[PATCH, ppc64] Fix stepping into calls via thunk

[Luis Machado <lgustavo at codesourcery dot com>]

Hi,

And finally this is the last failure i was seeing for 
gdb.cp/virtfunc.exp for ppc64.

The failure was related to not being able to step into a member function 
call via a thunk, ppc64 would step over that call.

Thunk skipping is handled by logic in 
gdb/gnu-v3-abi.c:gnuv3_skip_trampoline. We go from the address to a 
thunk symbol to the real function symbol and then to its final address, 
where GDB inserts a step-resume breakpoint.

For ppc64, though, what GDB thinks is the address of the real function 
isn't really the address of the function, but a pointer to a function 
descriptor. We need to fetch the real function's address from that 
descriptor.

This patch does exactly that, and it is very simple.

The use of current_target made me scratch my head a little. I seem to 
recall we dropped further uses of this variable in the past, but this 
hook still seems to require the target_ops to be passed. Is this OK?

Thanks,
Luis

2013-06-04  Luis Machado  <lgustavo@codesourcery.com>

	* gdb/gnu-v3-abi.c (gnuv3_skip_trampoline): Handle thunk
	minimal symbols pointing to function descriptors.

diff --git a/gdb/gnu-v3-abi.c b/gdb/gnu-v3-abi.c
index 6812c1d..1821f18 100644
--- a/gdb/gnu-v3-abi.c
+++ b/gdb/gnu-v3-abi.c
@@ -1207,7 +1207,7 @@ gnuv3_get_type_from_type_info (struct value *type_info_ptr)
 static CORE_ADDR 
 gnuv3_skip_trampoline (struct frame_info *frame, CORE_ADDR stop_pc)
 {
-  CORE_ADDR real_stop_pc, method_stop_pc;
+  CORE_ADDR real_stop_pc, method_stop_pc, func_addr;
   struct gdbarch *gdbarch = get_frame_arch (frame);
   struct minimal_symbol *thunk_sym, *fn_sym;
   struct obj_section *section;
@@ -1236,6 +1236,16 @@ gnuv3_skip_trampoline (struct frame_info *frame, CORE_ADDR stop_pc)
     return 0;
 
   method_stop_pc = SYMBOL_VALUE_ADDRESS (fn_sym);
+
+  /* Some targets have minimal symbols pointing to function descriptors
+     (powerpc 64 for example).  Make sure to retrieve the address
+     of the real function from the function descriptor before passing on
+     the address to other layers of GDB.  */
+  func_addr = gdbarch_convert_from_func_ptr_addr (gdbarch, method_stop_pc,
+                                                  &current_target);
+  if (func_addr != 0)
+    method_stop_pc = func_addr;
+
   real_stop_pc = gdbarch_skip_trampoline_code
 		   (gdbarch, frame, method_stop_pc);
   if (real_stop_pc == 0)