Tested x86_64-linux. Pushed to trunk.
-- >8 --
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.
---
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;
--
2.37.3
