Fix size check in dwarf2_evaluate_loc_desc_full

This Rust bug report:

https://github.com/rust-lang/rust/issues/41970

noted an error from gdb.  What is happening here (for me, the original
report had a different error) is that a pieced DWARF expression is not
writing to every byte in the resulting value.  GDB errors in this
case.  However, it seems to me that it is always valid to write fewer
bytes; the issue comes from writing too many -- that is, the test is
reversed.  The test was also checking the sub-object, but this also
seems incorrect, as it's expected for the expression to write the
entirety of the enclosing object.  So, this patch reverses the test
and applies it to the outer type, not the subobject type.

Regtested on the buildbot.

gdb/ChangeLog
2017-07-09  Tom Tromey  <tom@tromey.com>

	* dwarf2loc.c (dwarf2_evaluate_loc_desc_full): Reverse size
	check and apply to outer type.

gdb/testsuite/ChangeLog
2017-07-09  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/shortpiece.exp: New file.

1 files changed, 100 insertions, 0 deletions

diff --git a/gdb/testsuite/gdb.dwarf2/shortpiece.exp b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
new file mode 100644
index 0000000000..8526a70e7a
--- /dev/null
+++ b/gdb/testsuite/gdb.dwarf2/shortpiece.exp
@@ -0,0 +1,100 @@
+# Copyright 2017 Free Software Foundation, Inc.
+
+# This program is free software; you can redistribute it and/or modify
+# it under the terms of the GNU General Public License as published by
+# the Free Software Foundation; either version 3 of the License, or
+# (at your option) any later version.
+#
+# This program is distributed in the hope that it will be useful,
+# but WITHOUT ANY WARRANTY; without even the implied warranty of
+# MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+# GNU General Public License for more details.
+#
+# You should have received a copy of the GNU General Public License
+# along with this program.  If not, see <http://www.gnu.org/licenses/>.
+
+load_lib dwarf.exp
+
+# This test can only be run on targets which support DWARF-2 and use gas.
+if {![dwarf2_support]} {
+    return 0
+}
+
+standard_testfile main.c shortpiece-dw.S
+
+# Make some DWARF for the test.
+set asm_file [standard_output_file $srcfile2]
+Dwarf::assemble $asm_file {
+    global pair
+
+    cu { addr_size 4 } {
+	compile_unit {} {
+	    declare_labels int_label ushort_label struct_label
+
+	    int_label: DW_TAG_base_type {
+		{DW_AT_byte_size 4 DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_unsigned}
+		{DW_AT_name "myint"}
+	    }
+	    ushort_label: DW_TAG_base_type {
+		{DW_AT_byte_size 2 DW_FORM_udata}
+		{DW_AT_encoding @DW_ATE_unsigned}
+		{DW_AT_name "myushort"}
+	    }
+
+	    struct_label: DW_TAG_structure_type {
+		{DW_AT_name "S"}
+		{DW_AT_byte_size 8 DW_FORM_udata}
+	    } {
+		DW_TAG_member {
+		    {DW_AT_name "a"}
+		    {DW_AT_type :${int_label}}
+		    {DW_AT_data_member_location 0 DW_FORM_udata}
+		}
+
+		DW_TAG_member {
+		    {DW_AT_name "b"}
+		    {DW_AT_type :${ushort_label}}
+		    {DW_AT_data_member_location 4 DW_FORM_udata}
+		}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "s1"}
+		{DW_AT_type :${struct_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {
+		    DW_OP_constu 1
+		    DW_OP_stack_value
+		    DW_OP_piece 4
+		    DW_OP_constu 0
+		    DW_OP_stack_value
+		    DW_OP_piece 2
+		} SPECIAL_expr}
+	    }
+
+	    DW_TAG_variable {
+		{DW_AT_name "s2"}
+		{DW_AT_type :${struct_label}}
+		{DW_AT_external 1 DW_FORM_flag}
+		{DW_AT_location {
+		    DW_OP_constu 1
+		    DW_OP_stack_value
+		    DW_OP_piece 4
+		    DW_OP_constu 0
+		    DW_OP_stack_value
+		    DW_OP_piece 8
+		} SPECIAL_expr}
+	    }
+	}
+    }
+}
+
+if { [prepare_for_testing "failed to prepare" ${testfile} \
+			  [list $srcfile $asm_file] {nodebug}] } {
+    return -1
+}
+
+gdb_test "p s1" " = {a = 1, b = 0}"
+gdb_test "p s2" \
+    "access outside bounds of object referenced via synthetic pointer"