int-to-string conversions significantly slower in libc++ compared to MSVC STL

[tanderson-google]

Bugzilla Link	41131
Resolution	FIXED
Resolved on	Mar 19, 2019 13:35
Version	unspecified
OS	Windows NT
CC	@mclow

Extended Description

On my win10 machine, the below program runs in 2.647s with libc++ and 0.357s with MSVC's STL. Most of the time is spent converting int to string:

snprintf_l() src/support/win32/locale_win32.cpp
std::num_put<>::do_put() include/locale:1461
std::num_put<>::put() include/locale:1279
std::basic_ostream<>::operator<<() include/ostream:462
main() main.cc:6

The problem appears to be that snprintf_l() makes 10 calls to setlocale() via __libcpp_locale_guard.

---

#include
int main() {
int x = 1234;
std::ostringstream os;
for (int i = 0; i < 100000; i++) {
os << x;
os.clear();
}
}

[tanderson-google]

Seems like we could reduce this from 10 to 2 calls with setlocale(LC_ALL, ...) instead of explicitly setting every category:
https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/setlocale-wsetlocale?view=vs-2017

[tanderson-google]

patch
Hm.. so this patch reduces runtime from 2.647s to 1.324s. Still not as good as MSVC's 0.357s.

[tanderson-google]

Looks like the implementation of snprintf_l() could call out to the CRT's _sprintf_s_l() (https://docs.microsoft.com/en-us/cpp/c-runtime-library/reference/sprintf-s-sprintf-s-l-swprintf-s-swprintf-s-l?view=vs-2017). I wonder if that would be faster.

[tanderson-google]

Wow, yeah, that seems to have fixed it. In fact, now libc++ is 3x faster than MSVC.

[mclow]

Wow, yeah, that seems to have fixed it. In fact, now libc++ is 3x faster
than MSVC.

Yes, but is it correct?

BTW, the __libcpp_locale_guard stuff was added here: https://reviews.llvm.org/D40181

[tanderson-google]

I think so. All I changed was this:

diff --git a/src/support/win32/locale_win32.cpp b/src/support/win32/locale_win32.cpp
index c11adfe4c..f7fc972c7 100644
--- a/src/support/win32/locale_win32.cpp
+++ b/src/support/win32/locale_win32.cpp
@@ -87,10 +87,9 @@ int wctob_l( wint_t c, locale_t loc )

int snprintf_l(char *ret, size_t n, locale_t loc, const char *format, ...)
{

• __libcpp_locale_guard __current(loc);
  va_list ap;
  va_start( ap, format );
• int result = vsnprintf( ret, n, format, ap );

• int result = _sprintf_s_l( ret, n, format, loc, ap );
  va_end(ap);
  return result;
  }

[tanderson-google]

https://reviews.llvm.org/D59525

[tanderson-google]

Fixed by https://reviews.llvm.org/rCXX356512