EXPERIMENT: Avoid some extra bounds checks in read_{u8,u16}

[scottmcm]

Just the first commit from #109910, to see perf.

r? @ghost

[scottmcm]

@bors try @rust-timer queue

[bors]

⌛ Trying commit c434acd with merge cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20...

[bors]

☀️ Try build successful - checks-actions
Build commit: cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20 (cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20)

[bors]

☀️ Try build successful - checks-actions
Build commit: cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20 (cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20)

[rust-timer]

Finished benchmarking commit (cf26a991a2aacb7ce8e6daa3c5adf34aa97cda20): comparison URL.

Overall result: ❌ regressions - 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)	0.5%	[0.4%, 1.3%]	53
Regressions ❌ (secondary)	0.7%	[0.4%, 1.6%]	44
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.5%	[0.4%, 1.3%]	53

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)	1.0%	[1.0%, 1.0%]	1
Regressions ❌ (secondary)	3.5%	[3.3%, 3.7%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	1.0%	[1.0%, 1.0%]	1

Cycles

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