[TSan] Add CMake check for minimal SDK requirements on Darwin

[yln]

Summary

Fails with the following message in the error case:

CMake Error at /path/to/llvm-project/compiler-rt/lib/tsan/CMakeLists.txt:119 (message):
  Building the TSan runtime requires at least macOS SDK 10.12

Fixes #44682.
https://bugs.llvm.org/show_bug.cgi?id=44682

[yln]

@dmajor: are you okay with this?

[dmajor]

In D74501#1879277, @yln wrote:

@dmajor: are you okay with this?

This patch is fine from my side. I was hoping that someone more involved with this code would do the formal signoff, but it seems nobody is speaking up...

[delcypher]

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

128

@yln Why are you changing this? This is a different piece of functionality and this verbose-ness is also deliberate. Older versions of CMake do not support VERSION_GREATER_EQUAL and so this change will break the build for older CMake versions.

compiler-rt/lib/tsan/CMakeLists.txt

117

You need to guard this check on osx being present in ${TSAN_SUPPORTED_OS}. It's possible to have compiler-rt not target osx so we shouldn't fatal error when we aren't actually building for macOS.

[yln]

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

128

Ahh, I didn't realize VERSION_GREATER_EQUAL isn't available in older versions of CMake. Good catch!

compiler-rt/lib/tsan/CMakeLists.txt

117

I would like to fail the build for "macOS SDK < 10.12 or aligned SDK versions on other platforms". Do you know how to best express that?

[yln]

Add comment explaining that we use macOS to check aligned SDK versions. Improve error message.

[delcypher]

LGTM.

compiler-rt/cmake/Modules/CompilerRTDarwinUtils.cmake

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 "${macosx_sdk_version}" VERSION_EQUAL 10.15)
+      if ("${macosx_sdk_version}" VERSION_GREATER_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

compiler-rt/lib/tsan/CMakeLists.txt

set(TSAN_HEADERS
   rtl/tsan_sync.h
   rtl/tsan_trace.h
   rtl/tsan_update_shadow_word_inl.h)
 
 set(TSAN_RUNTIME_LIBRARIES)
 add_compiler_rt_component(tsan)
 
 if(APPLE)
+  find_darwin_sdk_version(macosx_sdk_version "macosx")
+  if ("${macosx_sdk_version}" VERSION_LESS 10.12)
+    message(FATAL_ERROR "Building the TSan runtime requires at least macOS SDK 10.12")
+  endif()
+
   add_asm_sources(TSAN_ASM_SOURCES rtl/tsan_rtl_amd64.S rtl/tsan_rtl_aarch64.S)
 
   set(TSAN_LINK_LIBS ${SANITIZER_COMMON_LINK_LIBS})
 
   add_weak_symbols("ubsan" WEAK_SYMBOL_LINK_FLAGS)
   add_weak_symbols("sanitizer_common" WEAK_SYMBOL_LINK_FLAGS)
 
   add_compiler_rt_runtime(clang_rt.tsan