Fix copy_bitwise()

When the user writes or reads a variable whose location is described
with DWARF pieces (DW_OP_piece or DW_OP_bit_piece), GDB's helper
function copy_bitwise is invoked for each piece.  The implementation of
this function has a bug that may result in a corrupted copy, depending
on alignment and bit size.  (Full-byte copies are not affected.)

This rewrites copy_bitwise, replacing its algorithm by a fixed version,
and adding an appropriate test case.  Without the fix the new test case
fails, e.g.:

  print def_t
  $2 = {a = 0, b = 4177919}
  (gdb) FAIL: gdb.dwarf2/nonvar-access.exp: print def_t

Written in binary, the wrong result above looks like this:

  01111111011111111111111

Which means that two zero bits have sneaked into the copy of the
original all-one bit pattern.  The test uses this simple all-one value
in order to avoid another GDB bug that causes the DWARF piece of a
DW_OP_stack_value to be taken from the wrong end on big-endian
architectures.

gdb/ChangeLog:

	* dwarf2loc.c (extract_bits_primitive): Remove.
	(extract_bits): Remove.
	(copy_bitwise): Rewrite.  Fixes a possible corruption that may
	occur for non-byte-aligned copies.

gdb/testsuite/ChangeLog:

	* gdb.dwarf2/nonvar-access.exp: Add a test for accessing
	non-byte-aligned bit fields.

2 files changed, 37 insertions, 1 deletions

diff --git a/gdb/testsuite/ChangeLog b/gdb/testsuite/ChangeLog
index e9dc36f..8147091 100644
--- a/gdb/testsuite/ChangeLog
+++ b/gdb/testsuite/ChangeLog
@@ -1,5 +1,10 @@
 2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
 
+	* gdb.dwarf2/nonvar-access.exp: Add a test for accessing
+	non-byte-aligned bit fields.
+
+2016-11-24  Andreas Arnez  <arnez@linux.vnet.ibm.com>
+
 	PR gdb/12616
 	* gdb.dwarf2/nonvar-access.exp: New testcase.  Check that GDB
 	respects the DW_AT_data_bit_offset attribute.
diff --git a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
index 21532a6..8910f6d 100644
--- a/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
+++ b/gdb/testsuite/gdb.dwarf2/nonvar-access.exp
@@ -32,7 +32,7 @@ Dwarf::assemble $asm_file {
 	    {DW_AT_name main.c}
 	} {
 	    declare_labels int_type_label short_type_label
-	    declare_labels struct_s_label
+	    declare_labels struct_s_label struct_t_label
 
 	    int_type_label: base_type {
 		{name "int"}
@@ -58,6 +58,24 @@ Dwarf::assemble $asm_file {
 		}
 	    }
 
+	    struct_t_label: structure_type {
+		{name t}
+		{byte_size 4 DW_FORM_sdata}
+	    } {
+		member {
+		    {name a}
+		    {type :$int_type_label}
+		    {data_member_location 0 DW_FORM_udata}
+		    {bit_size 9 DW_FORM_udata}
+		}
+		member {
+		    {name b}
+		    {type :$int_type_label}
+		    {data_bit_offset 9 DW_FORM_udata}
+		    {bit_size 23 DW_FORM_udata}
+		}
+	    }
+
 	    DW_TAG_subprogram {
 		{name main}
 		{DW_AT_external 1 flag}
@@ -84,6 +102,18 @@ Dwarf::assemble $asm_file {
 			bit_piece 24 0
 		    } SPECIAL_expr}
 		}
+		DW_TAG_variable {
+		    {name def_t}
+		    {type :$struct_t_label}
+		    {location {
+			const1u 0
+			stack_value
+			bit_piece 9 0
+			const1s -1
+			stack_value
+			bit_piece 23 0
+		    } SPECIAL_expr}
+		}
 	    }
 	}
     }
@@ -99,5 +129,6 @@ if ![runto_main] {
 }
 
 gdb_test "print def_s" " = \\{a = 0, b = -1\\}"
+gdb_test "print def_t" " = \\{a = 0, b = -1\\}"
 gdb_test "print undef_int" " = <optimized out>"
 gdb_test "print undef_s.a" " = <optimized out>"