unexpand: save stack size - perf +6.98%

[oech3]

Replace a stack array with vec to avoid stack spill seen at rust-lang/rust#148670 .

Probally fixes #10654

[codspeed-hq]

Merging this PR will improve performance by 6.98%

⚠️ Different runtime environments detected

Some benchmarks with significant performance changes were compared across different runtime environments,
which may affect the accuracy of the results.

Open the report in CodSpeed to investigate

⚡ 2 improved benchmarks
✅ 307 untouched benchmarks
⏩ 46 skipped benchmarks¹

Performance Changes

	Mode	Benchmark	BASE	HEAD	Efficiency
⚡	Simulation	unexpand_many_lines[100000]	145.8 ms	136.3 ms	+6.97%
⚡	Simulation	unexpand_large_file[10]	305.6 ms	285.6 ms	+6.98%

---

_{Comparing oech3:unexpand-stack (eda5aed) with main (44aff6b)}

Footnotes

1. 46 benchmarks were skipped, so the baseline results were used instead. If they were deleted from the codebase, click here and archive them to remove them from the performance reports. ↩

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/misc/io-errors. tests/misc/io-errors is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Skipping an intermittent issue tests/pr/bounded-memory (passes in this run but fails in the 'main' branch)
Note: The gnu test tests/seq/seq-epipe is now being skipped but was previously passing.
Congrats! The gnu test tests/tail/tail-n0f is now passing!

[sylvestre]

i wasn't expecting such wins for this kind of changes

[xtqqczze]

I don't understand why changing stack allocation to heap allocation is a performance improvement. Additionally, it may be possible that this isn't an improvement on targets other than x86_64-unknown-linux-gnu.

[oech3]

I'm OK to have cfg block.

[xtqqczze]

It would be good to understand why there is a performance difference; if necessary this could be reported upstream.

[oech3]

done at rust-lang/rust#148670

[xtqqczze]

done at rust-lang/rust#148670

Please add a comment linking to this issue; it would also have been helpful to include it in the PR description.

[xtqqczze]

I guess it would be good to explain in the comment that the regression is specifically with codegen-units=1 + LTO.

[github-actions]

GNU testsuite comparison:

GNU test failed: tests/timeout/timeout-group. tests/timeout/timeout-group is passing on 'main'. Maybe you have to rebase?
Skip an intermittent issue tests/tty/tty-eof (fails in this run but passes in the 'main' branch)
Congrats! The gnu test tests/expand/bounded-memory is now passing!

[xtqqczze]

I'm OK to have cfg block.

Like efd0f0c, or just x86_64-unknown-linux-gnu?