[Bug libstdc++/77582] Improve std::string::clear performace

[xiyou.wangcong at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77582

--- Comment #1 from Cong Wang  ---
Created attachment 39614
  --> https://gcc.gnu.org/bugzilla/attachment.cgi?id=39614&action=edit
A possible patch

This patch improves it by using _S_empty_rep directly when
_GLIBCXX_FULLY_DYNAMIC_STRING is not enabled.

[Bug libstdc++/77582] New: Improve std::string::clear performace

[xiyou.wangcong at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77582

Bug ID: 77582
   Summary: Improve std::string::clear performace
   Product: gcc
   Version: 6.2.1
Status: UNCONFIRMED
  Severity: normal
  Priority: P3
 Component: libstdc++
  Assignee: unassigned at gcc dot gnu.org
  Reporter: xiyou.wangcong at gmail dot com
  Target Milestone: ---

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.

[Bug libstdc++/77582] Improve std::string::clear performace

[xiyou.wangcong at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=77582

--- Comment #3 from Cong Wang  ---
Hi, Andrew

I just posted it:
https://gcc.gnu.org/ml/libstdc++/2016-09/msg00051.html

Please review.

I caught this when using Google protobuf on Fedora 21, _M_mutate() is shown in
perf top profile, inlined into clear(). AFAIK, only !_GLIBCXX_USE_CXX11_ABI
impl uses _M_mutate().

Thanks.