Use iter::repeat_n to implement Vec::extend_with

[paolobarbolini]

This replaces the Vec::extend_with manual implementation, which is used by Vec::resize and Vec SpecFromElem, with iter::repeat_n.

I've compared the codegen output between:

1. the current Vec::resize impl
2. this branch
3. this branch + Safer implementation of RepeatN #130887

3 gives the closest codegen output to 1, with some output improvements. 2 doesn't look good: https://rust.godbolt.org/z/Yrc83EhjY.
May also help #120050?

---

WARNING: DO NOT MERGE - in order to run the perf run in #133662 (comment) this PR currently also contains commits from #130887

[rustbot]

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[joboet]

Let's see what effect this has in the compiler!
@bors try @rust-timer queue

[bors]

⌛ Trying commit 4bf7072 with merge 243c5cc...

[Noratrieb]

given that this branch doesn't look good without #130887, let's block it on that.
@rustbot blocked

[bors]

☀️ Try build successful - checks-actions
Build commit: 243c5cc (243c5cc6905efbdf778394d96f4b83701962964f)

[rust-timer]

Finished benchmarking commit (243c5cc): comparison URL.

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

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 may lead 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-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.3%	[0.1%, 0.7%]	16
Regressions ❌ (secondary)	0.4%	[0.1%, 0.6%]	9
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-1.3%	[-1.8%, -0.5%]	8
All ❌✅ (primary)	0.3%	[-0.9%, 0.7%]	17

Max RSS (memory usage)

Results (primary 0.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.6%	[0.9%, 11.4%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-8.1%	[-11.1%, -5.0%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[-11.1%, 11.4%]	5

Cycles

Results (secondary -2.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.1%	[-2.1%, -2.1%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.4%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.5%	[0.0%, 1.3%]	43
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	7
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[-0.2%, 1.3%]	50

Bootstrap: 773.805s -> 774.205s (0.05%)
Artifact size: 332.32 MiB -> 331.64 MiB (-0.20%)

[paolobarbolini]

Could we do another perf run? I've cherry-picked the commit from #130887 since, at least via Compiler Explorer, the codegen looked better

[joboet]

Sure!
@bors try @rust-timer queue

[bors]

⌛ Trying commit b080e18 with merge 78d5ed4...

[bors]

☀️ Try build successful - checks-actions
Build commit: 78d5ed4 (78d5ed40d7c459a7e5ea4cb6bea9918a493a800f)

[rust-timer]

Finished benchmarking commit (78d5ed4): comparison URL.

Overall result: ❌ regressions - please read the text below

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 may lead 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-perf +perf-regression

Instruction count

This is the most reliable metric that we have; it was used to determine the overall result at the top of this comment. However, even this metric can sometimes exhibit noise.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.1%, 0.7%]	17
Regressions ❌ (secondary)	1.3%	[0.1%, 3.2%]	16
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.4%	[0.1%, 0.7%]	17

Max RSS (memory usage)

Results (primary -5.6%, secondary 0.3%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	5.3%	[5.3%, 5.3%]	1
Regressions ❌ (secondary)	1.5%	[1.0%, 1.8%]	3
Improvements ✅ (primary)	-11.1%	[-15.5%, -6.8%]	2
Improvements ✅ (secondary)	-3.2%	[-3.2%, -3.2%]	1
All ❌✅ (primary)	-5.6%	[-15.5%, 5.3%]	3

Cycles

Results (primary -1.5%, secondary 35.1%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.9%	[0.9%, 0.9%]	1
Regressions ❌ (secondary)	35.1%	[33.0%, 39.1%]	6
Improvements ✅ (primary)	-4.0%	[-4.0%, -4.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.5%	[-4.0%, 0.9%]	2

Binary size

Results (primary 0.6%)

This is a less reliable metric that may be of interest but was not used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.7%	[0.1%, 1.7%]	33
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	9
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.6%	[-0.2%, 1.7%]	42

Bootstrap: 767.632s -> 769.266s (0.21%)
Artifact size: 332.19 MiB -> 332.16 MiB (-0.01%)

[paolobarbolini]

Looking at performance data borrow checking time in the Cycles benchmark for wg-grammar is the main culprit. I'm confused why that is.

The fact that this also fixes #120050 makes these changes still worth it to me.

[paolobarbolini]

@joboet @scottmcm Do you think the combination of #130887 + this PR, plus the fact it could also fix #120050, could have a chance of working out? I'm not too sure how to interpret the benchmarks and if there's a way of fixing the wg-grammar Cycles regression.