Improve niche placement by trying two strategies and picking the better result

[the8472]

Fixes #104807
Fixes #105371

Determining which sort order is better requires calculating the struct size (so we can calculate the niche offset). But that in turn depends on the field order, so happens after sorting. So the simple way to solve that is to run the whole thing twice and pick the better result.

1st commit is just code motion, the meat is in the later ones.

[rustbot]

r? @Nilstrieb

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[the8472]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 552fd837688a2b7c9197527c90de7aa89475cfc7 with merge 38dc63170b19f9212a1cefea8fd5368ab4322886...

[bors]

☀️ Try build successful - checks-actions
Build commit: 38dc63170b19f9212a1cefea8fd5368ab4322886 (38dc63170b19f9212a1cefea8fd5368ab4322886)

[bors]

☀️ Try build successful - checks-actions
Build commit: 38dc63170b19f9212a1cefea8fd5368ab4322886 (38dc63170b19f9212a1cefea8fd5368ab4322886)

[rust-timer]

Finished benchmarking commit (38dc63170b19f9212a1cefea8fd5368ab4322886): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

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 a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.6%	[0.3%, 1.1%]	89
Regressions ❌ (secondary)	0.9%	[0.3%, 2.6%]	20
Improvements ✅ (primary)	-0.8%	[-1.1%, -0.5%]	2
Improvements ✅ (secondary)	-0.9%	[-1.7%, -0.2%]	17
All ❌✅ (primary)	0.5%	[-1.1%, 1.1%]	91

Max RSS (memory usage)

Results

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)	2.7%	[2.2%, 3.7%]	3
Improvements ✅ (primary)	-1.5%	[-1.5%, -1.5%]	1
Improvements ✅ (secondary)	-4.4%	[-5.8%, -3.1%]	3
All ❌✅ (primary)	-1.5%	[-1.5%, -1.5%]	1

Cycles

Results

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)	2.6%	[2.4%, 2.8%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

[the8472]

@bors try @rust-timer queue

[bors]

⌛ Trying commit f0e024d22db7492c0848a05e15da88300bb7de96 with merge 8123ebf2c5823201a7c31a8ad8c32332ea3b8b57...

[bors]

☀️ Try build successful - checks-actions
Build commit: 8123ebf2c5823201a7c31a8ad8c32332ea3b8b57 (8123ebf2c5823201a7c31a8ad8c32332ea3b8b57)

[bors]

☀️ Try build successful - checks-actions
Build commit: 8123ebf2c5823201a7c31a8ad8c32332ea3b8b57 (8123ebf2c5823201a7c31a8ad8c32332ea3b8b57)

[bors]

⌛ Testing commit a141f5b87bd94eb022e074cf7bce4fcb6d10d406 with merge 8f97ac75df44edcab355eabae3a6fbde6950c57e...

[bors]

💔 Test failed - checks-actions

[the8472]

excluded 32bit targets on a codegen tests because llvm seems to make different choices there (memset vs. stores)

@bors r=wesleywiser

[bors]

📌 Commit 61fb5a9 has been approved by wesleywiser

It is now in the queue for this repository.

[bors]

⌛ Testing commit 61fb5a9 with merge f229949...

[bors]

☀️ Test successful - checks-actions
Approved by: wesleywiser
Pushing f229949 to master...

[rust-timer]

Finished benchmarking commit (f229949): comparison URL.

Overall result: ❌✅ regressions and improvements - ACTION NEEDED

Next Steps: If you can justify the regressions found in this perf run, please indicate this with @rustbot label: +perf-regression-triaged along with sufficient written justification. If you cannot justify the regressions please open an issue or create a new PR that fixes the regressions, add a comment linking to the newly created issue or PR, and then add the perf-regression-triaged label to this PR.

@rustbot label: +perf-regression
cc @rust-lang/wg-compiler-performance

Instruction count

This is a highly reliable metric that was used to determine the overall result at the top of this comment.

	mean	range	count
Regressions ❌ (primary)	0.4%	[0.2%, 0.6%]	7
Regressions ❌ (secondary)	0.7%	[0.4%, 1.0%]	12
Improvements ✅ (primary)	-2.6%	[-8.5%, -0.2%]	4
Improvements ✅ (secondary)	-1.2%	[-2.8%, -0.3%]	31
All ❌✅ (primary)	-0.7%	[-8.5%, 0.6%]	11

Max RSS (memory usage)

Results

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)	0.8%	[0.8%, 0.8%]	1
Regressions ❌ (secondary)	2.3%	[2.3%, 2.3%]	1
Improvements ✅ (primary)	-5.1%	[-8.9%, -2.8%]	3
Improvements ✅ (secondary)	-2.1%	[-4.3%, -0.5%]	5
All ❌✅ (primary)	-3.6%	[-8.9%, 0.8%]	4

Cycles

Results

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)	1.9%	[1.1%, 3.8%]	60
Regressions ❌ (secondary)	2.9%	[2.1%, 4.2%]	14
Improvements ✅ (primary)	-3.7%	[-8.4%, -1.0%]	3
Improvements ✅ (secondary)	-6.3%	[-9.2%, -1.2%]	9
All ❌✅ (primary)	1.6%	[-8.4%, 3.8%]	63

[pnkfelix]

• addresses a real performance regression (#102750 regresses layout size of Cow<> #105371) to how we laid out niches.
• primary big benefit to syn. rest of primary changes may be noise; the secondary improvements are nice to have.
• marking as triaged.

@rustbot label: +perf-regression-triaged