gdb.trace: Fix string collection for 64-bit platforms.

String collection always used ref32 to fetch the string pointer.  Make it
use gen_fetch instead.

As a side effect, this patch changes dup+const+trace+pop sequence used
for collecting the string's address to a trace_quick opcode.  This
results in a shorter agent expression.

This appeared to work on x86_64 since it's a little-endian platform, and
malloc (used in gdb.trace/collection.exp) returns addresses in low 4GB.
Noticed and tested on s390x-ibm-linux-gnu, also tested on
i686-unknown-linux-gnu and x86_64-unknown-linux-gnu.

gdb/ChangeLog:

	* ax-gdb.c (gen_traced_pop): Use gen_fetch for string collection.

2 files changed, 15 insertions, 12 deletions

diff --git a/gdb/ChangeLog b/gdb/ChangeLog
index 45d8ef9..28173b0 100644
--- a/gdb/ChangeLog
+++ b/gdb/ChangeLog
@@ -1,3 +1,7 @@
+2016-01-21  Marcin Kościelnicki  <koriakin@0x04.net>
+
+	* ax-gdb.c (gen_traced_pop): Use gen_fetch for string collection.
+
 2016-01-21  Andrew Burgess  <andrew.burgess@embecosm.com>
 
 	* disasm.c (maybe_add_dis_line_entry): Rename to...
diff --git a/gdb/ax-gdb.c b/gdb/ax-gdb.c
index dd6eee6..7c6cb64 100644
--- a/gdb/ax-gdb.c
+++ b/gdb/ax-gdb.c
@@ -394,26 +394,25 @@ gen_traced_pop (struct gdbarch *gdbarch,
 
       case axs_lvalue_memory:
 	{
-	  if (string_trace)
-	    ax_simple (ax, aop_dup);
-
 	  /* Initialize the TYPE_LENGTH if it is a typedef.  */
 	  check_typedef (value->type);
 
-	  /* There's no point in trying to use a trace_quick bytecode
-	     here, since "trace_quick SIZE pop" is three bytes, whereas
-	     "const8 SIZE trace" is also three bytes, does the same
-	     thing, and the simplest code which generates that will also
-	     work correctly for objects with large sizes.  */
-	  ax_const_l (ax, TYPE_LENGTH (value->type));
-	  ax_simple (ax, aop_trace);
-
 	  if (string_trace)
 	    {
-	      ax_simple (ax, aop_ref32);
+	      gen_fetch (ax, value->type);
 	      ax_const_l (ax, ax->trace_string);
 	      ax_simple (ax, aop_tracenz);
 	    }
+	  else
+	    {
+	      /* There's no point in trying to use a trace_quick bytecode
+	         here, since "trace_quick SIZE pop" is three bytes, whereas
+	         "const8 SIZE trace" is also three bytes, does the same
+	         thing, and the simplest code which generates that will also
+	         work correctly for objects with large sizes.  */
+	      ax_const_l (ax, TYPE_LENGTH (value->type));
+	      ax_simple (ax, aop_trace);
+	    }
 	}
 	break;