Handle bit offset and bit size in base types

PR symtab/25470 points out that the Zig programming language allows
integers of various bit sizes (including zero), not just sizes that
are a multiple of 8.

This is supported in DWARF by applying both a byte size and a
DW_AT_bit_size.

This patch adds support for this feature to integer and boolean types.
Other base types are not handled -- for floating-point types, this
didn't seem to make sense, and for character types I didn't see much
need.  (These can be added later if desired.)

I've also added support for DW_AT_data_bit_offset at the same time.  I
don't know whether the Zig compiler requires this, but it was
described in the same section in the DWARF standard and was easy to
add.

A new test case is supplied, using the DWARF assembler.

gdb/ChangeLog
2020-09-23  Tom Tromey  <tom@tromey.com>

	PR symtab/25470:
	* value.c (unpack_long, pack_long, pack_unsigned_long): Handle bit
	offset and bit size.
	* printcmd.c (print_scalar_formatted): Handle zero-length
	integer.
	(print_scalar_formatted): Use bit_size_differs_p.
	* gdbtypes.h (enum type_specific_kind) <TYPE_SPECIFIC_INT>: New
	constant.
	(union type_specific): <int_stuff>: New member.
	(struct type) <bit_size_differs_p, bit_size, bit_offset>: New
	methods.
	* gdbtypes.c (init_integer_type, init_boolean_type): Initialize
	TYPE_SPECIFIC_FIELD.
	(recursive_dump_type, copy_type_recursive): Update.
	* dwarf2/read.c (read_base_type): Handle DW_AT_bit_size and
	DW_AT_data_bit_offset.

gdb/testsuite/ChangeLog
2020-09-23  Tom Tromey  <tom@tromey.com>

	* gdb.dwarf2/intbits.exp: New file.
	* gdb.dwarf2/intbits.c: New file.

1 files changed, 20 insertions, 0 deletions

diff --git a/gdb/dwarf2/read.c b/gdb/dwarf2/read.c
index a339141..977bdb8 100644
--- a/gdb/dwarf2/read.c
+++ b/gdb/dwarf2/read.c
@@ -18054,6 +18054,26 @@ read_base_type (struct die_info *die, struct dwarf2_cu *cu)
 
   type->set_endianity_is_not_default (gdbarch_byte_order (arch) != byte_order);
 
+  if (TYPE_SPECIFIC_FIELD (type) == TYPE_SPECIFIC_INT)
+    {
+      attr = dwarf2_attr (die, DW_AT_bit_size, cu);
+      if (attr != nullptr && DW_UNSND (attr) <= 8 * TYPE_LENGTH (type))
+	{
+	  unsigned real_bit_size = DW_UNSND (attr);
+	  attr = dwarf2_attr (die, DW_AT_data_bit_offset, cu);
+	  /* Only use the attributes if they make sense together.  */
+	  if (attr == nullptr
+	      || DW_UNSND (attr) + real_bit_size <= 8 * TYPE_LENGTH (type))
+	    {
+	      TYPE_MAIN_TYPE (type)->type_specific.int_stuff.bit_size
+		= real_bit_size;
+	      if (attr != nullptr)
+		TYPE_MAIN_TYPE (type)->type_specific.int_stuff.bit_offset
+		  = DW_UNSND (attr);
+	    }
+	}
+    }
+
   return set_die_type (die, type, cu);
 }