[PATCH] D19351: ELF: Add initial ThinLTO support.

[Peter Collingbourne via llvm-commits]

Thanks Rafael.

On Thu, May 12, 2016 at 4:32 PM, Rafael Espíndola <
rafael.espindola at gmail.com> wrote:

> On 12 May 2016 at 16:13, Peter Collingbourne <peter at pcc.me.uk> wrote:
> > pcc added a comment.
> >
> > I have updated the patch. I just want to make sure that people are happy
> with the situation around the ThinLTO API. I've been working on an
> LTO/ThinLTO API that could be used by both the gold plugin and lld which
> should address Mehdi's concerns, but it will take some time until that's
> ready.
> >
>
> Sorry I dropped out of this thread.
>
> I don't known a lot about ThinLTO, so most comments will be generic about
> LTO.
>
> The way the existing lib/LTO is implemented is fundamentally different
> from what we do in the gold plugin or lld. We want to make sure we
> don't even read and copy symbols if the linker decided to not keep it.
> The lib/LTO interface is designed to talk about symbols in the merged
> module.
>

Right, I certainly don't want to use any of the existing lib/LTO anywhere
we're not using it now. The new API I'm working on deals with symbols in
individual modules.

Having said that, I dislike duplicated code as much as anyone else.
> That is why I really don't want lld calling into Linker::linkModules:
> it duplicates symbol resolution. The same goes for the call to
> addPassesToEmitFile. We really should figure out a single place to
> populate the pipeline.
>
> If it is possible for lld/gold/LTO to share more code, excellent but:
>
> * I would probably not put that in lib/LTO. Lets keep that just for
> the existing api.
>

Putting the new API in some other lib makes sense to me.


> * It still seems a bit early for this. We still having more basic
> things to finish in lld and the thinLTO api is still moving.
>

Regarding the new API, I think there’s a secondary reason why we might want
to introduce it sooner rather than later. That reason is that we plan to
store symbol resolutions in a combined module index, which basically
describes which module contains the prevailing definition of each function.
Getting this right is basically a critical correctness issue, as we could
otherwise for example import the definition of a weak symbol instead of the
prevailing strong symbol. I also understand that Teresa plans to use this
information in distributed backends.

One way we could implement this could be to expose gold's symbol
resolutions to the ThinLTO importer in some form. The problem with this is
that it adds more complexity in a gold-specific layer, when we know that we
will eventually need to deal with that in every linker. Or we could
implement symbol resolution ourselves in the importer via the linkage info
in the summaries, but as I’m sure you are aware, this would be duplicating
logic from the linker. That’s why I feel it’s best to have the importer
deal with linker-agnostic symbol resolution info.

Getting this right in the gold plugin is basically orthogonal to whether we
implement anything in lld. So, the course of action I was considering was
1) Get the new interface in place, and port the gold plugin to it;
2) (in parallel with 1) Polish regular LTO in lld as necessary;
3) Port lld to the new interface.
4) (low priority for me, but maybe Mehdi can help) Port libLTO to the new
interface.

Thanks,
-- 
-- 
Peter
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