[SR-13425] 3-7x performance regression between 6-04-a and 7-11-a

[tayloraswift]

Previous ID	SR-13425
Radar	rdar://problem/67499431
Original Reporter	@Kelvin13
Type	Bug

Environment

Ubuntu, swift snapshots installed with swiftenv

Additional Detail from JIRA

Votes	0
Component/s
Labels	Bug
Assignee	None
Priority	Medium

md5: ec49b8b464e139278b981e5ca2601dd5

Issue Description:

There seems to be a large three- to sevenfold performance regression between the same framework (Swift PNG) compiled with the 6-04-a snapshot and the 7-11-a snapshot. As of the 8-04-a snapshot, the issue still appears to be present.

The performance regressions were noticed in the benchmarks of the Swift PNG library, at commit 87c8912d61910babd4ceff0786b256a7e78371be. (https://github.com/kelvin13/png/tree/87c8912d61910babd4ceff0786b256a7e78371be)

Instructions for how to run and interpret the benchmarks are in this forum post:

https://forums.swift.org/t/enormous-7x-performance-regression-between-6-04-a-and-7-11-a/39469

The function responsible for most of the benchmark execution time is here:

https://github.com/kelvin13/png/blob/87c8912d61910babd4ceff0786b256a7e78371be/sources/png4/deflate.swift#L1683

[typesanitizer]

@swift-ci create