public inbox for gcc-cvs@sourceware.org help / color / mirror / Atom feed
From: Jonathan Wakely <redi@gcc.gnu.org> To: gcc-cvs@gcc.gnu.org, libstdc++-cvs@gcc.gnu.org Subject: [gcc r13-2998] libstdc++: Optimize operator>> for std::bitset Date: Fri, 30 Sep 2022 20:56:55 +0000 (GMT) [thread overview] Message-ID: <20220930205656.750633858430@sourceware.org> (raw) https://gcc.gnu.org/g:1c12a3cfdfabf67c7962ee6532e001e4d48e7fc2 commit r13-2998-g1c12a3cfdfabf67c7962ee6532e001e4d48e7fc2 Author: Jonathan Wakely <jwakely@redhat.com> 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<N> 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<size_t>(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;
reply other threads:[~2022-09-30 20:56 UTC|newest] Thread overview: [no followups] expand[flat|nested] mbox.gz Atom feed
Reply instructions: You may reply publicly to this message via plain-text email using any one of the following methods: * Save the following mbox file, import it into your mail client, and reply-to-all from there: mbox Avoid top-posting and favor interleaved quoting: https://en.wikipedia.org/wiki/Posting_style#Interleaved_style * Reply using the --to, --cc, and --in-reply-to switches of git-send-email(1): git send-email \ --in-reply-to=20220930205656.750633858430@sourceware.org \ --to=redi@gcc.gnu.org \ --cc=gcc-cvs@gcc.gnu.org \ --cc=libstdc++-cvs@gcc.gnu.org \ /path/to/YOUR_REPLY https://kernel.org/pub/software/scm/git/docs/git-send-email.html * If your mail client supports setting the In-Reply-To header via mailto: links, try the mailto: linkBe sure your reply has a Subject: header at the top and a blank line before the message body.
This is a public inbox, see mirroring instructions for how to clone and mirror all data and code used for this inbox; as well as URLs for read-only IMAP folder(s) and NNTP newsgroup(s).