[perf experiment] try const dispatch instead of macros for slice::iter::next

[the8472]

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[rustbot]

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[the8472]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 505e294 with merge dac6212...

[bors]

☀️ Try build successful - checks-actions
Build commit: dac6212 (dac621274200ca2721505796b8d62f81cc2316c2)

[rust-timer]

Finished benchmarking commit (dac6212): 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)	1.1%	[0.3%, 3.5%]	4
Regressions ❌ (secondary)	0.4%	[0.3%, 0.4%]	3
Improvements ✅ (primary)	-0.6%	[-0.8%, -0.3%]	2
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[-0.8%, 3.5%]	6

Max RSS (memory usage)

Results (primary -0.5%)

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)	3.5%	[0.9%, 4.9%]	3
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-2.9%	[-4.4%, -1.7%]	5
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-4.4%, 4.9%]	8

Cycles

Results (primary 1.2%, secondary 1.8%)

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)	3.3%	[3.3%, 3.3%]	1
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-0.9%	[-0.9%, -0.9%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.2%	[-0.9%, 3.3%]	2

Binary size

Results (primary 0.1%, 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)	0.1%	[0.0%, 0.4%]	38
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.1%	[-0.2%, -0.0%]	16
Improvements ✅ (secondary)	-0.3%	[-1.1%, -0.0%]	4
All ❌✅ (primary)	0.1%	[-0.2%, 0.4%]	54

Bootstrap: 778.956s -> 779.107s (0.02%)
Artifact size: 329.06 MiB -> 328.99 MiB (-0.02%)

[saethlin]

Perf looks like usual inliner churn. It would be cool if this were a clean win but I don't think the result indicates it's a loss.

Do we get the desired MIR inlining now?

[the8472]

A good point, should have checked. We don't.

fn slice_iter(_1: &mut std::slice::Iter<'_, usize>) -> Option<&usize> {
    debug it => _1;
    let mut _0: std::option::Option<&usize>;

    bb0: {
        _0 = <std::slice::Iter<'_, usize> as Iterator>::next(move _1) -> [return: bb1, unwind continue];
    }

    bb1: {
        return;
    }
}