Experiment: Use packed query cache entries

[zetanumbers]

There are some prevalent cases of query value type being 8-byte aligned like references. This might add padding bytes to query cache structures, so we might get more cache misses. This PR packs cache entries to be 4-byte aligned to avoid padding. Might impact the performance.

Based on #155550 to distinguish perf impact of using insert_unique.

r? @petrochenkov

[petrochenkov]

@bors try @rust-timer queue

[rust-bors]

☀️ Try build successful (CI)
Build commit: 9c4c544 (9c4c5443cdbc95cab96b91e4df6fa8b8790aeed5, parent: e22c616e4e87914135c1db261a03e0437255335e)

[rust-timer]

Finished benchmarking commit (9c4c544): 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)	0.3%	[0.3%, 0.3%]	2
Regressions ❌ (secondary)	1.0%	[0.1%, 1.5%]	20
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-0.2%	[-0.4%, -0.0%]	2
All ❌✅ (primary)	0.3%	[0.3%, 0.3%]	2

Max RSS (memory usage)

Results (primary -1.1%, secondary 5.9%)

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)	5.9%	[4.6%, 7.3%]	2
Improvements ✅ (primary)	-1.1%	[-1.1%, -1.1%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-1.1%	[-1.1%, -1.1%]	1

Cycles

Results (secondary -3.5%)

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)	-3.5%	[-3.5%, -3.5%]	1
All ❌✅ (primary)	-	-	0

Binary size

This perf run didn't have relevant results for this metric.

Bootstrap: 507.073s -> 491.845s (-3.00%)
Artifact size: 394.31 MiB -> 394.32 MiB (0.00%)

[Zoxc]

Keep in mind that we erase query result types, so they always have byte alignment.

[zetanumbers]

Yeah, I remembered that too late.