[Dwarf-Discuss] address pool + offset representation

Robinson, Paul paul.robinson at sony.com
Wed Jul 26 12:00:17 PDT 2017


> From: Dwarf-Discuss [mailto:dwarf-discuss-bounces at lists.dwarfstd.org] On Behalf Of David Blaikie
> Sent: Wednesday, July 26, 2017 11:04 AM
> To: Doug Evans; dwarf-discuss at lists.dwarfstd.org
> Subject: Re: [Dwarf-Discuss] address pool + offset representation
>
>
> On Wed, Jul 26, 2017 at 10:51 AM Doug Evans <dje at google.com> wrote:
>> How about a new form that specifies two things: index in .debug_addr +
>> offset, where index and offset are any current constant representation
>> (fixed size or leb) ?
>
> Roughly what I had in mind - but wasn't sure how to encode that.
>
> Would the type of the representation for each component be encoded at
> the use? So this would be a variable length form (even when both the
> index+offset use fixed length forms - because the form would be at the
> use, so you'd have to read the bytes to figure out that they were using
> fixed length forms)?
>
> A two-value version of DW_FORM_indirect, I suppose?

This discussion is about reducing the number of .debug_addr entries you
need, so the place to look would be the "class address" attributes.
There aren't all that many of them, and the one most likely to benefit
would be DW_AT_low_pc.

We made it okay to have DW_AT_high_pc be a constant, which is implicitly
the sum of that constant with the same DIE's DW_AT_low_pc value.
How about if DW_AT_low_pc could also be a constant, which would be added
to the containing block/subprogram's DW_AT_low_pc?

>
> I know the LLDB folks seem to find that fixed-width forms especially
> help with parsing - not sure if GDB has a similar tendency/preference?

The producer can pick a fixed-size constant form to accommodate this.
It would have to be a big-enough form, as the assembler would be
computing the delta, but that's a QOI problem for the producer.
--paulr



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