Simplify and optimize VecCache's SlotIndex::from_index

[joshtriplett]

Simplify and optimize SlotIndex::from_index

Break out bucket 0 (containing idx < 4096) as an early return, which
simplifies the remainder of the function, and allows optimizing the
checked_ilog2 since it can no longer return None.

This reduces the runtime of vec_cache::tests::slot_index_exhaustive
(which calls SlotIndex::from_index for every u32, twice) from ~15.5s
to ~13.3s.

Separately, simplify the test case as well. (The old and new code passes with
the old and new test case.)

---

Noticed because slot_index_exhaustive stood out as taking unusually long compared to other tests, so I started investigating what it was doing.

[rustbot]

r? @SparrowLii

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[joshtriplett]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 9837c3c with merge 18aa6ca...

[bors]

☀️ Try build successful - checks-actions
Build commit: 18aa6ca (18aa6ca8c951e4ad5d9c94ee7ae3af8c80e4d939)

[rust-timer]

Finished benchmarking commit (18aa6ca): 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
Regressions ❌ (secondary)	0.3%	[0.2%, 0.5%]	4
Improvements ✅ (primary)	-0.8%	[-3.4%, -0.1%]	194
Improvements ✅ (secondary)	-1.0%	[-4.7%, -0.1%]	105
All ❌✅ (primary)	-0.8%	[-3.4%, -0.1%]	194

Max RSS (memory usage)

Results (primary 1.0%, secondary -5.1%)

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)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-5.1%	[-8.0%, -1.3%]	10
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Cycles

Results (primary -1.6%, secondary -0.8%)

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)	-	-	0
Regressions ❌ (secondary)	6.3%	[5.9%, 6.7%]	2
Improvements ✅ (primary)	-1.6%	[-2.0%, -1.1%]	2
Improvements ✅ (secondary)	-3.2%	[-3.9%, -2.3%]	6
All ❌✅ (primary)	-1.6%	[-2.0%, -1.1%]	2

Binary size

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

Bootstrap: 750.971s -> 749.964s (-0.13%)
Artifact size: 371.78 MiB -> 371.52 MiB (-0.07%)

[SparrowLii]

The result looks very nice!

[nnethercote]

@SparrowLii: you gave approval in the GitHub review, do you want to give r+? I'm happy with how the PR looks.

[SparrowLii]

@bors r+

[bors]

📌 Commit 8a56ed4 has been approved by SparrowLii

It is now in the queue for this repository.