[libc++] __vprint_unicode_posix() has unnecessary call to fflush()

[dimztimz]

llvm-project/libcxx/include/print

Lines 235 to 241 in 2f4328e

	__vprint_unicode_posix(FILE* __stream, string_view __fmt, format_args __args, bool __write_nl, bool __is_terminal) {
	// TODO PRINT Should flush errors throw too?
	if (__is_terminal)
	std::fflush(__stream);
	__print::__vprint_nonunicode(__stream, __fmt, __args, __write_nl);
	}

In this function the following two lines are not necessary and introduce performance pessimization.

llvm-project/libcxx/include/print

Lines 237 to 238 in 2f4328e

	if (__is_terminal)
	std::fflush(__stream);

Additionally, if you read the standard carefully http://www.eel.is/c++draft/print.fun#7.sentence-4 It says:

If the native Unicode API is used, the function flushes stream before writing out.

Although POSIX offers a way to query if a C stream goes to terminal or not, it does not have native Unicode API, so such API is never used, and flushing is not needed. A consequence of this is that on POSIX querying the C stream is not needed at all and it is another performance pessimization. The call to __is_terminal() is not needed here.

llvm-project/libcxx/include/print

Lines 312 to 313 in 2f4328e

	# ifndef _WIN32
	__print::__vprint_unicode_posix(__stream, __fmt, __args, __write_nl, __print::__is_terminal(__stream));

[dimztimz]

Ping @mordante, @vitaut

[vitaut]

You are right, flush shouldn't be here.

[dimztimz]

The simplest fix is to delete __vprint_unicode_posix() entirely, and call __print::__vprint_nonunicode() directly from __vprint_unicode() in the case of POSIX.

[mordante]

Note to self this has been introduced in #73262 too.

[dimztimz]

Note to self this has been introduced in #73262 too.

As it stands now the ostream overloads do two flushes, one is the os.flush() the other is fflush(). This too should be fixed.

The correct behaviour for the ostream vprint_unicode is the following:

• On POSIX don't call any flush or is_terminal()
• On Windows in the case where the stream prints to terminal call only os.flush(), no need for the the C fflush(),

[mordante]

Sorry for the late reply. I've been quite busy and wanted to investigate further was is considered a "native Unicode API". This is currently discussed in private e-mails but it indeed seems POSIX is considered not to have a "native Unicode API". In that case it's indeed not needed to call is_terminal nor to flush. I want to wait for that discussion to finish before moving on with the PR.