On Wed, Jul 26, 2017 at 12:00 PM Robinson, Paul <paul.robin...@sony.com> wrote:
> > From: Dwarf-Discuss [mailto:dwarf-discuss-boun...@lists.dwarfstd.org] > On Behalf Of David Blaikie > > Sent: Wednesday, July 26, 2017 11:04 AM > > To: Doug Evans; dwarf-discuss@lists.dwarfstd.org > > Subject: Re: [Dwarf-Discuss] address pool + offset representation > > > > > > On Wed, Jul 26, 2017 at 10:51 AM Doug Evans <d...@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? > It'd catch a fair few, maybe even most, of the cases - though still potentially many more relocations than needed: eg: two (or more) functions in the same section would end up using separate relocations (assuming they were in a CU with at least one function in some other section - such as an inline function in a comdat). Certainly having a low_pc works like high_pc, but uses the nearest enclosing low_pc would be a handy encoding size win as well, though, but I was partly motivated by looking at what the actual minimum number of relocations would be and looking at possible features/implementations that would get all the way there. > > > > > 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. > Not sure I quite follow - if the form is "pair of implicit values" then that's the only form here. Inside the debug_info data, then there could be constant forms for the two elements of the pair - but it's still a dynamically sized form from the perspective of a parser. Am I picturing this wrong? > 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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