[Darwin][compiler-rt] Test TARGET_OS_* conditionals using preprocessor directives

[veprbl]

Summary

This helps to avoid compilation errors like

lib/sanitizer_common/sanitizer_mac.cpp:617:7: error: use of undeclared identifier 'TARGET_OS_IOS'
  if (TARGET_OS_IOS || TARGET_OS_TV) return 6;
      ^

[yln]

Hi Dmitry, I am happy with the quality of this patch, but I don't understand why it is required.

These constant are defined in TargetConditionals.h (included via sanitizer_common/sanitizer_platform.h) which is part of the Apple SDK and should always be available.
Maybe the SDK paths weren't correctly configured or are you using manual -isysroot overrides?

[veprbl]

I can give some clarifications. The environment which I'm using is based on Nix/Nixpkgs where we are packaging our own SDK based on what is available from the open source releases that were done by Apple. Originally I googled the issue with the defines and saw that people outside of LLVM and Nix had similar issues, so I assumed it was a common issue with perhaps a slightly outdated SDK (we are using 10.12). Your response had prompted me to investigate this a little bit more, and by running the compiler with a nice -H option I learned that the particular instance of TargetConditionals.h that was using in compiler-rt's compilation wasn't coming from the SDK or the Swift CF framework. Instead it was coming from a hardcoded file [1] that may have been based on some SDK at the time, but definitely have not been updated in 6 years and is missing some of the definitions that are currently needed to build the compiler-rt.

I'm not really sure about all the moving parts involved, but, I think, I could try to fix the issue by updating TargetConditionals.h on our end.

[1] https://github.com/NixOS/nixpkgs/blame/5c770035c8fa1e84b1362dc2fe6da30be0eef363/pkgs/os-specific/darwin/apple-source-releases/Libsystem/default.nix#L63-L90

[veprbl]

Looking at the TargetConditionals.h from our 10.12 SDK, I see that there are actually branches that lead to some of the TARGET_OS_ definitions staying undefined for non-apple compilers, so it seems like this patch has some merit to it.

[veprbl]

Downstream PR https://github.com/NixOS/nixpkgs/pull/127229

compiler-rt/lib/sanitizer_common/sanitizer_mac.cpp

 // Offset example:
 // XNU 17 -- macOS 10.13 -- iOS 11 -- tvOS 11 -- watchOS 4
 constexpr u16 GetOSMajorKernelOffset() {
-  if (TARGET_OS_OSX) return 4;
-  if (TARGET_OS_IOS || TARGET_OS_TV) return 6;
-  if (TARGET_OS_WATCH) return 13;
+#  if TARGET_OS_OSX
+  return 4;
+#  endif
+#  if TARGET_OS_IOS || TARGET_OS_TV
+  return 6;
+#  endif
+#  if TARGET_OS_WATCH
+  return 13;
+#  endif
 }
 
 using VersStr = char[64];

   u16 os_major = kernel_major - offset;
 
   const char *format = "%d.0";
-  if (TARGET_OS_OSX) {
-    if (os_major >= 16) {  // macOS 11+
-      os_major -= 5;
-    } else {  // macOS 10.15 and below
-      format = "10.%d";
-    }
+#  if TARGET_OS_OSX
   }
+#  endif
   return internal_snprintf(vers, sizeof(VersStr), format, os_major);
 }
 

 // Aligned versions example:
 // macOS 10.15 -- iOS 13 -- tvOS 13 -- watchOS 6
 static void MapToMacos(u16 *major, u16 *minor) {
-  if (TARGET_OS_OSX)
-    return;
-
-  if (TARGET_OS_IOS || TARGET_OS_TV)
-    *major += 2;
-  else if (TARGET_OS_WATCH)
-    *major += 9;
-  else
-    UNREACHABLE("unsupported platform");
+#  if !TARGET_OS_OSX
+#    if TARGET_OS_IOS || TARGET_OS_TV
+  *major += 2;
+#    elif TARGET_OS_WATCH
+  *major += 9;
+#    else
+  UNREACHABLE("unsupported platform");
+#    endif
 
   if (*major >= 16) {  // macOS 11+
     *major -= 5;

     *minor = *major;
     *major = 10;
   }
+#  endif
 }
 
 static MacosVersion GetMacosAlignedVersionInternal() {