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

[Robinson, Paul via llvm-dev]

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).
--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<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

 > 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


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