[PATCH] D60226: [ThinLTO] Fix ThinLTOCodegenerator to export llvm.used symbols

Thu Apr 4 18:09:29 PDT 2019

steven_wu added a comment.

In D60226#1455688 <https://reviews.llvm.org/D60226#1455688>, @steven_wu wrote:

> In D60226#1455485 <https://reviews.llvm.org/D60226#1455485>, @pcc wrote:
>
> > In D60226#1455477 <https://reviews.llvm.org/D60226#1455477>, @tejohnson wrote:
> >
> > > In D60226#1453856 <https://reviews.llvm.org/D60226#1453856>, @steven_wu wrote:
> > >
> > > > Also do note, I did a quick benchmark on thin-link time for libLTO.dylib. thin-link was 2.6s before the change vs. 4.0s after the change. Building IRSymtab during thin-link does introduce non-negligible overhead. It might be possible to build them async in threads to reduce the overhead.
> > >
> > >
> > > @pcc is this surprising? Typically we should just be loading it out of the bitcode file. There are cases where it has to be rebuilt, but theoretically those should be exceptions.
> >
> >
> > Yes, we normally just load it out of the bitcode file. The irsymtab only needs to be rebuilt when the version of the producer and the consumer do not match. You might be hitting the "upgrade" path here: http://llvm-cs.pcc.me.uk/lib/Object/IRSymtab.cpp#330
>
>
> This is likely. I didn't bootstrap the build. I will try again with a matching version.

Without hitting the upgrade path, the time is almost negligible. Thanks for pointing that out.

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https://reviews.llvm.org/D60226