Improve alloc Vec::retain_mut performance

[fereidani]

Hi,

While reading the rustc source code, I noticed it uses smallvec and thin-vec in many places. I started reviewing those crates, optimized their retain_mut implementation, and then realized they were using the exact same algorithm as alloc::vec::Vec with less unsafe So now I’m back here with a PR for the standard library 😂.

In my benchmarks, this version is noticeably faster when retain_mut actually removes elements (thanks to fewer pointer operations, it just advances write_index), while performing identically to the current implementation when nothing is removed.

Let’s see if bors likes this change or not.

[rustbot]

r? @joboet

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[fereidani]

@bors try @rust-timer queue

[rust-bors]

@fereidani: 🔑 Insufficient privileges: not in try users

[Kivooeo]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 060d14c (060d14cd2be739e62dd1e1c6daeb2b80ec651c10, parent: 37aa2135b5d0936bd13aa699d941aaa94fbaa645)

[rust-timer]

Finished benchmarking commit (060d14c): comparison URL.

Overall result: ❌ regressions - 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)	0.4%	[0.3%, 0.5%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

Max RSS (memory usage)

Results (primary -3.0%, secondary 6.7%)

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)	6.7%	[6.7%, 6.7%]	1
Improvements ✅ (primary)	-3.0%	[-4.5%, -1.5%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.0%	[-4.5%, -1.5%]	2

Cycles

Results (primary 3.0%, secondary -2.8%)

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

	mean	range	count
Regressions ❌ (primary)	3.0%	[2.0%, 4.9%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.8%	[-3.1%, -2.4%]	3
All ❌✅ (primary)	3.0%	[2.0%, 4.9%]	3

Binary size

Results (primary -0.1%, secondary -0.1%)

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)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.1%, -0.0%]	41
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	24
All ❌✅ (primary)	-0.1%	[-0.1%, -0.0%]	41

Bootstrap: 470.452s -> 473.09s (0.56%)
Artifact size: 389.00 MiB -> 388.97 MiB (-0.01%)

[Kivooeo]

is it ready for another perf run?

[fereidani]

@Kivooeo Yes, Thank you!

[Kivooeo]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3ec64be (3ec64beb1b3da0ca553fcc7f6fb4e8f036199568, parent: d5525a73009e2c61b09daa69df79064530bd4dcf)

[rust-timer]

Finished benchmarking commit (3ec64be): comparison URL.

Overall result: ❌✅ regressions and 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)	0.3%	[0.1%, 0.5%]	3
Improvements ✅ (primary)	-0.2%	[-0.2%, -0.2%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.1%]	1
All ❌✅ (primary)	-0.2%	[-0.2%, -0.2%]	1

Max RSS (memory usage)

Results (primary -2.3%)

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)	-	-	0
Improvements ✅ (primary)	-2.3%	[-2.7%, -1.9%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.3%	[-2.7%, -1.9%]	2

Cycles

Results (secondary 1.0%)

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)	2.7%	[2.3%, 3.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.5%	[-2.5%, -2.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

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

Bootstrap: 472.272s -> 473.265s (0.21%)
Artifact size: 389.04 MiB -> 388.98 MiB (-0.02%)