[cmake] Cache results of find_darwin_sdk_dir

[kubamracek]

Summary

This improves find_darwin_sdk_dir to cache the results of executing xcodebuild to find the SDK. Should significantly reduce the CMake re-configure time.

[kubamracek]

ping

[george.karpenkov]

cmake/Modules/CompilerRTDarwinUtils.cmake

11

Could you move this change to a separate review and additionally move the variable declaration outside of the function?

36

Can we move the declaration to top of the file, and then use FORCE to set it at the end of the function?

[kubamracek]

I've split out the separate change into https://reviews.llvm.org/D35071.

[george.karpenkov]

LGTM

[kubamracek]

cmake/Modules/CompilerRTDarwinUtils.cmake

3–4

Actually, this doesn't work, as ${sdk_name} isn't available. I'm afraid I'll have to keep this declaration inside the function...

cmake/Modules/CompilerRTDarwinUtils.cmake

 include(CMakeParseArguments)
 
 # On OS X SDKs can be installed anywhere on the base system and xcode-select can
 # set the default Xcode to use. This function finds the SDKs that are present in
 # the current Xcode.
 function(find_darwin_sdk_dir var sdk_name)
+  if(NOT "${DARWIN_${sdk_name}_CACHED_SYSROOT}" STREQUAL "")
+    set(${var} ${DARWIN_${sdk_name}_CACHED_SYSROOT} PARENT_SCOPE)
+    return()
+  endif()
+  set(DARWIN_PREFER_PUBLIC_SDK OFF CACHE BOOL "Prefer Darwin public SDK, even when an internal SDK is present.")
+  if(NOT DARWIN_PREFER_PUBLIC_SDK)
     # Let's first try the internal SDK, otherwise use the public SDK.
     execute_process(
       COMMAND xcodebuild -version -sdk ${sdk_name}.internal Path
       RESULT_VARIABLE result_process
       OUTPUT_VARIABLE var_internal
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_FILE /dev/null
     )
+  endif()
   if((NOT result_process EQUAL 0) OR "" STREQUAL "${var_internal}")
     execute_process(
       COMMAND xcodebuild -version -sdk ${sdk_name} Path
       RESULT_VARIABLE result_process
       OUTPUT_VARIABLE var_internal
       OUTPUT_STRIP_TRAILING_WHITESPACE
       ERROR_FILE /dev/null
     )
   else()
     set(${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}.")
 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 ld -v