[llvm-dev] Supporting Regular and Thin LTO with a Single LTO Bitcode Format

[Mehdi AMINI via llvm-dev]

Hi,

I actually just watched the presentation, but it is a bit too high level
for me to really understand what is this new "format" in practice.
As far as I remember ThinLTO bitcode should be a super-set of information
compared to FullLTO. It isn't clear to me what prevents a linker
implementation to take the ThinLTO bitcode and perform FullLTO on these,
this was designed to allow this originally.

The optimization pipelines are set up differently, but that does not really
affect the format I believe. A good FullLTO link implementation based on
the ThinLTO bitcode would likely involve running a heavier pipeline during
LTO, this is something I tried to do at the time to align the pipeline and
ensure there is only one, however it was making the LTO link slower and we
considered it wasn't a good tradeoff.
We could also run part of the pipeline on individual modules (in parallel)
before linking them together during FullLTO, which would address the
performance question. It wouldn't however help when the same modules are
linked into different binaries, like it is the case for LLVM test binaries
which are linking over and over the same set of libraries. For this setup
(which may not be common though) it is better to do as much work as
possible in the first compilation phase and as little as possible during
the link phase.

> 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.

For such a case, the "ideal" setup would be to link these in a ThinLTO mode
where we disable cross-module importing and just optimize/codegen in
parallel. We could guarantee this way that each file is codegened once and
cached and every test binary would have a cache hit on the files. This
makes it "almost zero" cost over non-LTO I think.

Best,

-- 
Mehdi

On Tue, Apr 13, 2021 at 2:46 PM via llvm-dev <llvm-dev at lists.llvm.org>
wrote:

> Hi Petr,
>
>
>
> This does sound like a good use case for our pipeline. We’ve seen good
> runtime performance overall, as we stated in the talk. I’ve been working on
> upstreaming our patches off and on for a couple months now. Our pipeline
> needs to be ported to the NPM, but once that work is done, the patch is
> ready for review. I should be able to finish that work within the next
> month or two and would love to get some feedback on our approach.
>
>
>
> Thanks,
>
> Matthew
>
>
>
>
>
> *From:* Steven Wu <stevenwu at apple.com>
> *Sent:* Tuesday, April 13, 2021 2:39 PM
> *To:* David Blaikie <dblaikie at gmail.com>
> *Cc:* Petr Hosek <phosek at google.com>; Voss, Matthew <Matthew.Voss at sony.com>;
> Teresa Johnson <tejohnson at google.com>; llvm-dev <llvm-dev at lists.llvm.org>
> *Subject:* Re: [llvm-dev] Supporting Regular and Thin LTO with a Single
> LTO Bitcode Format
>
>
>
> This is a really good thread to read:
> 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?
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