[llvm-dev] DWARF .debug_aranges data objects and address spaces

[Robinson, Paul via llvm-dev]

> -----Original Message-----
> From: Pavel Labath <pavel at labath.sk>
> Sent: Tuesday, March 17, 2020 3:02 AM
> To: David Blaikie <dblaikie at gmail.com>; Robinson, Paul
> <paul.robinson at sony.com>
> Cc: llvm-dev at lists.llvm.org
> Subject: Re: [llvm-dev] DWARF .debug_aranges data objects and address
> spaces
> 
> On 16/03/2020 18:20, David Blaikie via llvm-dev wrote:
> > On Mon, Mar 16, 2020 at 9:31 AM Robinson, Paul <paul.robinson at sony.com
> > <mailto:paul.robinson at sony.com>> wrote:
> >
> >     With AVR being affected, upstreaming a patch to put segment
> >     selectors into .debug_aranges becomes completely reasonable.  There
> >     would likely want to be a target hook somewhere to return a value
> >     saying what size to use, with the default implementation returning
> zero.
> >
> >
> > *nod* something along those lines
> >
> 
> Does that mean putting the selector *only* into debug_aranges (and not
> debug_line, debug_frame, etc.)?

That was my thought, yes.  It's the only section where there is no other
context to determine whether a raw address is for code or for data.

> 
> Even though they are not really needed if the target only ever has one
> code address space, it seems somewhat odd to have different values for
> segment_selector_size in different sections.
> 
> In the DWARF spec these are described as "... containing the size in
> bytes of a segment selector on the _target system_". I would interpret
> the "target system" portion of that as meaning that the segment selector
> size is a property of a target, and hence, it should be consistent
> across all relevant sections.

For a target with actual segments (like 80x86) the selector would always
have to be present.

For a Harvard target there is no explicit selector in the machine code, 
and a strict reading of the DWARF spec would require the segement selector
size to be zero in all cases; but that leaves us where we are today, with
.debug_aranges being impossible to interpret correctly.

IMO, having a segment selector in .debug_aranges and nowhere else, for a
Harvard architecture, falls within the "permissive" aspect of DWARF.  It
solves an actual problem using what is IMO a reasonable interpretation of
the existing DWARF feature set.  If the AVR (+other Harvard-like) targets
prefer, I wouldn't stop them from adding a segment selector to all DWARF
sections, but it seems like a waste of space in other sections.

I'd be happy to propose a DWARF wiki item or even a non-normative bit of
text in the spec, to codify this.  It would affect consumers that target
a Harvard architecture, but they have to contend with this somehow in any
case.
--paulr

> 
> pl