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

[Adam Barth]

Ah, you're think of operator+, which is now quite efficient.  This
thread is about operator+=, which is sadly slower than molasses.

Adam


On Tue, Sep 4, 2012 at 4:38 PM, Dirk Schulze <dschulze at adobe.com> wrote:
> 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
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>>>
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