Optimize Extend for String

[paolobarbolini]

Implements the idea in #148604 (comment).
Enhancement to #149694.

Could we get a perf run?

[rustbot]

r? @joboet

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[Urgau]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 3f3acdf (3f3acdfbe44242af2a9a5419227d7c20c118425e, parent: fbab541a7ad1c22fc51783d03c7d75fa577f5633)

[rust-timer]

Finished benchmarking commit (3f3acdf): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.1%	[0.1%, 0.2%]	3
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-0.4%	[-1.3%, -0.1%]	4
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Max RSS (memory usage)

Results (secondary -2.4%)

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)	1.1%	[0.8%, 1.7%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.7%	[-6.4%, -0.9%]	11
All ❌✅ (primary)	-	-	0

Cycles

Results (primary -3.2%, secondary 1.4%)

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)	3.7%	[2.1%, 6.6%]	7
Improvements ✅ (primary)	-3.2%	[-3.2%, -3.2%]	1
Improvements ✅ (secondary)	-6.5%	[-6.8%, -6.2%]	2
All ❌✅ (primary)	-3.2%	[-3.2%, -3.2%]	1

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%	[-1.1%, -0.0%]	30
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	23
All ❌✅ (primary)	-0.1%	[-1.1%, -0.0%]	30

Bootstrap: 471.701s -> 471.964s (0.06%)
Artifact size: 388.90 MiB -> 388.53 MiB (-0.09%)

[paolobarbolini]

Interesting perf results. Let's see what happens when we specialize on array and Vec iterators...

@Urgau: Could we do another perf run please?

I hope this is ok for you @lolbinarycat since it takes from #148604, #148604 (comment) and #149694

[Urgau]

@bors try @rust-timer queue

[rust-bors]

💥 Test timed out after 21600s

[lolbinarycat]

I would appreciate a Co-Authored-by field on the first commit if you could (git rebase -i then edit on that commit)

[paolobarbolini]

Could we run perf again? The previous attempt failed after the try build timed out.

[lolbinarycat]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: ed1b0fb (ed1b0fb66582f4a21549e973103af3f8652d8b66, parent: 198328ad7960b1bece0dc48496bff6c62dd5d339)

[rust-timer]

Finished benchmarking commit (ed1b0fb): comparison URL.

Overall result: ❌✅ regressions and improvements - please read the text below

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.

Next Steps: If you can justify the regressions found in this try perf run, please do so in sufficient writing along with @rustbot label: +perf-regression-triaged. If not, please fix the regressions and do another perf run. If its results are neutral or positive, the label will be automatically removed.

@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)	0.5%	[0.3%, 0.8%]	3
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-0.1%	[-0.1%, -0.0%]	6
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

Max RSS (memory usage)

Results (primary 0.0%, secondary -3.5%)

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

	mean	range	count
Regressions ❌ (primary)	3.5%	[3.5%, 3.5%]	1
Regressions ❌ (secondary)	0.9%	[0.8%, 1.0%]	2
Improvements ✅ (primary)	-1.1%	[-1.5%, -0.9%]	3
Improvements ✅ (secondary)	-4.4%	[-6.9%, -1.6%]	10
All ❌✅ (primary)	0.0%	[-1.5%, 3.5%]	4

Cycles

Results (secondary 3.7%)

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)	4.4%	[1.8%, 7.5%]	9
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.0%	[-2.0%, -2.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary -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)	-	-	0
Improvements ✅ (primary)	-0.4%	[-1.1%, -0.1%]	4
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-1.1%, -0.1%]	4

Bootstrap: 471.708s -> 474.648s (0.62%)
Artifact size: 389.07 MiB -> 388.75 MiB (-0.08%)