[Dwarf-Discuss] modeling different address spaces

[Todd Allen]

Markus, Michael, David, Xing,

I always assumed that the segment support in DWARF was meant to be more general,
and support architectures where there was no single flat memory, and so the
segments were necessary for memory accesses.  I personally have not dealt with
any architectures where DW_AT_segment came into play, though.

Certainly x86 does not fall into that "truly distinct segments" category, at
least not in modern times.  The segment registers there (fs & gs, for example)
are an indirect way of specifying a base address within the flat address space.
They usually end up being used for thread-specific data structures where each
thread has a different segment selector which implies a different base address.
And it requires a syscall to interact with the base addresses, at least on
Linux.  The other segment registers (cs, ds, ss) are set-and-forget by the OS
typically.

The CUDA architecture is an interesting case.  It doesn't use DW_AT_segment at
all.  But it does use the DW_AT_address_class attribute to specify CUDA segments
(e.g. Global, Local, Shared, among many others) for variables and/or types.  So
it's fairly fine-grained.  You can, for example, have a shared pointer to a
global pointer to a local integer, and the DW_AT_address_class attribute can
convey that.

Some of those CUDA segments are for radically different sorts of memory
(e.g. very low latency Shared memory vs. high latency Global memory).  But other
distinctions seem more gratuitous (e.g. Param vs. Global memory).  I assume that
there's a CUDA under-the-hood mapping of many of the segments to regions of a
flat Global address space in there, but the CUDA architectures & drivers
deliberately hide that mapping.  So effectively you end up with all the segments
being distinct, as far as a debugger can tell.

On Thu, Jul 16, 2020 at 09:23:51AM +0000, Dwarf Discussion wrote:
>    Hello,
> 
> 
> 
>    What would be the recommended way to model variables that are allocated to
>    different address spaces?
> 
> 
> 
>    I found DW_OPT_xderef for dereferencing address-space qualified pointers
>    but the resulting memory location description wouldn't have an
>    address-space qualifier.
> 
> 
> 
>    I found DW_AT_address_class, which allows attaching an integer, which
>    could represent the address-space.  This sounds pretty close.  I'm a bit
>    thrown off by the example, though.
> 
> 
> 
>    Thanks,
> 
>    Markus.
> 

-- 
Todd Allen
Concurrent Real-Time