[PATCH] Improve string::clear() performance

[Jonathan Wakely]

On 14/09/16 09:09 -0700, Cong Wang wrote:
>On Wed, Sep 14, 2016 at 4:06 AM, Jonathan Wakely <jwakely@redhat.com> wrote:
>> On 13/09/16 11:02 -0700, Cong Wang wrote:
>>>
>>> In !_GLIBCXX_USE_CXX11_ABI implementation, string::clear() calls
>>> _M_mutate(), which could allocate memory as we do COW. This hurts
>>> performance when string::clear() is on the hot path.
>>>
>>> This patch improves it by using _S_empty_rep directly when
>>> _GLIBCXX_FULLY_DYNAMIC_STRING is not enabled. And Linux distro like
>>> Fedora doesn't enable this, this is why we caught it.
>>>
>>> The copy-and-clear test shows it improves by 50%.
>>>
>>> Ran all testsuites on Linux-x64.
>>
>>
>> Thank you for the patch (and changelog and test results!).
>>
>> Do you have a GCC copyright assignment in place? See
>> https://gcc.gnu.org/contribute.html#legal for details.
>
>Oh, didn't notice this, I thought gcc has something as quick
>as the 'Signed-off-by' for Linux kernel (I am a Linux kernel developer).
>I will do it.
>
>>
>> If I understand the purpose of the change correctly, it's similar to
>> https://gcc.gnu.org/ml/gcc-patches/2014-06/msg00278.html - is that
>> right?
>>
>
>Oh, yes, I didn't know your patch because I don't subscribe
>gcc mailing list. I am wondering why your patch is not merged
>after 2+ years?

As I said in the patch email, I'm not sure if it's conforming, due to
clearing the string's cpacity() as well as its size().

>Please let me know what you prefer: 1) You update your patch
>and get it merged; 2) Use my patch if it looks good. I am fine with
>either way. :)

I'll refresh my memory of the various issues and reconsider applying
my patch (that way we don't need to wait for your copyright
assignment). The performance benefits you measured make it more
compelling.