[Darwin] Clarify fix and continue support (NFC).

This updates the description of the support for fix and continue
debugging.

gcc/ChangeLog:

2019-10-15  Iain Sandoe  <iain@sandoe.co.uk>

	* config/darwin.c: Update description of fix and continue.

From-SVN: r277010

gcc/ChangeLog

 2019-10-15  Iain Sandoe  <iain@sandoe.co.uk>
 
+	* config/darwin.c: Update description of fix and continue.
+
+2019-10-15  Iain Sandoe  <iain@sandoe.co.uk>
+
 	* config/darwin.c (darwin_binds_local_p): Update to call
 	default_binds_local_p_3 () directly. amend comments.
 

gcc/config/darwin.c

@@ -49,18 +49,24 @@ along with GCC; see the file COPYING3.  If not see
 #include "intl.h"
 #include "optabs.h"
 
-/* Darwin supports a feature called fix-and-continue, which is used
-   for rapid turn around debugging.  When code is compiled with the
-   -mfix-and-continue flag, two changes are made to the generated code
-   that allow the system to do things that it would normally not be
-   able to do easily.  These changes allow gdb to load in
-   recompilation of a translation unit that has been changed into a
-   running program and replace existing functions and methods of that
-   translation unit with versions of those functions and methods
-   from the newly compiled translation unit.  The new functions access
-   the existing static symbols from the old translation unit, if the
-   symbol existed in the unit to be replaced, and from the new
-   translation unit, otherwise.
+/* Fix and Continue.
+
+   NOTES:
+   1) this facility requires suitable support from a modified version
+   of GDB, which is not provided on any system after MacOS 10.7/Darwin11.
+   2) There is no support for this in any X86 version of the FSF compiler.
+
+   Fix and continue was used on some earlier MacOS systems for rapid turn
+   around debugging.  When code is compiled with the -mfix-and-continue
+   flag, two changes are made to the generated code that allow the system
+   to do things that it would normally not be able to do easily.  These
+   changes allow gdb to load in recompilation of a translation unit that
+   has been changed into a running program and replace existing functions
+   and methods of that translation unit with versions of those functions
+   and methods from the newly compiled translation unit.  The new functions
+   access the existing static symbols from the old translation unit, if the
+   symbol existed in the unit to be replaced, and from the new translation
+   unit, otherwise.
 
    The changes are to insert 5 nops at the beginning of all functions
    and to use indirection to get at static symbols.  The 5 nops