[PATCH] Fix PR target/104871 (macosx-version-min wrong for macOS >= Big Sur (darwin20))

[Simon Wright <simon@pushface.org>]

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(resent with commit message format update)

This is the same sort of problem as in PR80204: at present, GCC 11 & 12 assume that if the 
OS version is >= 20, the compiler should see --mmacosx-version-min={major - 9}.{minor -1}.0, 
e.g. for OS version 21.3.0 that would be 12.2.0 (the linker sees -macosx-version-min, same 
arguments).

However, the native compiler clang treats 21.3.0 as 12.0.0: the compiler sees
  -triple x86_64-apple-macosx12.0.0
and the linker sees
  -platform_version macos 12.0.0 
the result of which is that linking an object file built with clang and one built with gcc gives e.g.

  ld: warning: object file (null.o) was built for newer macOS version (12.2) than being linked (12.0)

I propose the following patch, which works fine for me (darwin 21.3.0).

gcc/ChangeLog:
	
	2022-06-02  Simon Wright  <simon@pushface.org>
	
	PR target/104871
	* config/darwin-driver.cc (darwin_find_version_from_kernel): If the OS version is
           20 (macOS 11) or greater, report the minor version and the patch level as 0
           to match Apple clang’s behaviour.


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diff --git a/gcc/config/darwin-driver.cc b/gcc/config/darwin-driver.cc
index 30e0e64f280..c47599e3fcb 100644
--- a/gcc/config/darwin-driver.cc
+++ b/gcc/config/darwin-driver.cc
@@ -160,19 +160,13 @@ darwin_find_version_from_kernel (void)
     goto parse_failed;
 
   /* Darwin20 sees a transition to macOS 11.  In this, it seems that the
-     mapping to macOS minor version is now shifted to the kernel minor
-     version - 1 (at least for the initial releases).  */
+     mapping to macOS minor version and patch level is now always 0, 0
+     (at least for macOS 11 and 12).  */
   if (major_vers >= 20)
     {
-      int minor_vers = *version_p++ - '0';
-      if (ISDIGIT (*version_p))
-	minor_vers = minor_vers * 10 + (*version_p++ - '0');
-      if (*version_p++ != '.')
-	goto parse_failed;
-      if (minor_vers > 0)
-	minor_vers -= 1; /* Kernel 20.3 => macOS 11.2.  */
-      /* It's not yet clear whether patch level will be considered.  */
-      asprintf (&new_flag, "%d.%02d.00", major_vers - 9, minor_vers);
+      /* Apple clang doesn't include the minor version or the patch level
+         in the object file, nor does it pass it to ld  */
+      asprintf (&new_flag, "%d.00.00", major_vers - 9);
     }
   else if (major_vers - 4 <= 4)
     /* On 10.4 and earlier, the old linker is used which does not