[EXPERIMENT] Measure the perf impact of PackedFingerprint

[Zalathar]

I noticed that the PackedFingerprint optimization in #78646 is only applied on x86-family hosts, and not on other host architectures, even if they can be expected to support unaligned memory access efficiently (e.g. modern aarch64).

Before I propose expanding that optimization to some or all other architectures, let's measure the perf and memory impact that it currently has on x86-64.

[Zalathar]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 7d1c6cc (7d1c6cc07ebd305aa8f6b75d8e18744dd373109b, parent: 139651428df86cf88443295542c12ea617cbb587)

[rust-timer]

Finished benchmarking commit (7d1c6cc): comparison URL.

Overall result: ❌ regressions - 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)	0.3%	[0.3%, 0.3%]	3
Regressions ❌ (secondary)	0.3%	[0.1%, 0.5%]	22
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	3

Max RSS (memory usage)

Results (primary 3.7%, secondary 4.4%)

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

	mean	range	count
Regressions ❌ (primary)	3.7%	[0.7%, 6.7%]	116
Regressions ❌ (secondary)	4.4%	[1.5%, 6.5%]	35
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	3.7%	[0.7%, 6.7%]	116

Cycles

Results (primary -2.0%, secondary 2.8%)

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)	2.8%	[2.2%, 3.4%]	5
Improvements ✅ (primary)	-2.0%	[-2.0%, -2.0%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-2.0%	[-2.0%, -2.0%]	1

Binary size

This benchmark run did not return any relevant results for this metric.

Bootstrap: 482.657s -> 482.803s (0.03%)
Artifact size: 397.96 MiB -> 395.93 MiB (-0.51%)