[CMake] Disable building all Darwin libraries (except builtins) for macOS i386 when the SDK is >= 10.15.

[delcypher]

Summary

In the macOS 10.15 SDK the ability to link i386 binaries was removed and
in the corresponding OS it is not possible to run macOS i386 binaries.

The consequence of these changes meant that targets like check-asan
would fail because:

• Unit tests could not be linked for i386
• Lit tests for i386 would fail due to not being able to execute compiled binaries.

The simplest fix to this is to simply disable building for i386 for
macOS when using the 10.15 SDK (or newer). This disables building the
i386 slice for most compiler-rt libraries and consequently disables the
unit and lit tests for macOS i386.

Note that because the DARWIN_osx_ARCHS CMake variable is a cache
variable this patch will have no affect on existing builds unless
the existing cache variable is deleted. The simplest way to deal with
this is delete existing builds and just do a fresh configure.

Note this should not affect the builtins which are managed with
the DARWIN_osx_BUILTIN_ARCHS CMake cache variable.

For those who wish to force using a particular set of architectures when
using newer SDKs passing -DDARWIN_osx_ARCHS=i386;x86_64;x86_64h to
CMake should provide a usable (but completely unsupported) workaround.

rdar://problem/55668535
rdar://problem/47939978

[delcypher]

I'm including so many reviewers here because this affects anyone who ships a Clang toolchain that wishes to target macOS i386.

[yln]

I confirmed that ninja check-asan works with this patch on my macOS 10.15 machine.

[delcypher]

@yln Thanks for the review!

[arichardson]

This breaks the build for me. I don't have XCode installed. Can we get the same information using xcrun instead?

$ xcrun --show-sdk-version
10.14.6

[vchuravy]

This broke cross-compilation from Linux for us, since we don't have xcodebuild/xcrun . We currently build with

CMAKE_FLAGS="-DDARWIN_macosx_CACHED_SYSROOT:STRING=/opt/x86_64-apple-darwin14/x86_64-apple-darwin14/sys-root"

export MACOSX_DEPLOYMENT_TARGET=10.9
export LDFLAGS=-mmacosx-version-min=10.9

I would be happy to provide a -DDARWIN_macosx_CACHED_SDKVERSION

cmake/Modules/CompilerRTDarwinUtils.cmake

else()
     set(${var}_INTERNAL ${var_internal} PARENT_SCOPE)
   endif()
   if(result_process EQUAL 0)
     set(${var} ${var_internal} PARENT_SCOPE)
   endif()
   set(DARWIN_${sdk_name}_CACHED_SYSROOT ${var_internal} CACHE STRING "Darwin SDK path for SDK ${sdk_name}." FORCE)
 endfunction()
 
+function(find_darwin_sdk_version var sdk_name)
+  # We deliberately don't cache the result here because
+  # CMake's caching causes too many problems.
+  set(result_process 1)
+  if(NOT DARWIN_PREFER_PUBLIC_SDK)
+    # Let's first try the internal SDK, otherwise use the public SDK.
+    execute_process(
+      COMMAND xcodebuild -version -sdk ${sdk_name}.internal SDKVersion
+      RESULT_VARIABLE result_process
+      OUTPUT_VARIABLE var_internal
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_FILE /dev/null
+    )
+  endif()
+  if((NOT ${result_process} EQUAL 0) OR "" STREQUAL "${var_internal}")
+    execute_process(
+      COMMAND xcodebuild -version -sdk ${sdk_name} SDKVersion
+      RESULT_VARIABLE result_process
+      OUTPUT_VARIABLE var_internal
+      OUTPUT_STRIP_TRAILING_WHITESPACE
+      ERROR_FILE /dev/null
+    )
+  endif()
+  if(NOT result_process EQUAL 0)
+    message(FATAL_ERROR
+      "Failed to determine SDK version for \"${sdk_name}\" SDK")
+  endif()
+  # Check reported version looks sane.
+  if (NOT "${var_internal}" MATCHES "^[0-9]+\\.[0-9]+(\\.[0-9]+)?$")
+    message(FATAL_ERROR
+      "Reported SDK version \"${var_internal}\" does not look like a version")
+  endif()
+  set(${var} ${var_internal} PARENT_SCOPE)
+endfunction()
+
 # There isn't a clear mapping of what architectures are supported with a given
 # target platform, but ld's version output does list the architectures it can
 # link for.
 function(darwin_get_toolchain_supported_archs output_var)
   execute_process(
     COMMAND "${CMAKE_LINKER}" -v
     ERROR_VARIABLE LINKER_VERSION)
 
if(${valid_archs})
     return()
   endif()
 
   set(archs ${ARGN})
   if(NOT TEST_COMPILE_ONLY)
     message(STATUS "Finding valid architectures for ${os}...")
     set(SIMPLE_C ${CMAKE_BINARY_DIR}${CMAKE_FILES_DIRECTORY}/src.c)
     file(WRITE ${SIMPLE_C} "#include <stdio.h>\nint main() { printf(__FILE__); return 0; }\n")
-  
     set(os_linker_flags)
     foreach(flag ${DARWIN_${os}_LINK_FLAGS})
       set(os_linker_flags "${os_linker_flags} ${flag}")
     endforeach()
+
+    # Disable building for i386 for macOS SDK >= 10.15. The SDK doesn't support
+    # linking for i386 and the corresponding OS doesn't allow running macOS i386
+    # binaries.
+    if ("${os}" STREQUAL "osx")
+      find_darwin_sdk_version(macosx_sdk_version "macosx")
+      if ("${macosx_sdk_version}" VERSION_GREATER 10.15 OR "${macosx_sdk_version}" VERSION_EQUAL 10.15)
+        message(STATUS "Disabling i386 slice for ${valid_archs}")
+        list(REMOVE_ITEM archs "i386")
+      endif()
+    endif()
   endif()
 
   # The simple program will build for x86_64h on the simulator because it is 
   # compatible with x86_64 libraries (mostly), but since x86_64h isn't actually
   # a valid or useful architecture for the iOS simulator we should drop it.
   if(${os} MATCHES "^(iossim|tvossim|watchossim)$")
     list(REMOVE_ITEM archs "x86_64h")
   endif()