Issue 080112.2: FORM Reform Proposal 2: Block Class Forms and Expressions
Background
Since expressions can contain reference to DIEs, it's important for any
generic DWARF consumer to be able to identify and parse expressions without
a complete understanding of what attributes may have blocks that should be
treated as expressions. This proposal adds a new form, DW_FORM_expr, which
takes a ULEB128 length parameter, followed by "length" bytes interpreted as
an expression.
I don't believe that fixed 1-, 2-, 4-, and 8-byte lengths are necessary
here -- any producer should be able to determine how long the expression
is before it needs to emit the length field.
This proposal also deprecates DW_FORM_block1, block2, and block4 in favor
of DW_FORM_block. For blocks that aren't expressions (e.g., constant values
expressed as a block of bytes), the length of the block can just as easily
be expressed as a ULEB128. Any producer can reasonably be expected to know
how long the block is before it needs to emit the length.
Proposal
The proposed changes to the DWARF specification are detailed below, keyed
to the section numbering for the December 20, 2005, version of the DWARF-3
document.
In Section 2.2, "Attribute Types," add the following row to the table of
Figure 3:
expr An arbitrary number of bytes of data, interpreted
as a DWARF expression (see Section 2.5).
[Note: The terminology for DWARF expressions has been under discussion.
This proposal may need to be updated to be consistent with the final
terminology.]
In Section 6.4.2.2, "CFA Definition Instructions," Item 6,
"DW_CFA_def_cfa_expression", change "DW_FORM_block" to "DW_FORM_expr".
In Section 7.5.4, "Attribute Encodings," add the following paragraph to
the description of class block:
The use of forms DW_FORM_block1, DW_FORM_block2,
and DW_FORM_block4 is deprecated. Producers should
use DW_FORM_block instead.
Add the following paragraphs immediately preceding the description for
class flag:
expr
Represented as an unsigned LEB128 length followed by the
number of bytes specified by the length (DW_FORM_expr).
The information bytes contain a DWARF expression (see
Section 2.5).
In FIgure 20, "Attribute encodings," change "block" to "expr" in the
"Classes" column for the following attributes:
DW_AT_location
DW_AT_byte_size
DW_AT_bit_offset
DW_AT_bit_size
DW_AT_string_length
DW_AT_lower_bound
DW_AT_return_addr
DW_AT_upper_bound
DW_AT_count
DW_AT_data_member_location
DW_AT_discr_list
DW_AT_frame_base
DW_AT_namelist_item
DW_AT_segment
DW_AT_static_link
DW_AT_use_location
DW_AT_vtable_elem_location
DW_AT_allocated
DW_AT_associated
DW_AT_data_location
DW_AT_byte_stride
In Figure 21, add the following row:
Form name Value Class
DW_FORM_expr 0x18 expr
--
Accepted with changes:
1. Remove DW_AT_namelist_item from list of changes
2. Fix typo on pg. 136 -- DW_AT_namelist_item is reference, not block
3. No deprecation of block1-4
4. Clarify block form description (pg. 126) with reference to expr.
