On 10/21/2015 06:19 AM, Ulrich Weigand wrote:
Richard Henderson wrote:
+@deftypefn {Target Hook} int TARGET_ADDR_SPACE_DEBUG (addr_space_t @var{as})
+Define this to define how the address space is encoded in dwarf.
+The result, @var{r}, should be positive to indicate
+@code{DW_AT_address_class @var{r}} should be emitted; or negative
+to indicate @code{DW_AT_segment} with location in register @code{~@var{r}}.
+@end deftypefn
From my reading of the DWARF specs, DW_AT_segment and DW_AT_address_class
are intended to be used for different purposes:
Any debugging information entry that contains a description of the location
of an object or subroutine may have a DW_AT_segment attribute, whose value
is a location description. The description evaluates to the segment selector
of the item being described.
vs.
Any debugging information entry representing a pointer or reference type or
a subroutine or subroutine type may have a DW_AT_address_class attribute,
whose value is an integer constant. The set of permissible values is
specific
to each target architecture.
Back when we implemented address space support for Cell/B.E. I interpreted
this to mean that DW_AT_segment was intended to be used for DIEs *defining*
an object, while DW_AT_address_class was intended to be used for pointer
types. (Since the former wasn't possible on Cell/B.E., we never actually
implemented DW_AT_segment back then.)
So you would recommend continuing to use address_class for these x86 segments?
I suppose the DW_AT_segment can go onto the base type, just like it does in the
GCC type system. E.g.
int __seg_fs *f;
<a> DW_TAG_base_type
DW_AT_byte_size 4
DW_AT_encoding 5
DW_AT_segment DW_OP_regx 58
<b> DW_TAG_pointer_type
DW_AT_byte_size 8
DW_AT_type <a>
But I'm not really sure that's any better.
B.t.w. Appendix A of the DWARF4 standard also does *not* list DW_AT_segment
as a valid attribute for a DW_TAG_pointer_type or DW_TAG_reference_type DIE.
Hmm, yes, that is suggestive that segment is supposed to be used for something
completely different, not especially relevant to what I'm attempting to do.
r~
