bug#58558: 29.0.50; re-search-forward is slow in some buffers

[Stefan Monnier]

> The benchmark itself does not trigger the breakpoint.

Does that mean that `Fmatch_data` is not called during a single
`re-search-forward` (not a surprise: you'd need to put a breakpoint on
`build_marker` to see the markers built by `buf_bytepos_to_charpos`)
but is called between `re-search-forward`, or that it's not called at
all during the whole benchmark where you perform several
`re-search-forward` which grow progressively slower?

If it's the latter, then those calls can't explain the slowdown, right?

> If I read the backtrace correctly, something in my custom mode-line is
> triggering Fmatch_data that creates markers.

The most common calls to `match-data` are from `save-match-data`.
And most uses of `save-match-data` are ill-advised (as the docstring
explains `save-match-data' should be used to save *your* match data
rather than your caller's match data), so you might like to double check
whether that call to `match-data` can be eliminated altogether.


        Stefan