[Dwarf-discuss] Enhancement: Expression Operation Vendor Extensibility Opcode

[Linder, Scott]

[AMD Official Use Only - General]

Background
==========

The vendor extension encoding space for DWARF expression operations
accommodates only 32 unique operations. In practice, the lack of a central
registry and a desire for backwards compatibility means vendor extensions are
never retired, even when standard versions are accepted into DWARF proper. This
has produced a situation where the effective encoding space available for new
vendor extensions is miniscule today.

To expand this encoding space we propose defining one DWARF operation in the
official encoding space which acts as a "prefix" for vendor extensions. It is
followed by a ULEB128 encoded vendor extension opcode, which is then followed
by the operands of the corresponding vendor extension operation.

This scheme opens up an infinite encoding space for arbitrary vendor
extensions, and in practical terms is no less compact than if a fixed-size
encoding were chosen, as was done for DW_LNS_extended_op. That is to say, when
compared with an alternative scheme which encodes the opcode with a single
unsigned byte: for the first 127 opcodes our approach is indistinguishable from
the alternative scheme; for the next 128 opcodes it requires one more byte than
that alternative scheme; and after 255 opcodes the alternative scheme is
exhausted.

Since vendor extension operations can have arbitrary semantics, the consumer
must understand them to be able to continue evaluating the expression. The only
use for a size operand would be for a consumer that only needs to print the
expression. Omitting a size operand makes the operation encoding more compact,
and this was deemed more important than the limited printing use case.
Therefore no ULEB128 size operand is present to provide the number of bytes of
following operands, unlike DW_LNS_extended_op.

A centralized registry of vendor extension opcodes which are in use, maintained
on the dwarfstd.org website or another suitable location, could also be
implemented as a part of this proposal. This would remove the need for vendors
to coordinate allocation themselves, and make it simpler to use more than one
vendor extension at a time. As there is support for an infinite number of
opcodes, the registration process could involve very limited review, and would
therefore pose a minimal burden to the maintainer of such a registry.

Proposal
========

1) In Section 2.5.1.7, p38, add a new code at the end of the list:

    3. DW_OP_user
        The DW_OP_user opcode encodes a vendor extension operation. It has at
        least one operand: a ULEB128 constant identifying a vendor extension
        operation. The remaining operands are defined by the vendor extension.
        The vendor extension opcode 0 is reserved and cannot be used by any
        vendor extension.

        <i>The DW_OP_user encoding space can be understood to supplement the
        space defined by DW_OP_lo_user and DW_OP_hi_user that is allocated by
        the standard for the same purpose.</i>

2) In Section 7.7.1, p226, add a new row to table 7.9:

    DW_OP_user  |  TBD  |  1+  | ULEB128 vendor extension opcode, followed by
                |       |      | vendor-extension-defined operands