[PATCH] D28441: [libc++] [CMake] Link with /nodefaultlibs on Windows

[Eric Fiselier via Phabricator via cfe-commits]

EricWF added inline comments.


================
Comment at: CMakeLists.txt:388
+# non-debug DLLs
+remove_flags("/D_DEBUG" "/MTd" "/MDd" "/MT" "/Md" "/RTC1")
+
----------------
smeenai wrote:
> cl (and presumably clang-cl) also accepts all these flags with a leading dash instead of a leading slash, and cmake at least has a tendency to do `-D` instead of `/D`, so you might need to take those into account as well. Also, what about the other potential `/RTC` options?
My goal here is only to remove flags which CMake adds by default as part of `CMAKE_CXX_FLAGS_<BUILD_TYPE>_INIT`.


================
Comment at: lib/CMakeLists.txt:108-109
+if (LIBCXX_TARGETING_WINDOWS)
+  add_compile_flags(/Zl)
+  add_link_flags(/nodefaultlib)
+  add_library_flags(ucrt) # Universal C runtime
----------------
smeenai wrote:
> halyavin wrote:
> > smeenai wrote:
> > > These should be guarded under a check for a cl or cl-like frontend rather than `LIBCXX_TARGETING_WINDOWS` (since in theory we could be using the regular clang frontend to compile for Windows as well).
> > Regular clang supports both gcc-like and cl-like options (there are 2 compilers: clang.exe and clang-cl.exe). I think it is not worth it to support both considering they differ only in command line options handling.
> I'm aware of the separate drivers, but I still think it's worthwhile specifying appropriate conditionals when it's easy enough to do. (In this case, the inverse check of https://reviews.llvm.org/diffusion/L/browse/libcxx/trunk/CMakeLists.txt;291339$394 should do the trick.)
Is it alright if libc++'s CMakeLists.txt only supports targeting `clang-cl` for the time being? I would love to also support `clang++` but that seems like a ways off.

For now my only concern is getting `clang-cl` working. Expanding to include supporting `clang++` seems like it's a ways away. @smeenai Would this be a regression for you?


================
Comment at: lib/CMakeLists.txt:111
+  add_library_flags(ucrt) # Universal C runtime
+  add_library_flags(vcruntime) # C++ runtime
+  add_library_flags(msvcrt) # C runtime startup files
----------------
smeenai wrote:
> halyavin wrote:
> > smeenai wrote:
> > > Idk if there's anything specific to C++ in vcruntime; it's more compiler runtime functions as far as I know.
> > It contains exception handling stuff.
> You're right, but it also contains `longjmp`, `memcpy`, `memmove`, `memset`, etc, which is why I found the comment slightly weird initially. I guess it's fairly accurate as far as the usage of vcruntime in libc++ goes though.
FYI here is the documentation I was reading when deciding what libraries to link: https://msdn.microsoft.com/en-us/library/abx4dbyh.aspx


================
Comment at: lib/CMakeLists.txt:113
+  add_library_flags(msvcrt) # C runtime startup files
+  add_library_flags(iso_stdio_wide_specifiers)
+endif()
----------------
smeenai wrote:
> Maybe add a comment explaining the purpose of this one as well?
Not sure what the comment should read. I originally implemented this patch without this library, but during testing it generated a undefined reference to `___PLEASE_LINK_WITH_iso_stdio_wide_specifiers.lib`.  So should the comment read "MSVC told me to link this"?


https://reviews.llvm.org/D28441