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Postings from February 2020
On 2/11/20 10:58 AM, Karl Williamson wrote:
> I have been looking at the code in regcomp.c in regpiece() that deals
> with qwuantifiers.
>
> After reordering things so that goto's don't cause it to jump back then
> forth, some anonmalies became clear. I also found some potential easy
> optimizations.
>
> I would expect that the results of parsing {1,} would be the same as
> '+', and they both do generate the PLUS regnode, but the flags passed to
> the higher level aren't set the same. This is true of the other
> shortcuts '*' and '?' as well.
>
> I then tried to figure out what the consequences of those differences
> are. Two of the flags WORST and SPSTART do not appear to ever be looked
> at. Should we remove them, or dig to find out how they used to be used,
> or might they come back again, and we should set them consistently?
>
> regpiece assumes that any quantifier whose upper limit is non-zero
> causes the construct to not match the null string, and sets HASWIDTH.
> That simply isn't true when quantifying a zero-width assertion. I
> didn't look at what the optimizer does with that, but when I change that
> a higher level warning is emitted:
>
> "Quantifier unexpected on zero-length expression "
>
> Now to the optimizations: I believe the quantifier {1,1} can simply be
> optimized out. There are occurrences in our test suite of this; I
> believe from Abigail. And I can see machine generated or interpolated
> code ending up with this. So we don't need to create a loop that gets
> executed precisely once. But there is {1,1}+, that has to be
> considered; and that's easy to do.
>
> Generally, in {m,m}? the ? is a no-op and can be omitted.
I think that research like this is important -- but we need to be
mindful of where we are in our annual development cycle. Your findings
and recommendations are likely to be at such a deep level inside the
codebase that their implications will take time to learn. So we
shouldn't be expecting to implement such recommendations in perl-5.32.0.
Thank you very much.
Jim Keenan
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