[Bug gdb/34426] New: [gdb] DW_AT_bit_size-only base types lose their exact bit width

https://sourceware.org/bugzilla/show_bug.cgi?id=34426

            Bug ID: 34426
           Summary: [gdb] DW_AT_bit_size-only base types lose their exact
                    bit width
           Product: gdb
           Version: HEAD
            Status: UNCONFIRMED
          Severity: normal
          Priority: P2
         Component: gdb
          Assignee: unassigned at sourceware dot org
          Reporter: firmiana402 at gmail dot com
  Target Milestone: ---

GDB loses the exact width of integer base types whose sole size
attribute is DW_AT_bit_size and whose width is not a multiple of 8.
It records the byte-rounded storage width as the logical width.

DWARF v5 Section 5.1 states:

    "A base type entry has a DW_AT_byte_size attribute or a
     DW_AT_bit_size attribute whose integer constant value (see
     Section 2.21 on page 56) is the amount of storage needed to hold
     a value of the type."

This permits DW_AT_bit_size as the sole size attribute.  Therefore,
this is a valid 31-bit integer type:

    DW_TAG_base_type
      DW_AT_encoding  DW_ATE_signed
      DW_AT_bit_size  31

In gdb/dwarf2/read.c, read_base_type does not preserve that exact
width when DW_AT_byte_size is absent.  It first computes:

    if (byte_size.has_value ())
      bits = TARGET_CHAR_BIT * *byte_size;
    else if (bit_size.has_value ())
      bits = align_up (*bit_size, 8);

For this type, bits becomes 32 and is passed to init_integer_type.  In
gdb/gdbtypes.c, that function records the same rounded value as the
logical bit size:

    t = alloc.new_type (TYPE_CODE_INT, bit, name);
    ...
    TYPE_MAIN_TYPE (t)->type_specific.int_stuff.bit_size = bit;

The type therefore has both a 4-byte storage size and a 32-bit logical
width.  Later in read_base_type, the original DW_AT_bit_size is
restored only when DW_AT_byte_size is also present:

    if (TYPE_SPECIFIC_FIELD (type) == TYPE_SPECIFIC_INT
        && byte_size.has_value ()
        && bit_size.has_value ())
      {
        ...
        type->main_type->type_specific.int_stuff.bit_size = *bit_size;
      }

Because the valid type above has no DW_AT_byte_size, this block is
skipped and the logical width remains 32 instead of 31.

This might be an implementation/specification mismatch in read_base_type.

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