Remove a large amount of leb128-coded integers

[Mark-Simulacrum]

This removes ~41%¹ of the leb128-encoded integers serialized during libcore compilation by changing enum tags to opportunistically use u8 where feasible instead of the leb128 coding via usize.

This should have effectively zero impact on metadata file sizes, since almost all or all enum tags fit into the 7 bits available in leb128 for single-byte encodings. Perf results indicate this is basically neutral across the board except for an improvement in bootstrap time.

Footnotes

1. More than half the remaining integers still fit into <= 128, so the leb128 coding still makes sense. 32% are zero, and 46% are <= 4. ↩

[rustbot]

r? @cjgillot

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[Mark-Simulacrum]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 1d2005b with merge 78bd79d...

[bors]

☀️ Try build successful - checks-actions
Build commit: 78bd79d (78bd79dedcac84ebf4531777a0dd4a436723736c)

[rust-timer]

Finished benchmarking commit (78bd79d): comparison URL.

Overall result: ❌✅ regressions and improvements - 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)	0.3%	[0.3%, 0.3%]	2
Regressions ❌ (secondary)	0.6%	[0.6%, 0.6%]	1
Improvements ✅ (primary)	-0.3%	[-0.5%, -0.2%]	7
Improvements ✅ (secondary)	-0.4%	[-0.7%, -0.1%]	14
All ❌✅ (primary)	-0.2%	[-0.5%, 0.3%]	9

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)	1.2%	[1.2%, 1.2%]	1
Regressions ❌ (secondary)	1.8%	[1.8%, 1.8%]	1
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-3.0%	[-3.0%, -3.0%]	1
All ❌✅ (primary)	0.4%	[-0.5%, 1.2%]	2

Cycles

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)	-	-	0
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.5%, -0.5%]	1
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-0.5%	[-0.5%, -0.5%]	1

Binary size

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

Bootstrap: 667.55s -> 665.141s (-0.36%)
Artifact size: 308.58 MiB -> 308.49 MiB (-0.03%)

[Mark-Simulacrum]

Seems like this is pretty much neutral on everything except bootstrap times, presuming the delta there isn't noise. I think we should move ahead with it though.

[cjgillot]

@bors r+

[bors]

📌 Commit 1d2005b has been approved by cjgillot

It is now in the queue for this repository.

[bors]

⌛ Testing commit 1d2005b with merge 68acb39...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing 68acb39 to master...

[rust-timer]

Finished benchmarking commit (68acb39): comparison URL.

Overall result: ✅ improvements - no action needed

@rustbot label: -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
Regressions ❌ (secondary)	1.1%	[1.1%, 1.1%]	1
Improvements ✅ (primary)	-0.3%	[-0.3%, -0.2%]	5
Improvements ✅ (secondary)	-0.3%	[-0.5%, -0.1%]	12
All ❌✅ (primary)	-0.3%	[-0.3%, -0.2%]	5

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)	1.7%	[1.7%, 1.7%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.4%, -0.4%]	1
Improvements ✅ (secondary)	-1.4%	[-1.4%, -1.4%]	2
All ❌✅ (primary)	0.7%	[-0.4%, 1.7%]	2

Cycles

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

Binary size

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

Bootstrap: 666.77s -> 665.325s (-0.22%)
Artifact size: 308.45 MiB -> 308.34 MiB (-0.04%)

[pnkfelix]

• the 1.1% hit is to deep-vector debug full. It may be transient; the history is pretty up-and-down at the time of this PR, and has settled at a lower level than where it was when this PR landed.
• in any case, the gains elsewhere, especially bootstrap, outweigh the loss to that one secondary benchmark. (Which ... I guess is what the rustc-perf bot now computes as well?)