PR71951: Fix unwinding with -fomit-frame-pointer

As described in PR71951, if libgcc is built with -fomit-frame-pointer,
unwinding crashes, for example while doing a backtrace.  The underlying
reason is the Dwarf unwinder does not setup the frame pointer register
in the initialization code.  When later unwinding a function that uses
the frame pointer, it tries to read FP using _Unwind_GetGR, and this
crashes if has never restored FP.  To unwind correctly the first frame
must save and restore FP (it is unwound in a special way so that it
uses SP instead of FP).  This is done by adding -fno-omit-frame-pointer.

    gcc/
	PR target/71951
	* config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define.

From-SVN: r253061

gcc/ChangeLog

+2017-09-21  Wilco Dijkstra  <wdijkstr@arm.com>
+
+	PR target/71951
+	* config/aarch64/aarch64.h (LIBGCC2_UNWIND_ATTRIBUTE): Define.
+
 2017-09-21  Richard Biener  <rguenther@suse.de>
 
 	* graphite-isl-ast-to-gimple.c (graphite_regenerate_ast_isl):

gcc/config/aarch64/aarch64.h

@@ -954,4 +954,12 @@ extern const char *host_detect_local_cpu (int argc, const char **argv);
 extern tree aarch64_fp16_type_node;
 extern tree aarch64_fp16_ptr_type_node;
 
+/* The generic unwind code in libgcc does not initialize the frame pointer.
+   So in order to unwind a function using a frame pointer, the very first
+   function that is unwound must save the frame pointer.  That way the frame
+   pointer is restored and its value is now valid - otherwise _Unwind_GetGR
+   crashes.  Libgcc can now be safely built with -fomit-frame-pointer.  */
+#define LIBGCC2_UNWIND_ATTRIBUTE \
+  __attribute__((optimize ("no-omit-frame-pointer")))
+
 #endif /* GCC_AARCH64_H */