[llvm-dev] DWARF Fission + ThinLTO

[David Blaikie via llvm-dev]

Alrighty, a little fuzzy on how best to implement this - Adrian, you've
probably got the most context here as to how to wrangle this.

My first attempt was in IRMover.cpp, IRLinker::linkFunctionBody - after
metadata is copied over, create a new subprogram derived from
Dst.getSubprogram, only changing the CU over (to the first, if any, CU on
the llvm.dbg.cu named metadata).

This is insufficient because all the metadata in the function (variables,
etc) doesn't get updated and chains up to the old DISubprogram still.

Next attempt was to RAUW the foreign CU with the desired CU, but that fails
with "MDNode::replaceAllUsesWith - expected temporary node"

So - any ideas?

On Thu, May 4, 2017 at 7:40 AM Teresa Johnson <tejohnson at google.com> wrote:

> On Thu, May 4, 2017 at 7:22 AM, Rafael Avila de Espindola via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
>> David Blaikie via llvm-dev <llvm-dev at lists.llvm.org> writes:
>>
>> > So Dehao and I have been dealing with some of the nitty gritty details
>> of
>> > debug info with ThinLTO, specifically with Fission(Split DWARF).
>> >
>> > This applies to LTO as well, so I won't single out ThinLTO here.
>>
>> When are the .dwo files produced? If at link time, what is the advantage
>> of using fission? Normally the advantage is the faster link, but if it
>> is the linker producing them, what is left?
>>
>
> For ThinLTO they are produced by the backends, and only the split .o part
> needs to be linked. I think that benefit would occur with in-process
> ThinLTO as well (the .o files handed back to the linker are smaller), but
> for us where we have a distributed ThinLTO build, the backends are
> producing split .o and .dwo files and so the final native link process gets
> all the normal advantages of fission.
>
> For regular LTO, I suppose the same benefit could occur - the backend
> produces a split .o and .dwo file and only passes the split .o file to the
> link.
>
> Teresa
>
>
>> Cheers,
>> Rafael
>
>
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>> llvm-dev at lists.llvm.org
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>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413 <(408)%20460-2413>
>
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