gcc/libstdc++-v3
Jonathan Wakely e4fe6dba90 libstdc++: Optimize std::ostream inserters for single characters
On the libsdc++ mailing list Lewis Hyatt pointed out the performance
overhead of using sputn in stream inserters, rather than writing
directly to the streambuf's put area:
https://gcc.gnu.org/pipermail/libstdc++/2021-July/052877.html

As Lewis noted, the standard explicitly requires a call to sputn for
inserting a std::basic_string_view or std::basic_string.  But for
inserting single characters or null-terminated strings it is more vague,
and so we can improve performance by not using the __ostream_insert
function.

This is a minimal change that avoids __ostream_insert for single
characters. We can use the unformatted basic_ostream::put(charT)
function when we don't need the additional effects of a formatted output
function (i.e. padding and resetting the width). The put function will
insert into the buffer if possible, and only make a virtual call (to
overflow) if the buffer is full.

We could also avoid sputn when inserting null-terminated character
strings, but that would require using a new function for inserting
null-terminated strings, so the existing code using sputn is still used
for basic_string and basic_string_view. My preference is to leave that
for now, and try to improve the standard. We could either remove the
requirement to call sputn, or allow sputn to write directly to the
buffer instead of calling xsputn.

libstdc++-v3/ChangeLog:

	* include/std/ostream (operator<<(basic_ostream&, charT)):
	Use unformatted input if no padding is needed.
	(operator<<(basic_ostream<char>&, char)): Likewise.
2022-01-11 13:28:46 +00:00
..
config libstdc++: Add %j, %U, %w, %W time_get support, fix %y, %Y, %C, %p [PR77760] 2022-01-10 15:38:47 +01:00
doc libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
include libstdc++: Optimize std::ostream inserters for single characters 2022-01-11 13:28:46 +00:00
libsupc++ libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
po libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
python libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
scripts Update copyright years. 2022-01-03 10:42:10 +01:00
src libstdc++: Add %j, %U, %w, %W time_get support, fix %y, %Y, %C, %p [PR77760] 2022-01-10 15:38:47 +01:00
testsuite libstdc++: Make std::common_iterator completely constexpr-able (LWG 3574) 2022-01-11 13:22:34 +00:00
acinclude.m4 libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
aclocal.m4
ChangeLog Daily bump. 2022-01-11 00:16:36 +00:00
ChangeLog-1998
ChangeLog-1999
ChangeLog-2000
ChangeLog-2001
ChangeLog-2002
ChangeLog-2003
ChangeLog-2004
ChangeLog-2005
ChangeLog-2006
ChangeLog-2007
ChangeLog-2008
ChangeLog-2009
ChangeLog-2010
ChangeLog-2011
ChangeLog-2012
ChangeLog-2013
ChangeLog-2014
ChangeLog-2015
ChangeLog-2016
ChangeLog-2017
ChangeLog-2018
ChangeLog-2019
ChangeLog-2020
ChangeLog-2021 Rotate ChangeLog files - part 1 - add ChangeLog-2021. 2022-01-03 10:18:16 +01:00
config.h.in libstdc++: Fix definition of _GLIBCXX_NO_SLEEP config macro 2021-12-10 14:05:46 +00:00
configure libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
configure.ac libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
configure.host
crossconfig.m4
fragment.am
linkage.m4
Makefile.am Update copyright years. 2022-01-03 10:42:10 +01:00
Makefile.in libstdc++: Fix and simplify freestanding configuration [PR103866] 2022-01-10 12:18:14 +00:00
README

file: libstdc++-v3/README

New users may wish to point their web browsers to the file
index.html in the 'doc/html' subdirectory.  It contains brief
building instructions and notes on how to configure the library in
interesting ways.