Rewrite MemDecoder around pointers not a slice

[saethlin]

This is basically #109910 but I'm being a lot more aggressive. The pointer-based structure means that it makes a lot more sense to absorb more complexity into MemDecoder, most of the diff is just complexity moving from one place to another.

The primary argument for this structure is that we only incur a single bounds check when doing multi-byte reads from a MemDecoder. With the slice-based implementation we need to do those with data[position..position + len] , which needs to account for position + len wrapping. It would be possible to dodge the first bounds check if we stored a slice that starts at position, but that would require updating the pointer and length on every read.

This PR also embeds the failure path in a separate function, which means that this PR should subsume all the perf wins observed in #109867.

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 708c71bc9fed44627872cff4d24018b17905f0fa with merge ea67749271a3bfca6996cc4b8becfa7a0591f9ab...

[bors]

☀️ Try build successful - checks-actions
Build commit: ea67749271a3bfca6996cc4b8becfa7a0591f9ab (ea67749271a3bfca6996cc4b8becfa7a0591f9ab)

[rust-timer]

Finished benchmarking commit (ea67749271a3bfca6996cc4b8becfa7a0591f9ab): comparison URL.

Overall result: ✅ improvements - no 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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.7%, -0.3%]	41
Improvements ✅ (secondary)	-0.5%	[-0.6%, -0.2%]	10
All ❌✅ (primary)	-0.5%	[-0.7%, -0.3%]	41

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)	-	-	0
Regressions ❌ (secondary)	2.2%	[2.2%, 2.2%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	2.7%	[2.6%, 2.8%]	4
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

[saethlin]

@bors try @rust-timer queue

[bors]

⌛ Trying commit 2995eb38309270f12efb8a8d43ba82cf6fdb4000 with merge 441c638841629089f54e4aa6da3960f27bbfac6d...

[bors]

☀️ Try build successful - checks-actions
Build commit: 441c638841629089f54e4aa6da3960f27bbfac6d (441c638841629089f54e4aa6da3960f27bbfac6d)

[bors]

☀️ Try build successful - checks-actions
Build commit: 441c638841629089f54e4aa6da3960f27bbfac6d (441c638841629089f54e4aa6da3960f27bbfac6d)

[rust-timer]

Finished benchmarking commit (441c638841629089f54e4aa6da3960f27bbfac6d): comparison URL.

Overall result: ✅ improvements - no 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.

@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
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.4%	[-0.6%, -0.3%]	33
Improvements ✅ (secondary)	-0.4%	[-0.5%, -0.3%]	7
All ❌✅ (primary)	-0.4%	[-0.6%, -0.3%]	33

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)	-	-	0
Regressions ❌ (secondary)	2.6%	[2.6%, 2.6%]	1
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-2.2%	[-2.2%, -2.2%]	1
All ❌✅ (primary)	-	-	0

Cycles

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

[saethlin]

r? compiler

[bors]

⌛ Testing commit 1f67ba6 with merge f5d087af3c1f1f1cd92ce34419bd874002eca742...

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

[bors]

💔 Test failed - checks-actions

[cjgillot]

@bors retry

[bors]

⌛ Testing commit 1f67ba6 with merge 9abd3bce4eec20d550601dbfd2f024f5e896f635...

[bors]

💔 Test failed - checks-actions

[rust-log-analyzer]

A job failed! Check out the build log: (web) (plain)

Click to see the possible cause of the failure (guessed by this bot)

Prepare workflow directory
Prepare all required actions
Getting action download info
Download action repository 'actions/checkout@v3' (SHA:8e5e7e5ab8b370d6c329ec480221332ada57f0ab)
##[warning]Failed to download action 'https://api.github.com/repos/actions/checkout/tarball/8e5e7e5ab8b370d6c329ec480221332ada57f0ab'. Error: The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing.
##[warning]Back off 18.465 seconds before retry.
##[warning]Failed to download action 'https://api.github.com/repos/actions/checkout/tarball/8e5e7e5ab8b370d6c329ec480221332ada57f0ab'. Error: The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing.
##[warning]Back off 26.406 seconds before retry.
##[error]The request was canceled due to the configured HttpClient.Timeout of 100 seconds elapsing.

[compiler-errors]

@bors retry

[bors]

⌛ Testing commit 1f67ba6 with merge adaac6b...

[bors]

☀️ Test successful - checks-actions
Approved by: cjgillot
Pushing adaac6b to master...

[rust-timer]

Finished benchmarking commit (adaac6b): 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)	0.4%	[0.4%, 0.4%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-0.5%	[-0.7%, -0.3%]	52
Improvements ✅ (secondary)	-0.5%	[-0.8%, -0.3%]	16
All ❌✅ (primary)	-0.4%	[-0.7%, 0.4%]	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)	-	-	0
Regressions ❌ (secondary)	1.9%	[0.7%, 3.1%]	2
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-	-	0
All ❌✅ (primary)	-	-	0

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)	0.6%	[0.6%, 0.6%]	1
Regressions ❌ (secondary)	-	-	0
Improvements ✅ (primary)	-	-	0
Improvements ✅ (secondary)	-10.2%	[-11.7%, -4.6%]	7
All ❌✅ (primary)	0.6%	[0.6%, 0.6%]	1