[webkit-dev] String::operator+= considered harmful

[Dirk Schulze]

With a short search in the logs I found optimizations for  at least operator+, but didn't search further:

http://trac.webkit.org/changeset/86330
https://bugs.webkit.org/show_bug.cgi?id=58420

Greetings,
Dirk

On Sep 4, 2012, at 4:31 PM, Adam Barth <abarth at webkit.org> wrote:

> Do you have a proposal for how that would work and/or a link to the
> previous discussion?
> 
> Adam
> 
> 
> On Tue, Sep 4, 2012 at 4:27 PM, Dirk Schulze <dschulze at adobe.com> wrote:
>> I thought we had efforts to make String::operator+= use StringBuilder somehow? I can remember that we had a discussion on webkit-dev and definitely on bugzilla about improving String::operator+= instead of replacing it with StringBuilder.
>> 
>> Greetings,
>> Dirk
>> 
>> On Sep 4, 2012, at 4:22 PM, Adam Barth <abarth at webkit.org> wrote:
>> 
>>> As part of the work to deploy efficient string patterns [1] throughout
>>> WebKit, Benjamin and I noticed a bunch of very inefficient code that
>>> uses operator+= with Strings:
>>> 
>>> String foo;
>>> for (...)
>>> foo += [...];
>>> return foo;
>>> 
>>> This pattern is particularly inefficient because += mallocs an
>>> entirely new buffer for the result and copies the the string into the
>>> new buffer.  That means that this pattern makes O(n) calls to malloc
>>> and does O(n^2) work.  A more efficient pattern is to use
>>> StringBuilder:
>>> 
>>> StringBuilder foo;
>>> for (...)
>>> foo.append([...]);
>>> return foo.toString();
>>> 
>>> I'm in the process of going through WebCore and removing all callers
>>> of WTF::String::operator+=.  Once that's complete, my plan is to
>>> remove WTF::String::operator+= from WTFString.h.  Hopefully that will
>>> nudge contributors and reviewers towards more efficient string
>>> patterns.
>>> 
>>> Removing operator+= will likely require changes to a number of
>>> port-specific files.  I'll do my best to remove these, but I might
>>> need some help from maintainers of individual ports.  If you're
>>> interested in helping out, please let me know.
>>> 
>>> Many thanks,
>>> Adam
>>> 
>>> [1] http://trac.webkit.org/wiki/EfficientStrings
>>> _______________________________________________
>>> webkit-dev mailing list
>>> webkit-dev at lists.webkit.org
>>> http://lists.webkit.org/mailman/listinfo/webkit-dev
>> 
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