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[PATCH] Improve string::clear() performance

From: Cong Wang <xiyou dot wangcong at gmail dot com>
To: libstdc++ at gcc dot gnu dot org, gcc-patches at gcc dot gnu dot org
Date: Tue, 13 Sep 2016 11:02:54 -0700
Subject: [PATCH] Improve string::clear() performance
Authentication-results: sourceware.org; auth=none

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.

2016-09-13  Cong Wang  <xiyou.wangcong@gmail.com>

    PR libstdc++/77582
    * libstdc++-v3/include/bits/basic_string.h: Change inline to a declaration.
    * libstdc++-v3/include/bits/basic_string.tcc: Inline the
implementation based on _M_mutate, and use _S_empty_rep when possible.
    * libstdc++-v3/testsuite/performance/21_strings/copy_and_clear.cc: New.

Attachment: gcc.diff
Description: Text document

Follow-Ups:
- Re: [PATCH] Improve string::clear() performance
  - From: Jonathan Wakely

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