perf probe: Fix perf probe to find correct variable DIE

Fix perf probe to find correct variable DIE which has location or
external instance by tracking down the lexical blocks.

Current die_find_variable() expects that the all variable DIEs
which has DW_TAG_variable have a location. However, since recent
dwarf information may have declaration variable DIEs at the
entry of function (subprogram), die_find_variable() returns it.

To solve this problem, it must track down the DIE tree to find
a DIE which has an actual location or a reference for external
instance.

e.g. finding a DIE which origin is <0xdc73>;

<1><11496>: Abbrev Number: 95 (DW_TAG_subprogram)
<11497> DW_AT_abstract_origin: <0xdc42>
<1149b> DW_AT_low_pc : 0x1850
[...]
<2><114cc>: Abbrev Number: 119 (DW_TAG_variable) <- this is a declaration
<114cd> DW_AT_abstract_origin: <0xdc73>
<2><114d1>: Abbrev Number: 119 (DW_TAG_variable)
[...]
<3><115a7>: Abbrev Number: 105 (DW_TAG_lexical_block)
<115a8> DW_AT_ranges : 0xaa0
<4><115ac>: Abbrev Number: 96 (DW_TAG_variable) <- this has a location
<115ad> DW_AT_abstract_origin: <0xdc73>
<115b1> DW_AT_location : 0x486c (location list)

Signed-off-by: Masami Hiramatsu <[e-mail]>
Tested-by: Arnaldo Carvalho de Melo <[e-mail]>
Acked-by: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Arnaldo Carvalho de Melo <[e-mail]>
Cc: Peter Zijlstra <[e-mail]>
Cc: Paul Mackerras <[e-mail]>
Cc: Ingo Molnar <[e-mail]>
Cc: Namhyung Kim <[e-mail]>
Link: http://lkml.kernel.org/r/[e-mail]
Signed-off-by: Jiri Olsa <[e-mail]>

tools/perf/util/dwarf-aux.c

···
 static int __die_find_variable_cb(Dwarf_Die *die_mem, void *data)
 {
 	struct __find_variable_param *fvp = data;
+	Dwarf_Attribute attr;
 	int tag;
 
 	tag = dwarf_tag(die_mem);
 	if ((tag == DW_TAG_formal_parameter ||
 	     tag == DW_TAG_variable) &&
-	    die_compare_name(die_mem, fvp->name))
+	    die_compare_name(die_mem, fvp->name) &&
+	/* Does the DIE have location information or external instance? */
+	    (dwarf_attr(die_mem, DW_AT_external, &attr) ||
+	     dwarf_attr(die_mem, DW_AT_location, &attr)))
 		return DIE_FIND_CB_END;
-
 	if (dwarf_haspc(die_mem, fvp->addr))
 		return DIE_FIND_CB_CONTINUE;
 	else