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

[James Henderson via llvm-dev]

On Thu, 12 Nov 2020 at 02:10, Fangrui Song <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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