Build thread_win32.cpp only if LIBCXX_HAS_PTHREAD_API is not set.

[ColinFinck]

Summary

This allows building libc++ against winpthreads from mingw-w64 to support operating systems older than Windows 7.
The remaining libc++ code already supports WIN32 with LIBCXX_HAS_PTHREAD_API.

Note that there is also the older "pthreads-win32". However, that support library implements pthread_t as a struct, which violates the libc++ assumption that pthread_t is always a scalar and can be compared, ordered, and set to zero.

[mstorsjo]

The change looks fine to me (although I didn't try it myself - and with the additional request to add another condition to it), but someone else has to approve it as well. Can you provide your desired git author string (Full Name <email@domain>) to be used when it would be applied?

libcxx/src/CMakeLists.txt

82

This is the cmake define for when pthread api is explicitly forced to be used, instead of autodetection based on platform and __has_include(<pthread.h>) and such - right? In that case I guess it's ok.

There's also been a desire (in https://reviews.llvm.org/D88124#change-1LdJXjC8YEPj) to make compilation of this file optional based on LIBCXX_ENABLE_THREADS - I guess it'd be nice to do that at the same time while adding conditionals here.

[ColinFinck]

Colin Finck <[e-mail]>

[ldionne]

libcxx/src/CMakeLists.txt

82

Actually, it's impossible for LIBCXX_HAS_PTHREAD_API to be set to ON if LIBCXX_ENABLE_THREADS is OFF, so I think this is OK as-is.

[mstorsjo]

libcxx/src/CMakeLists.txt

82

I meant for the configuration with LIBCXX_HAS_PTHREAD_API disabled and LIBCXX_ENABLE_THREADS disabled too - in that case we still compile thread_win32.cpp (just as before this change). Not sure if there is an issue with such a config or not though - if such a patch is submitted (split out from D88124), I guess it would come with an explanation of exactly what issue it fixes.

libcxx/src/CMakeLists.txt

list(APPEND LIBCXX_SOURCES
     strstream.cpp
     )
 endif()
 
 if(WIN32)
   list(APPEND LIBCXX_SOURCES
     support/win32/locale_win32.cpp
     support/win32/support.cpp
+    )
+
+  if (NOT LIBCXX_HAS_PTHREAD_API)
+    list(APPEND LIBCXX_SOURCES
       support/win32/thread_win32.cpp
       )
+  endif()
 elseif("${CMAKE_SYSTEM_NAME}" STREQUAL "SunOS")
   list(APPEND LIBCXX_SOURCES
     support/solaris/mbsnrtowcs.inc
     support/solaris/wcsnrtombs.inc
     support/solaris/xlocale.cpp
     )
 endif()