OOK hack example in perlguts is outdated with COW #13990
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From @ntyniThis is a bug report for perl from Niko Tyni <ntyni@debian.org>, Trying to fix Devel-Size [rt.cpan.org #95493] for Perl 5.20, % perl5182 -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)' % perl5200 -MDevel::Peek -le '$a="12345"; $a=~s/.//; Dump($a)' This changed with commit v5.19.0-246-g13b0f67, "re-enable Copy-on-Write The example is from the 'Offsets' section in perlguts, so at least that Flags: Site configuration information for perl 5.18.2: Configured by Debian Project at Mon Jul 14 20:39:08 UTC 2014. Summary of my perl5 (revision 5 version 18 subversion 2) configuration: Locally applied patches: @INC for perl 5.18.2: Environment for perl 5.18.2: |
From @demerphqOn 18 July 2014 08:58, Niko Tyni <perlbug-followup@perl.org> wrote:
I would assume this is a serious performance failure. COW clashing with Yves |
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The RT System itself - Status changed from 'new' to 'open' |
From @arcdemerphq <demerphq@gmail.com> wrote:
Yes, it seems so: $ for f in 5.18.2 5.20.0; do time perl$f -wE 'say for "", $^V; for (1 v5.18.2 real 0m19.769s v5.20.0 real 0m39.145s -- |
From @iabynOn Mon, Jul 21, 2014 at 03:34:00PM +0100, Aaron Crane wrote:
Note that this is in fact a bug fix that has performance loss as a $ perl5180 -le'$_="ABC"; s/.//; print ord eval q{$&}' Basically in general, a pattern match has to make a temporary copy of the However, when the match includes a substitution, things become more Note that s/.// is a bit of special case. Replace it with s/(.)// say, But I do rather wonder whether COW could be improved to still share the But its all very messy and too much for my brain at this time of night. -- |
From @demerphqOn 23 July 2014 23:19, Dave Mitchell <davem@iabyn.com> wrote:
Removing characters from the front or back of a string efficiently is I really think that the long history of "you dont need $&" shows that On the other hand correctness is pleasing. :-) I was wondering if we could wriggle out of this conundrum with a new I cant help but observe that I already used a modifier to work around the I wonder if we would have success if we had the opposite modifier (say /P) Yves |
From @cpansproutOn Thu Jul 24 00:18:47 2014, demerphq wrote:
Wasn’t this suggested as /n or something similar last year? I seem to remember you supported the idea and there were several messages back and forth in which you might have thought I disagreed, though I did not. -- Father Chrysostomos |
From @cpansproutOn Wed Jul 23 14:19:47 2014, davem wrote:
OOK offsets are stored in the string buffer itself. But perhaps we could bring the old IV system back at least for COW scalars. -- Father Chrysostomos |
From @cpansproutOn Thu Jul 17 23:58:03 2014, ntyni@debian.org wrote:
Here is one way: $ ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= ""; Dump($a)' Note the LEN. Then extend the string to one less than that: $ ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= "123456789"; Dump($a)' OK, it’s still 10. Now try substitution: $ ./perl -Ilib -MDevel::Peek -le '$a=""; $a .= "123456789"; $a=~s/.//; Dump($a)' -- Father Chrysostomos |
From @iabynOn Sun, Sep 07, 2014 at 07:17:57AM -0700, Father Chrysostomos via RT wrote:
But as I pointed out earlier in this thread, strings are normally expected -- |
From @cpansproutOn Mon Sep 08 04:51:05 2014, davem wrote:
I have done s/// on just the beginning enough times that I think it would be worth optimising it. I have updated the docs in commit f942a0d, but I think this ticket should stay open (but not block 5.22) until the optimisation can be implemented. -- Father Chrysostomos |
From david@justatheory.comCan I just say how much I love the subject of this thread? D |
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