perf: store splat arg index in a single byte

[teor2345]

This is a perf test for a variation of #153697.
It's part of the splat lang experiment, tracking issue #153629.

[JonathanBrouwer]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 51cfa71 (51cfa71c5e3563738f2ed8b82f18684e522dbc8d)
Base parent: 9030e34 (9030e345fe92df1ccefd0a8cdf61a9a9a5b73cb0)

[rust-timer]

Finished benchmarking commit (51cfa71): comparison URL.

Overall result: ❌ regressions - please read:

Benchmarking means the PR may be perf-sensitive. It's automatically marked not fit for rolling up. Overriding is possible but disadvised: it risks changing compiler perf.

Next, please: If you can, justify the regressions found in this try perf run in writing along with @rustbot label: +perf-regression-triaged. If not, fix the regressions and do another perf run. Neutral or positive results will clear the label automatically.

@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)	0.1%	[0.1%, 0.1%]	5
Regressions ❌ (secondary)	0.2%	[0.1%, 0.3%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.1%	[0.1%, 0.1%]	5

Max RSS (memory usage)

Results (secondary -2.4%)

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)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.4%	[-3.3%, -1.7%]	6
All ❌✅ (primary)	-	-	0

Cycles

Results (secondary 4.3%)

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)	4.9%	[1.7%, 18.5%]	11
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	-	-	0

Binary size

Results (primary 0.0%, secondary 0.1%)

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

	mean	range	count
Regressions ❌ (primary)	0.0%	[0.0%, 0.1%]	56
Regressions ❌ (secondary)	0.1%	[0.0%, 0.3%]	75
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	0.0%	[0.0%, 0.1%]	56

Bootstrap: 503.321s -> 509.023s (1.13%)
Artifact size: 353.40 MiB -> 353.69 MiB (0.08%)

[teor2345]

0.1% regression on 5 primary and 0.2% on 4 secondary benchmarks seems like as good as we'll get.

I'll replace #153697 with this branch once I've checked with Oli.

[teor2345]

I'm going to push these perf fixes to #153697