d3cd650)
Base parent: 3659db0 (3659db0d3e2cd634c766fcda79ed118eca31a9fd)

rust-timer · 2026-07-06T11:01:46Z

Finished benchmarking commit (24ab8cd): 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
@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)	0.3%	[0.2%, 0.4%]	85
Regressions ❌ (secondary)	0.4%	[0.2%, 0.9%]	35
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.1%]	4
Improvements ✅ (secondary)	-0.3%	[-0.9%, -0.1%]	29
All ❌✅ (primary)	0.3%	[-0.3%, 0.4%]	89

Max RSS (memory usage)

Results (primary -1.0%, secondary 0.4%)

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)	5.0%	[1.5%, 8.5%]	2
Improvements ✅ (primary)	-1.0%	[-1.4%, -0.7%]	4
Improvements ✅ (secondary)	-2.6%	[-3.0%, -2.2%]	3
All ❌✅ (primary)	-1.0%	[-1.4%, -0.7%]	4

Cycles

Results (secondary 4.1%)

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)	4.1%	[4.1%, 4.1%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -0.0%, secondary -0.0%)

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
Improvements ✅ (primary)	-0.0%	[-0.1%, -0.0%]	31
Improvements ✅ (secondary)	-0.0%	[-0.1%, -0.0%]	5
All ❌✅ (primary)	-0.0%	[-0.1%, -0.0%]	31

Bootstrap: 491.532s -> 488.059s (-0.71%)
Artifact size: 388.93 MiB -> 388.51 MiB (-0.11%)

…revious green This lookup is the most common operation in try_mark_green

panstromek · 2026-07-06T20:13:50Z

@bors try @rust-timer queue

…try> [experiment] Try to make index vec smaller when possible

rust-bors · 2026-07-06T22:20:45Z

☀️ Try build successful (CI)
Build commit: 07d35df (07d35df71cb619d79cefd6d19713307996292218)
Base parent: 36714a9 (36714a9983d6ba11203d8bb87a1b372247fbcf06)

rust-timer · 2026-07-06T23:14:16Z

Finished benchmarking commit (07d35df): 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
@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)	0.4%	[0.2%, 0.7%]	96
Regressions ❌ (secondary)	0.4%	[0.2%, 0.9%]	42
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	2
Improvements ✅ (secondary)	-0.3%	[-0.8%, -0.1%]	24
All ❌✅ (primary)	0.4%	[-0.2%, 0.7%]	98

Max RSS (memory usage)

Results (primary -1.0%, secondary -3.9%)

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
Improvements ✅ (primary)	-1.0%	[-1.2%, -0.6%]	4
Improvements ✅ (secondary)	-3.9%	[-8.0%, -1.6%]	6
All ❌✅ (primary)	-1.0%	[-1.2%, -0.6%]	4

Cycles

Results (secondary -11.3%)

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)	3.5%	[3.5%, 3.5%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-18.6%	[-34.2%, -3.1%]	2
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 489.375s -> 488.234s (-0.23%)
Artifact size: 388.97 MiB -> 389.03 MiB (0.02%)

…y_mark_previous green" This reverts commit c89fc82.

panstromek · 2026-07-07T06:29:39Z

It seems like no matter what I do in try_mark_previous_green, the result is slower, which is pretty weird given that the changes are usually just extracting as_slice call out of a hot loop. Let's try to get back and re-measure the most positive result so far (the exec_cache_promotions change), maybe something got changed on main and changes the baseline measurement.

@bors try @rust-timer queue

…try> [experiment] Try to make index vec smaller when possible

rust-bors · 2026-07-07T08:36:00Z

☀️ Try build successful (CI)
Build commit: c7db7b7 (c7db7b71ea016e508e29d99a6f84aff65f37a5f4)
Base parent: b960fcf (b960fcf2ff0f04967b30b947be8fc155fb067901)

rust-timer · 2026-07-07T09:31:48Z

Finished benchmarking commit (c7db7b7): 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
@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)	0.2%	[0.2%, 0.3%]	23
Regressions ❌ (secondary)	0.2%	[0.1%, 0.4%]	13
Improvements ✅ (primary)	-0.2%	[-0.3%, -0.1%]	5
Improvements ✅ (secondary)	-0.3%	[-0.9%, -0.1%]	37
All ❌✅ (primary)	0.1%	[-0.3%, 0.3%]	28

Max RSS (memory usage)

Results (primary 0.5%, secondary -2.2%)

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

	mean	range	count
Regressions ❌ (primary)	4.0%	[2.1%, 5.9%]	2
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-1.3%	[-1.9%, -0.6%]	4
Improvements ✅ (secondary)	-2.2%	[-2.6%, -1.7%]	5
All ❌✅ (primary)	0.5%	[-1.9%, 5.9%]	6

Cycles

Results (primary -2.3%, secondary 0.2%)

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)	3.8%	[3.8%, 3.8%]	1
Improvements ✅ (primary)	-2.3%	[-2.3%, -2.3%]	1
Improvements ✅ (secondary)	-3.4%	[-3.4%, -3.4%]	1
All ❌✅ (primary)	-2.3%	[-2.3%, -2.3%]	1

Binary size

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

Bootstrap: 489.183s -> 493.103s (0.80%)
Artifact size: 388.47 MiB -> 388.61 MiB (0.04%)

rust-bors · 2026-07-28T01:11:11Z

☔ The latest upstream changes (presumably #158059) made this pull request unmergeable. Please resolve the merge conflicts by rebasing.

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 Jun 12, 2026