[Dwarf-Discuss] debug_aranges use and overhead

[Greg Clayton]

> On Mar 11, 2021, at 1:12 PM, Paul Robinson via Dwarf-Discuss <dwarf-discuss at lists.dwarfstd.org> wrote:
> 
> Tom Russell could perhaps speak to this better, but my understanding is that our debugger guys like having .debug_aranges, because parsing the CU DIE does take that extra effort.  I am unfamiliar with their code so I have to take their word on it.  But I can certainly imagine that probing hundreds to thousands of CUs in order to collect range information with lengthy range lists would be more expensive than running through a comparatively compact .debug_aranges list.  If Tom tells me I?m wrong, well, wouldn?t be the first time.

We will use them if they are there, but one interesting issue that we ran into with LLDB is some compile units might be in .debug_aranges because the compiler made a .debug_aranges section in the .o file, but others might not. So we had to add code to LLDB to figure out which compile units have any entries in the .debug_aranges section, and read the DW_AT_ranges from the DW_TAG_compile_unit if it exist, and if it doesn't, manually index the DWARF to create one on the fly each time.

>  
> One thing we have encountered (see issue 210113.1) is that when we?ve done dead-stripping, .debug_aranges entries (one per function, typically, because -ffunction-sections) can end up pointing to nothing.  In our proprietary linker I believe we compress/rewrite .debug_aranges to minimize the number of entries, which by coincidence ends up producing a conforming aranges list; LLD doesn?t do that, which means it produces a non-conforming list (with zero-length entries), hence the issue. 
>  
> I?ll have to think about what a ?modern? .debug_aranges might want to look like.

A big issue with any of the DWARF sections is we are subject to making the contents work with linkers that just want to concatenate + relocate. This often leads to information being kept around when dead stripping occurs because anything that is dead stripped will just have its address zero'ed out or -1'ed out, but this bogus info is still in the data.

If we don't need a format that can simply be concatenated and relocated, the GSYM format, which is open sourced in llvm.org <http://llvm.org/> already, might be good inspiration for a .debug_aranges successor section that has very efficient lookups. The GSYM format could actually be used as is by adding only a new DIE offset IntoType. 

Besides ".debug_names", all other DWARF accelerator tables are really just random indexes that must be linearly scanned or pre-indexed prior to being used because of the concatenate + relocate style that is used for these DWARF sections. It would be great if any future accelerator tables are "map into memory and use as is" kind of tables like ".debug_names" and the ".apple_XXX" name accelerator tables. 


> Thanks,
> --paulr
>  
> From: David Blaikie <dblaikie at gmail.com> 
> Sent: Thursday, March 11, 2021 3:48 PM
> To: Robinson, Paul <paul.robinson at sony.com>
> Cc: Cary Coutant <ccoutant at gmail.com>; DWARF Discuss <dwarf-discuss at lists.dwarfstd.org>
> Subject: debug_aranges use and overhead
>  
> On Thu, Mar 11, 2021 at 5:48 AM <paul.robinson at sony.com <mailto:paul.robinson at sony.com>> wrote:
> Hopefully not to side-track things too much... maybe wants its own
> thread, if there's more to debate here.
> 
> Yeah, how about we spin it off into another thread (done here)
>  
> >> For the case you suggested where it would be useful to keep the range
> >> list for the CU in the .o file, I think .debug_aranges is what you're
> >> looking for.
> >
> > aranges has been off by default in LLVM for a while - it adds a lot of
> > overhead (doesn't have all the nice rnglist encodings for instance -
> > nor can it use debug_addr, and if it did it'd still be duplicate with
> > the CU ranges wherever they were).
> 
> Did you want to file an issue to improve how .debug_aranges works?
> 
> I don't currently understand the value it provides, and I at least don't have a use case for it, so I'm not sure I'd be the best person to advocate/drive that work.
> 
> Complaining that it duplicates CU ranges is missing the point, though;
> it's an index, like .debug_names, of course it duplicates other info.
> If you want to suggest an improved index, like we did with .debug_names,
> that would be great too.
> 
> .debug_names is quite different though - it collects information from across the DIE tree - information that is expensive to otherwise gather (walking the whole DIE tree).
> 
> .debug_aranges is not like that for most producers (producers that do include the address ranges on the CU DIE) - the data is readily available immediately on the CU. That does involve reading some of .debug_abbrev, and interpreting a handful of attributes - but at least for the use cases I'm aware of, that overhead isn't worth the size increase.
> 
> Do you have numbers on the benefits of .debug_aranges compared to parsing the ranges from CU DIEs?
> 
> (one possible issue: the CU doesn't /have/ to contain low/high/ranges if its children DIEs contain addresses - having that as a guarantee, or some preferred way of encoding zero length (high/low of 0 would be acceptable, I guess) would be nice & make it cheap to skip over CUs that don't have any address ranges)
> 
> Roughly, a modern debug_aranges to me would look something like:
> 
> <length>
> <version>
> <CU sec_offset>
> <addr_base>
> <rnglist sec_offset>
> 
> So it could fully re-use the rnglist encoding. If this was going to be as compact as possible, it'd need to be configurable which encodings it uses - ranges V high/low, addrx V addr - at which point it'd probably look like a small DIE with an inline abbrev (similar to the way DWARFv5 encodes the file and directory entries now, and how debug_names is self-describing) - at which point it looks to me a lot like parsing the CU DIEs.
>  
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