[PATCH] D22356: [ThinLTO] Perform conservative weak/linkonce resolution in distributed backend case

[Teresa Johnson via llvm-commits]

On Mon, Jul 18, 2016 at 7:02 PM, Xinliang David Li <davidxl at google.com>
wrote:

>
>
> On Mon, Jul 18, 2016 at 5:37 PM, Mehdi AMINI <mehdi.amini at apple.com>
> wrote:
>
>> mehdi_amini added a comment.
>>
>> In https://reviews.llvm.org/D22356#487800, @tejohnson wrote:
>>
>> > In https://reviews.llvm.org/D22356#487784, @mehdi_amini wrote:
>> >
>> > > > That's a good question and an idea I thought about briefly but
>> discarded for a couple reasons. I was concerned about requiring
>> communication between the ThinLink and final link to build the link line
>> (it would be more difficult to support in a build system, and also seems
>> conceptually more complicated).
>> > >
>> > >
>> > > How is the final link invocation computed right now?
>> >
>> >
>> > The link line is essentially the same, but with native .o instead of
>> the bitcode .o. (See the new test case for an example)
>>
>>
>> The question is: in the presence of static archives, how do you generates
>> --start-lib/--end-lib? This seems to already require some build-system
>> integration?
>>
>
> There is one to one mapping from bitcode .o and native .o -- so the
> command line can be formed.
>
>
>>
>> > >
>>
>> >
>>
>> > >
>>
>> >
>>
>> > > > Also I'm not 100% convinced that removing the
>> --start-lib/--end-lib, even if we only include those object files the
>> linker decided to select symbols from, would result in the same linking
>> behavior.
>>
>> >
>>
>> > >
>>
>> >
>>
>> > >
>>
>> >
>>
>> > > Your observations about the linker picking different symbols seem to
>> indicate that the --start-lib/--end-lib model is already broken.
>>
>> >
>>
>> >
>>
>> > When you say "is already broken" do you mean even in non-ThinLTO mode?
>> I'm not sure why - it is just like having an archive of the objects between
>> each start/end pair.
>>
>>
>> I'm only talking about ThinLTO and the two-stage linking, i.e. the second
>> invocation of the linker does not end-up with the same prevailing
>> resolution as the first invocation. Your current patches are working around
>> this deficiency.
>>
>
>
> Since they are orginally from LinkOnceODR defs, whatever definition is
> picked up is implementation dependent.
>

Right for the multiply defined ODR refs, it shouldn't matter that the
second link picks a different copy (we just need to do the conservative
thing as in this and my other patch to ensure we don't introduce backwards
refs).

I do have a worry about non-ODR multiply defined case. E.g. In the example
if the first copy originally chosen as prevailing is weak (non-odr), and
due to the importing of strong references to that object we no longer
linked in its symbols in the second link, we might end up linking with a
different weak copy that has a different body - resulting in different
program behavior. If that is a real concern then I suppose we must do
something else to ensure that the same copy is prevailing in the second
link...


>> >
>>
>> >
>>
>> > > If a list of `.o` on the command line is not enough for relinking,
>> there's gonna be a need for a "linker resolution map" file that drives the
>> linker.
>>
>> >
>>
>> >
>>
>> > In ThinLTO it is because of the change (between the ThinLink and native
>> object link) in which strong references exist between objects/libraries due
>> to importing and inlining. But I believe with this patch and the follow-on
>> https://reviews.llvm.org/D22467 the importing and symbol resolution is
>> made suitably conservative.
>>
>>
>> I don't see any justification for --start-lib/--end-lib right now.
>>
>>
> Can you clarify what do you mean by 'justification' here?
>

Reading through Mehdi's comments, I think he didn't realize we were already
using --start-lib/--end-lib instead of regular archive libraries.


>
> thanks,
>
> David
>
>
>>
>> https://reviews.llvm.org/D22356
>>
>>
>>
>>
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
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