Workaround allocation regression caused by missing specialized [Substring].joined

[simonjbeaumont]

Motivation

The specialization for [Substring].joined was dropped in Swift 5.9¹ and consequently we regressed in allocations².

Modifications

Workaround the lack of specialized [Substring].joined by handwriting the comma-separated concatenation of the SSH algorithms.

Result

Reduces the allocations to below what they were prior to the toolchain regression.

Notes

As a baseline, I was using d23c142 of this repo, which is a commit from around the time of the toolchain regression.

The allocation results are as follows:

swift-nio-ssh	swift toolchain	allocs
d23c142 (Aug 11)	5.9-2023-07-25	1,007,000
	5.9-2023-07-29	1,100,000
	5.9-RELEASE	1,100,000
d23c142 + patch	5.9-2023-07-25	989,000
	5.9-2023-07-29	989,000
	5.9-RELEASE	989,000
d33c701 (main)	5.9-RELEASE	1,060,000
main + patch	5.9-RELEASE	949,000

From this we infer:

1. The allocation regression was caused by a regression in Swift 5.9 between nightly versions 2023-07-25 and 2023-07-29.
2. The same regression is still present in 5.9-RELEASE.
3. The patch seems to mitigate the regression we're seeing as a result of the new 5.9 behavior.
4. Since the original regression we actually saved some 40k allocations by some other change in the code (either here, or more likely in a dependency).

For more information on the Swift issue, including reproducer package and heaptrack outputs, see the Swift issue¹.

Footnotes

1. https://github.com/apple/swift/issues/69883 ↩ ↩²

2. Update allocation results #157 ↩

[dnadoba]

@swift-server-bot add to allowlist

[simonjbeaumont]

CI expected to fail because allocations are now much less than the figure in the CI config. The runs from the above commit show these observed allocations:

• 5.7: 952,000¹
• 5.8: 939,000²
• 5.9: 949,000³
• nightly: 939,000⁴

Weirdly, 5.10 failed because the allocations for a different tests were out of the allowed range⁵:

16:06:14 ++ echo 'info: client_server_many_small_commands_per_connection: total number of mallocs: 193748'

Footnotes

1. https://ci.swiftserver.group/job/swift-nio-ssh-swift57-prb/137/console ↩

2. https://ci.swiftserver.group/job/swift-nio-ssh-swift58-prb/66/console ↩

3. https://ci.swiftserver.group/job/swift-nio-ssh-swift59-prb/48/console ↩

4. https://ci.swiftserver.group/job/swift-nio-ssh-nightly-prb/211/console ↩

5. https://ci.swiftserver.group/job/swift-nio-ssh-swift510-prb/2/console ↩