[llvm-dev] [LLD] Support DWARF64, debug_info "sorting"

[Alexander Yermolovich via llvm-dev]

Looks like there is an agreement that this path, modifying lld to order sections using relocations, should be explored.
If Igor doesn't object, since he was primary one driving DWARF64 so far, I would like to give it a shot at implementing and collecting some performance numbers. 🙂

Alex

________________________________
From: James Henderson <jh7370.2008 at my.bristol.ac.uk>
Sent: Thursday, November 12, 2020 2:20 AM
To: Fangrui Song <maskray at google.com>
Cc: Alexander Yermolovich <ayermolo at fb.com>; Robinson, Paul <paul.robinson at sony.com>; David Blaikie <dblaikie at gmail.com>; Eric Christopher <echristo at gmail.com>; Igor Kudrin <ikudrin at accesssoftek.com>; llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info "sorting"



On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at google.com<mailto:maskray at google.com>> wrote:
On 2020-11-12, Alexander Yermolovich wrote:
>Thanks for feedback.
>
>I agree with patch and numbers this will be a more concrete discussion, but I wanted to judge overall receptiveness to this approach and see maybe there was a better way.
>
>"Whilst the majority of objects will only have a single CU in them, there will be exceptions (LTO-generated objects, -r merged objects etc), so we do need to consider this approach."
>David can you elaborate under which conditions LTO-generated objects will have a mix of DWARF32/64 in same .debug_info? Looking at how dwarf64 was implemented same flag will be used for the entirety of the dwarf output, even if multiple CUs are included.

Thinking about it, I wouldn't expect an LTO generated object itself to have a mixture of DWARF32/64, although I guess the 32/64 bit state could be encoded in the IR (I am not familiar enough with it to know if it actually is or not). It might be necessary to find ways to configure LTO to generate DWARF64, possibly via a link-time option.

>
>On one hand since this is only applicable for when DWARF64 is used, special option would be the way to go. Although the user will need to be aware of yet another LLD option. Maybe an error when relocations overflow occur can be modified to display this option along with -fdebug-types-section

I am quite happy with the relocation approach under a linker option. I'd still
want to know generic-abi folks's thoughts, though. James may have prepared something
he wants to share with generic-abi:) Let's wait...

I hadn't prepared anything if I'm honest (though if there's widespread agreement that this would be useful, I certainly can - it would have other positive improvements too, reducing the need for tools to rely on section names to identify debug data for example). It was more a case of bouncing ideas off of people to see what they thought. Any discussion we have will probably also need circulating on the DWARF mailing list too, since it is more a DWARF issue than a gABI issue (unless the solution is a new section type). Further refinements to this idea that might make it more appealing to the generic group: `SHT_DEBUG` for the section type name, with the first N bytes of the sh_info used to specify the variant of debug data it represents (e.g. 0x1 for DWARF, 0x2 for SOME_OTHER_STANDARD etc), and the remainder for use as flags as defined by the standard (I'm thinking for DWARF you could encode the 64-bit/32-bit state in there, possibly the section variant (info/rnglists/line etc) and the DWARF version too), on the understanding that consumers like the linker wouldn't combine sections in a potentially broken way. This has the advantage that it could be retrofitted to the existing standard versions, but as has been pointed out, this won't help those with linker scripts - that could only be solved with a new DWARF standard and separate names for 64/32 bit sections, at least if we wanted to avoid the linker needing to do anything beyond reading the section header.

The relocation approach sounds like a reasonable solution for the current situation - even if we do decide to go the route of changing producers to start emitting a new section type/update the standard etc, it doesn't resolve the problem people may currently face.
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