[committed] libstdc++: Use std::from_chars to speed up parsing subsecond durations
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Commit Message
Tested x86_64-linux. Pushed to trunk.
-- >8 --
With std::from_chars we can parse subsecond durations much faster than
with std::num_get, as shown in the microbenchmarks below. We were using
std::num_get and std::numpunct in order to parse a number with the
locale's decimal point character. But we copy the chars from the input
stream into a new buffer anyway, so we can replace the locale's decimal
point with '.' in that buffer, and then we can use std::from_chars on
it.
Benchmark Time CPU Iterations
----------------------------------------------------------
from_chars_millisec 158 ns 158 ns 4524046
num_get_millisec 192 ns 192 ns 3644626
from_chars_microsec 164 ns 163 ns 4330627
num_get_microsec 205 ns 205 ns 3413452
from_chars_nanosec 173 ns 173 ns 4072653
num_get_nanosec 227 ns 227 ns 3105161
libstdc++-v3/ChangeLog:
* include/bits/chrono_io.h (_Parser::operator()): Use
std::from_chars to parse fractional seconds.
---
libstdc++-v3/include/bits/chrono_io.h | 28 +++++++++++++++++----------
1 file changed, 18 insertions(+), 10 deletions(-)
@@ -37,6 +37,7 @@
#include <sstream> // ostringstream
#include <iomanip> // setw, setfill
#include <format>
+#include <charconv> // from_chars
#include <bits/streambuf_iterator.h>
@@ -3597,13 +3598,17 @@ namespace __detail
__err |= ios_base::eofbit;
else
{
- auto& __np = use_facet<numpunct<_CharT>>(__loc);
- auto __dp = __np.decimal_point();
+ _CharT __dp = '.';
+ if (__loc != locale::classic())
+ {
+ auto& __np = use_facet<numpunct<_CharT>>(__loc);
+ __dp = __np.decimal_point();
+ }
_CharT __c = _Traits::to_char_type(__i);
if (__c == __dp)
{
(void) __is.get();
- __buf.put(__c);
+ __buf.put('.');
int __prec
= hh_mm_ss<_Duration>::fractional_width;
do
@@ -3618,14 +3623,17 @@ namespace __detail
}
}
- if (!__is_failed(__err))
+ if (!__is_failed(__err)) [[likely]]
{
- auto& __ng = use_facet<num_get<_CharT>>(__loc);
- long double __val;
- ios_base::iostate __err2{};
- __ng.get(__buf, {}, __buf, __err2, __val);
- if (__is_failed(__err2)) [[unlikely]]
- __err |= __err2;
+ long double __val{};
+ string __str = std::move(__buf).str();
+ auto __first = __str.data();
+ auto __last = __first + __str.size();
+ using enum chars_format;
+ auto [ptr, ec] = std::from_chars(__first, __last,
+ __val, fixed);
+ if ((bool)ec || ptr != __last) [[unlikely]]
+ __err |= ios_base::failbit;
else
{
duration<long double> __fs(__val);