[compiler-rt] Fix running tests on macOS when XCode is not installed

[arichardson]

Summary

If XCode is not installed, xcodebuild -version -sdk macosx Path will give
xcode-select: error: tool 'xcodebuild' requires Xcode, but active developer directory '/Library/Developer/CommandLineTools' is a command line tools instance
In this case the variable OSX_SYSROOT will be empty and
OSX_SYSROOT_FLAG is set to "-isysroot" (without a path).
This then causes the CompilerRTUnitTestCheckCxx target failed to for me
because "${COMPILER_RT_TEST_COMPILER} ${OSX_SYSROOT_FLAG} -E" expanded to
"clang -isysroot -E". This results in a warning "sysroot -E does not exist"
and the target fails to run because the C++ headers cannot be found.

[beanz]

compiler-rt/cmake/base-config-ix.cmake

104

Might as well just call xcrun always instead. I think really old versions of xcrun don't support this flag, but I doubt that is a problem worth worrying about.

[arichardson]

Always use xcrun

[beanz]

LGTM.

compiler-rt/cmake/base-config-ix.cmake

 else(LLVM_ENABLE_PER_TARGET_RUNTIME_DIR)
   set(COMPILER_RT_LIBRARY_OUTPUT_DIR
     ${COMPILER_RT_OUTPUT_DIR}/lib/${COMPILER_RT_OS_DIR})
   set(COMPILER_RT_LIBRARY_INSTALL_DIR
     ${COMPILER_RT_INSTALL_PATH}/lib/${COMPILER_RT_OS_DIR})
 endif()
 
 if(APPLE)
-  # On Darwin if /usr/include doesn't exist, the user probably has Xcode but not
-  # the command line tools. If this is the case, we need to find the OS X
-  # sysroot to pass to clang.
-  if(NOT EXISTS /usr/include)
+  # On Darwin if /usr/include/c++ doesn't exist, the user probably has Xcode but
+  # not the command line tools (or is using macOS 10.14 or newer). If this is
+  # the case, we need to find the OS X sysroot to pass to clang.
+  if(NOT EXISTS /usr/include/c++)
     execute_process(COMMAND xcodebuild -version -sdk macosx Path
        OUTPUT_VARIABLE OSX_SYSROOT
        ERROR_QUIET
        OUTPUT_STRIP_TRAILING_WHITESPACE)
+    # Fall back to xcrun if XCode is not installed
+    if (NOT OSX_SYSROOT)
+      execute_process(COMMAND xcrun -sdk macosx --show-sdk-path
+         OUTPUT_VARIABLE OSX_SYSROOT
+         ERROR_QUIET
+         OUTPUT_STRIP_TRAILING_WHITESPACE)
+    endif()
+    if (NOT OSX_SYSROOT OR NOT EXISTS ${OSX_SYSROOT})
+      message(WARNING "Detected OSX_SYSROOT ${OSX_SYSROOT} does not exist")
+    else()
+      message(STATUS "Found OSX_SYSROOT: ${OSX_SYSROOT}")
       set(OSX_SYSROOT_FLAG "-isysroot${OSX_SYSROOT}")
     endif()
+  else()
+    set(OSX_SYSROOT_FLAG "")
+  endif()
 
   option(COMPILER_RT_ENABLE_IOS "Enable building for iOS" On)
   option(COMPILER_RT_ENABLE_WATCHOS "Enable building for watchOS - Experimental" Off)
   option(COMPILER_RT_ENABLE_TVOS "Enable building for tvOS - Experimental" Off)
 
 else()
   option(COMPILER_RT_DEFAULT_TARGET_ONLY "Build builtins only for the default target" Off)
 endif()