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

[David Blaikie via llvm-dev]

On Mon, Mar 16, 2020 at 10:50 AM Robinson, Paul <paul.robinson at sony.com>
wrote:

> SCE tuning does turn on the .debug_aranges section.  Our debugger team
> really cares about startup cost. Turnaround time in general is huge for our
> licensees, to the point where we support edit-and-continue (minimal
> rebuild, live-patch the running process).
>

Ah, good to know! I'd be curious to know about the performance tradeoff
when they're disabled if you ever happen to have data around that.
I guess a related question: Does SCE use the non-.text entries (or
otherwise have an opinion on having them) in debug_aranges?


> --paulr
>
>
>
> *From:* David Blaikie <dblaikie at gmail.com>
> *Sent:* Monday, March 16, 2020 1:20 PM
> *To:* Robinson, Paul <paul.robinson at sony.com>
> *Cc:* Dylan McKay <me at dylanmckay.io>; llvm-dev at lists.llvm.org
> *Subject:* Re: [llvm-dev] DWARF .debug_aranges data objects and address
> spaces
>
>
>
> On Mon, Mar 16, 2020 at 9:31 AM Robinson, Paul <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
>
>
>
>  > If the producer has put ranges on the CU it's not a lot of work - it's
> parsing one DIE & looking for a couple of attributes.
>
>
>
> It’s walking through all the CUs, picking up the associated abbrevs,
> trolling down the list of attributes… “not a lot” indeed, but not as
> trivial as running through a single section linearly, which is what
> .debug_aranges gets you.  I’ve been lectured by @clayborg on what consumers
> really want for performance gains.
>
>
> Sure enough - though I don't believe aranges is used by default on any
> target/platform LLVM supports, so this time/space tradeoff doesn't seem to
> have been important to any of them?
>
>
>  > It's enough at least at Google for us to not use them & use CU ranges
> for the same purpose.
>
>
>
> Google is much more seriously concerned about debug-info size than about
> debugger performance, IIRC.  This is not universally the preferred
> tradeoff.  Just sayin’.
>
>
> Sure enough.
>
> I've just had a couple of people ask about aranges recently (~year or so)
> & when pressing a little further, using the CU's address ranges turned out
> to be sufficient for their needs without having to change Clang's defaults
> or have their users specify extra flags to explicitly request them, etc.
>
> Out of curiosity/for data/usage/etc - does Sony use aranges? (changing the
> default when targeting SCE or the like)
>
> - Dave
>
>
>
>
>
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20200316/ea0e7012/attachment.html>