Use index based drop loop for slices and arrays

[tmiasko]

Instead of building two kinds of drop pair loops, of which only one will be eventually used at runtime in a given monomorphization, always use index based loop.

[tmiasko]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 5e5d676de9ddf29daa6b3f3f35fb3a3b84d32473 with merge 4ab473db23ab00fd0f20e0d2e533fff23afed969...

[bors]

☀️ Try build successful - checks-actions
Build commit: 4ab473db23ab00fd0f20e0d2e533fff23afed969 (4ab473db23ab00fd0f20e0d2e533fff23afed969)

[rust-timer]

Finished benchmarking commit (4ab473db23ab00fd0f20e0d2e533fff23afed969): 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)	0.3%	[0.3%, 0.3%]	1
Regressions ❌ (secondary)	0.6%	[0.4%, 0.7%]	2
Improvements ✅ (primary)	-0.6%	[-0.7%, -0.5%]	4
Improvements ✅ (secondary)	-0.4%	[-0.4%, -0.3%]	7
All ❌✅ (primary)	-0.5%	[-0.7%, 0.3%]	5

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)	2.9%	[2.9%, 2.9%]	1
Improvements ✅ (primary)	-3.8%	[-5.3%, -2.8%]	3
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-3.8%	[-5.3%, -2.8%]	3

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)	-	-	0
Improvements ✅ (primary)	-0.8%	[-0.8%, -0.8%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.8%	[-0.8%, -0.8%]	1

[tmiasko]

The pointer based variant was present in the first MIR implementation, but it
doesn't seem to lead to any optimization opportunities. For simplicity of the
implementation, and to avoid generating code for both variants, this proposes
to keep only index based variant.

r? compiler

[WaffleLapkin]

For reference, I've reproduced index/pointer based drop impls for arrays and they seem to produce exactly the same assembly: https://godbolt.org/z/T89evrqK6

[WaffleLapkin]

The perf looks mostly like noise, codegen behaves a bit differently with different input, maybe.

[WaffleLapkin]

r? @WaffleLapkin
@bors r+

[bors]

📌 Commit cf0e78b has been approved by WaffleLapkin

It is now in the queue for this repository.

[bors]

⌛ Testing commit cf0e78b with merge 2d64f22...

[bors]

☀️ Test successful - checks-actions
Approved by: WaffleLapkin
Pushing 2d64f22 to master...

[rust-timer]

Finished benchmarking commit (2d64f22): comparison URL.

Overall result: ❌✅ regressions and improvements - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	0.9%	[0.9%, 0.9%]	1
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.4%	[-0.4%, -0.4%]	1

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)	1.1%	[0.1%, 2.6%]	4
Regressions ❌ (secondary)	3.2%	[3.2%, 3.2%]	1
Improvements ✅ (primary)	-2.7%	[-6.4%, -0.5%]	3
Improvements ✅ (secondary)	-2.8%	[-2.8%, -2.8%]	1
All ❌✅ (primary)	-0.5%	[-6.4%, 2.6%]	7

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)	-	-	0
Improvements ✅ (primary)	-2.0%	[-3.6%, -0.5%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-3.6%, -0.5%]	2