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libstdc++: Use std::from_chars to speed up parsing subsecond durations

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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.
This commit is contained in:
Jonathan Wakely 2024-03-07 13:47:46 +00:00
parent 5f9d7a5b6c
commit 715127b63d
1 changed files with 18 additions and 10 deletions
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28
libstdc++-v3/include/bits/chrono_io.h
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@ -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);