llvm dev - Apr 2021 - Supporting Regular and Thin LTO with a Single LTO Bitcode Format

+Matthew and Teresa for any context they might have

High level sounds like a reasonable thing to me, for what it's worth.

On Tue, Apr 13, 2021 at 2:19 PM Petr Hosek via llvm-dev
<llvm-dev at lists.llvm.org> wrote:
>
> We're using regular LTO for our Clang toolchain build because we
don't mind spending more resources to squeeze out as much performance as
possible. However, when looking into our build performance, I've noticed
that we only spent about 1/3 of the total build time in building distribution
components, the rest is spent on building unit tests and tools that are only
used by lit tests. For the latter, we don't care about the performance, so
it'd be nice to avoid doing regular LTO to speed up the build.
>
> The idea I had would be to use a single LTO bitcode format for all
translation units, and then decide only at link time whether to use regular LTO
for distribution components or ThinLTO for everything else.
>
> After doing some research, I found the "Supporting Regular and Thin
LTO with a Single LTO Bitcode Format" talk presented by Matthew Voss at
LLVM Developers’ Meeting 2019 which does exactly what I described, but it seems
like this was only implemented downstream.
>
> Has there been any progress on upstreaming the implementation? Is there any
way to do what I described using the in-tree LTO implementation?
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev

This is a really good thread to read:
https://lists.llvm.org/pipermail/llvm-dev/2018-April/122469.html
<https://lists.llvm.org/pipermail/llvm-dev/2018-April/122469.html>

There is no fundamental technical reasons why this cannot happen but it requires
lots of work to fine tuning the pipeline (yes, fullLTO and thinLTO uses
different pipeline) so that it reaches a good balance of performance/build
overhead for general users.

Steven
> On Apr 13, 2021, at 2:23 PM, David Blaikie via llvm-dev <llvm-dev at
lists.llvm.org> wrote:
> 
> +Matthew and Teresa for any context they might have
> 
> High level sounds like a reasonable thing to me, for what it's worth.
> 
> On Tue, Apr 13, 2021 at 2:19 PM Petr Hosek via llvm-dev
> <llvm-dev at lists.llvm.org> wrote:
>> 
>> We're using regular LTO for our Clang toolchain build because we
don't mind spending more resources to squeeze out as much performance as
possible. However, when looking into our build performance, I've noticed
that we only spent about 1/3 of the total build time in building distribution
components, the rest is spent on building unit tests and tools that are only
used by lit tests. For the latter, we don't care about the performance, so
it'd be nice to avoid doing regular LTO to speed up the build.
>> 
>> The idea I had would be to use a single LTO bitcode format for all
translation units, and then decide only at link time whether to use regular LTO
for distribution components or ThinLTO for everything else.
>> 
>> After doing some research, I found the "Supporting Regular and
Thin LTO with a Single LTO Bitcode Format" talk presented by Matthew Voss
at LLVM Developers’ Meeting 2019 which does exactly what I described, but it
seems like this was only implemented downstream.
>> 
>> Has there been any progress on upstreaming the implementation? Is there
any way to do what I described using the in-tree LTO implementation?
>> _______________________________________________
>> LLVM Developers mailing list
>> llvm-dev at lists.llvm.org
>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
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