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, 34 insertions, 3 deletions

diff --git a/gdb/value.c b/gdb/value.c
index 4b28213..7c36c31 100644
--- a/gdb/value.c
+++ b/gdb/value.c
@@ -2776,10 +2776,27 @@ unpack_long (struct type *type, const gdb_byte *valaddr)
     case TYPE_CODE_MEMBERPTR:
       {
 	LONGEST result;
-	if (nosign)
-	  result = extract_unsigned_integer (valaddr, len, byte_order);
+
+	if (type->bit_size_differs_p ())
+	  {
+	    unsigned bit_off = type->bit_offset ();
+	    unsigned bit_size = type->bit_size ();
+	    if (bit_size == 0)
+	      {
+		/* unpack_bits_as_long doesn't handle this case the
+		   way we'd like, so handle it here.  */
+		result = 0;
+	      }
+	    else
+	      result = unpack_bits_as_long (type, valaddr, bit_off, bit_size);
+	  }
 	else
-	  result = extract_signed_integer (valaddr, len, byte_order);
+	  {
+	    if (nosign)
+	      result = extract_unsigned_integer (valaddr, len, byte_order);
+	    else
+	      result = extract_signed_integer (valaddr, len, byte_order);
+	  }
 	if (code == TYPE_CODE_RANGE)
 	  result += type->bounds ()->bias;
 	return result;
@@ -3339,6 +3356,13 @@ pack_long (gdb_byte *buf, struct type *type, LONGEST num)
     case TYPE_CODE_FLAGS:
     case TYPE_CODE_BOOL:
     case TYPE_CODE_MEMBERPTR:
+      if (type->bit_size_differs_p ())
+	{
+	  unsigned bit_off = type->bit_offset ();
+	  unsigned bit_size = type->bit_size ();
+	  num &= ((ULONGEST) 1 << bit_size) - 1;
+	  num <<= bit_off;
+	}
       store_signed_integer (buf, len, byte_order, num);
       break;
 
@@ -3381,6 +3405,13 @@ pack_unsigned_long (gdb_byte *buf, struct type *type, ULONGEST num)
     case TYPE_CODE_BOOL:
     case TYPE_CODE_RANGE:
     case TYPE_CODE_MEMBERPTR:
+      if (type->bit_size_differs_p ())
+	{
+	  unsigned bit_off = type->bit_offset ();
+	  unsigned bit_size = type->bit_size ();
+	  num &= ((ULONGEST) 1 << bit_size) - 1;
+	  num <<= bit_off;
+	}
       store_unsigned_integer (buf, len, byte_order, num);
       break;