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libstdc++: Optimize operator>> for std::bitset

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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.
This commit is contained in:
Jonathan Wakely 2022-09-30 17:06:15 +01:00
parent 4eb46f453c
commit 1c12a3cfdf
1 changed files with 27 additions and 9 deletions
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36
libstdc++-v3/include/std/bitset
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@ -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;