From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: by sourceware.org (Postfix, from userid 2181) id 750633858430; Fri, 30 Sep 2022 20:56:55 +0000 (GMT) DKIM-Filter: OpenDKIM Filter v2.11.0 sourceware.org 750633858430 DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=gcc.gnu.org; s=default; t=1664571416; bh=sKBco2Y1hzhBm5ybahe+wzsEgcwlqgGfrqjCw4biKq0=; h=From:To:Subject:Date:From; b=NSX4XVu2WgMKKv0u9sky/vpUrgNYc1umkhnGILlpSkFlL9mSJjblFvkT5WUZdZKpJ aBxAG/pwgZxcPdQQAFGi+Kc/4SJ5VW71yLCgsp0aMFU8QEqQl51vB4ZDsE1TBiYEAg 0v+2TWE968teYQvVkPj8nQ1MBhBW3dPA7cgW90pg= MIME-Version: 1.0 Content-Transfer-Encoding: 7bit Content-Type: text/plain; charset="utf-8" From: Jonathan Wakely To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-2998] libstdc++: Optimize operator>> for std::bitset X-Act-Checkin: gcc X-Git-Author: Jonathan Wakely X-Git-Refname: refs/heads/master X-Git-Oldrev: 4eb46f453cc74adf0055dae35cec41f4a4c4be5b X-Git-Newrev: 1c12a3cfdfabf67c7962ee6532e001e4d48e7fc2 Message-Id: <20220930205656.750633858430@sourceware.org> Date: Fri, 30 Sep 2022 20:56:55 +0000 (GMT) List-Id: https://gcc.gnu.org/g:1c12a3cfdfabf67c7962ee6532e001e4d48e7fc2 commit r13-2998-g1c12a3cfdfabf67c7962ee6532e001e4d48e7fc2 Author: Jonathan Wakely Date: Fri Sep 30 17:06:15 2022 +0100 libstdc++: Optimize operator>> for std::bitset We can improve performance by using a char buffer instead of basic_string. The loop bound already means we can't overflow the buffer, and we don't need to keep writing a null character after every character written to the buffer. We could just use basic_string::resize(N) to zero-init the whole string, then overwrite those chars. But that zero-init of all N chars would be wasted in the case where we are writing to a bitset with large N, but only end up extracting one or two chars from the stream. With this change we just use buffer of uninitialized chars. For a small-ish bitset (currently <= 256) we can improve performance further by using alloca instead of the heap. libstdc++-v3/ChangeLog: * include/std/bitset (operator>>): Use a simple buffer instead of std::basic_string. Diff: --- libstdc++-v3/include/std/bitset | 36 +++++++++++++++++++++++++++--------- 1 file changed, 27 insertions(+), 9 deletions(-) diff --git a/libstdc++-v3/include/std/bitset b/libstdc++-v3/include/std/bitset index 757da020ffe..1038cc65138 100644 --- a/libstdc++-v3/include/std/bitset +++ b/libstdc++-v3/include/std/bitset @@ -1563,8 +1563,22 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER typedef std::basic_istream<_CharT, _Traits> __istream_type; typedef typename __istream_type::ios_base __ios_base; - std::basic_string<_CharT, _Traits> __tmp; - __tmp.reserve(_Nb); + struct _Buffer + { + _Buffer() + : _M_base(_Nb > 256 ? new _CharT[_Nb] : (_CharT*)__builtin_alloca(_Nb)) + { } + + ~_Buffer() + { + if _GLIBCXX17_CONSTEXPR (_Nb > 256) + delete[] _M_base; + } + + _CharT* const _M_base; + }; + _Buffer __buf; + _CharT* __ptr = __buf._M_base; // _GLIBCXX_RESOLVE_LIB_DEFECTS // 303. Bitset input operator underspecified @@ -1591,9 +1605,9 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER { const char_type __c2 = _Traits::to_char_type(__c1); if (_Traits::eq(__c2, __zero)) - __tmp.push_back(__zero); + *__ptr++ = __zero; else if (_Traits::eq(__c2, __one)) - __tmp.push_back(__one); + *__ptr++ = __one; else if (_Traits:: eq_int_type(__is.rdbuf()->sputbackc(__c2), __eof)) @@ -1613,11 +1627,15 @@ _GLIBCXX_BEGIN_NAMESPACE_CONTAINER { __is._M_setstate(__ios_base::badbit); } } - if (__tmp.empty() && _Nb) - __state |= __ios_base::failbit; - else if _GLIBCXX17_CONSTEXPR (_Nb) - __x._M_copy_from_string(__tmp, static_cast(0), _Nb, - __zero, __one); + if _GLIBCXX17_CONSTEXPR (_Nb) + { + if (size_t __len = __ptr - __buf._M_base) + __x.template _M_copy_from_ptr<_CharT, _Traits>(__buf._M_base, __len, + 0, __len, + __zero, __one); + else + __state |= __ios_base::failbit; + } if (__state) __is.setstate(__state); return __is;