# Complete Record Objects Allocated in Registers

## Part II: Complete Record Objects Allocated in Registers

### Introduction

For structure/union/class types, DWARF defines a `DW_AT_location`
attribute used in `DW_TAG_member` components to specify the layout of the
components in memory.

But what happens if an object of such a type is (desired to be)
allocated in one or more registers?  There is no guidance in DWARF about
whether this can be described. Further, it is easy to see that the
byte-oriented offsets used for in-memory layout descriptions do not map
naturally or reasonably to registers.

There is one assumption that we do make: the ABI-defined register
numbers can be usefully assumed to group registers with similar
properties so they have contiguous numbers. Thus it is reasonable to
think of register R1 as the successor of register R0 in an architecture
that has at least two generally similar registers. Conversely, while
dissimilar registers may have adjacent numbers, for example, vector
register V0 may happen to have the next higher register number after
general register R15 (in an architecture with 16 general registers),
this is arbitrary and of no significance. We assume the ABI group will
assign register numbers in a useful way.


### Concept Overview

For data member entries (`DW_TAG_member`):

- Rename `DW_AT_data_member_location` to `DW_AT_data_member_address`,
but otherwise give it the same semantics and keep the same DIE code.

- Add `DW_AT_data_member_regseg`, whose value is a location
description that yields a register segment name. For most purposes, an
attribute of form `DW_FORM_regseg` will be appropriate.

A data member may have either or both of these attributes. The
`DW_AT_data_member_address` attribute will be used if/when the object is
allocated in memory while `DW_AT_data_member_regseg` applies if/when the
object is allocated in one or more registers.

In the later case, the register number part of the register segment name
value is treated as a relative register number, with respect to the base
register allocation of the object.

It is expected that if one member of a structure/union/class has a
`DW_AT_data_member_address` attribute then all will. Similarly, if one
member has a `DW_AT_data_member_regseg` attribute then all will.

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2025-04-11: Withdrawn in favor of [250408.1][250408.1].

[250408.1]: https://dwarfstd.org/issues/250408.1.html