gdb/rs6000: Read backchain as unsigned.

Previously, backchain was read as a signed quantity, resulting in
addresses like 0xfffffffffffeded0 instead of 0xfffeded0 returned by
unwinder on 32-bit powerpc.  While normally such addresses are masked
off, this causes problems for tracepoints, since 0xfffffffffffeded0
is considered unavailable.

Fixes a test failure in gdb.trace/entry-values.exp.

gdb/ChangeLog:

	* corefile.c (safe_read_memory_unsigned_integer): New function.
	* gdbcore.h (safe_read_memory_unsigned_integer): New prototype.
	* rs6000-tdep.c (rs6000_frame_cache): Read backchain as unsigned.

gdb/ChangeLog

@@ -1,3 +1,9 @@
+2016-03-09  Marcin Kościelnicki  <koriakin@0x04.net>
+
+	* corefile.c (safe_read_memory_unsigned_integer): New function.
+	* gdbcore.h (safe_read_memory_unsigned_integer): New prototype.
+	* rs6000-tdep.c (rs6000_frame_cache): Read backchain as unsigned.
+
 2016-03-09  Marcin Kościelnicki  <koriakin@0x04.net>
 
 	* rs6000-tdep.c: Add "ax.h" and "ax-gdb.h" includes.

gdb/corefile.c

@@ -306,6 +306,24 @@ safe_read_memory_integer (CORE_ADDR memaddr, int len,
   return 1;
 }
 
+/* Read memory at MEMADDR of length LEN and put the contents in
+   RETURN_VALUE.  Return 0 if MEMADDR couldn't be read and non-zero
+   if successful.  */
+
+int
+safe_read_memory_unsigned_integer (CORE_ADDR memaddr, int len,
+				   enum bfd_endian byte_order,
+				   ULONGEST *return_value)
+{
+  gdb_byte buf[sizeof (ULONGEST)];
+
+  if (target_read_memory (memaddr, buf, len))
+    return 0;
+
+  *return_value = extract_unsigned_integer (buf, len, byte_order);
+  return 1;
+}
+
 LONGEST
 read_memory_integer (CORE_ADDR memaddr, int len,
 		     enum bfd_endian byte_order)

gdb/gdbcore.h

@@ -76,6 +76,9 @@ extern int safe_read_memory_integer (CORE_ADDR memaddr, int len,
 extern ULONGEST read_memory_unsigned_integer (CORE_ADDR memaddr,
 					      int len,
 					      enum bfd_endian byte_order);
+extern int safe_read_memory_unsigned_integer (CORE_ADDR memaddr, int len,
+					      enum bfd_endian byte_order,
+					      ULONGEST *return_value);
 
 /* Read an integer from debugged code memory, given address,
    number of bytes, and byte order for code.  */

gdb/rs6000-tdep.c

@@ -3336,10 +3336,10 @@ rs6000_frame_cache (struct frame_info *this_frame, void **this_cache)
   if (!fdata.frameless)
     {
       /* Frameless really means stackless.  */
-      LONGEST backchain;
+      ULONGEST backchain;
 
-      if (safe_read_memory_integer (cache->base, wordsize,
-				    byte_order, &backchain))
+      if (safe_read_memory_unsigned_integer (cache->base, wordsize,
+					     byte_order, &backchain))
         cache->base = (CORE_ADDR) backchain;
     }