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

[David Blaikie via llvm-dev]

On Fri, Nov 13, 2020 at 8:35 AM Wenlei He via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> >  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.
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> I don’t think we need to encode dwarf32/64 in IR as attribute for each module. We’re not going to emit mixed dwarf32/64 for merged LTO module anyways, so allowing each module to express its dwarf setting would only introduce burden for LTO to deal with inconsistency (warning?) among input modules. Having a linker switch to pass the setting from driver to LTO sounds better to me.

Usually the issue there is that existing build systems may be setup
only to pass such flags to the compilations, and not to the link
invocations - like DWARF version, we pass that down through IR, emit
warnings/errors when two IR modules with different DWARF versions are
linked together, and then emit only one (the higher, I believe) DWARF
version out the other end.

We aren't 100% consistent on this "anything you could do without LTO,
you shuold be able to do with LTO/passing the same flags to the same
actions" kind of strategy (eg: type units and DWARF compression aren't
passed down through IR - if you want those you have to pass them to
the link invocation (via the clang driver) yourself). So it's more "is
there a systemic use of these flags already for the compilation and
would not supporting it there be a pain"? It's probably not for
DWARF64, since we haven't had any flag to support it at the moment
anyway.

- Dave

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> From: llvm-dev <llvm-dev-bounces at lists.llvm.org>
> Date: Thursday, November 12, 2020 at 2:21 AM
> To: Fangrui Song <maskray at google.com>
> Cc: llvm-dev at lists.llvm.org <llvm-dev at lists.llvm.org>
> Subject: Re: [llvm-dev] [LLD] Support DWARF64, debug_info "sorting"
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> On Thu, 12 Nov 2020 at 02:10, Fangrui Song <maskray at google.com> wrote:
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> 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.
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> 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.
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> >
> >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
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> 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...
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> 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.
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> 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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