[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 ↗

(On Diff #104348)

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

36 ↗

(On Diff #104348)

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 ↗

(On Diff #105472)

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

compiler-rt/trunk/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)
+  set(DARWIN_${sdk_name}_CACHED_SYSROOT "" CACHE STRING "Darwin SDK path for SDK ${sdk_name}.")
+  if(DARWIN_${sdk_name}_CACHED_SYSROOT)
+    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 ${sdk_name}.internal Path
       RESULT_VARIABLE result_process
       OUTPUT_VARIABLE var_internal
       OUTPUT_STRIP_TRAILING_WHITESPACE
execute_process(
       ERROR_FILE /dev/null
     )
   else()
     set(${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}." FORCE)
 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