[llvm-dev] Question about thinLTO

[Teresa Johnson via llvm-dev]

Hi Christu,

Thanks for the note!

On Wed, Jul 12, 2017 at 9:56 AM, Christudasan D via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Hello,
>
>
>
> My impression on *thinLTO* when I first heard of it, (EuroLLVM2015) was
> about achieving Cross Module Optimization (CMO) at the IR level.
>
>
>
> Having parallel front-end compilation & initial optimization first, a
> thin-link of individual input units, more optimization by calling opt again
> on the combined IR, and finally the target codegen using llc.
>
>
>
> A transformation similar to the following:
>
> Input File 1: Clang+opt (with thinLTO)
>
> Input File 2: Clang+opt (with thinLTO)           ---
> llvm-link       ----       opt (for CMO)      ---             llc for
> target codegen.
>
>                        …..
>
> Input File n: Clang+opt (with thinLTO)
>
>
>
> But from the presentation on LLVM Developers’ Meeting 2016, I believe
> thinLTO is more than that. The full advantage of this optimization should
> require a significant changes in the backend as well (I suppose).
>

Right, the model in the EuroLLVM talk was just an initial prototype that
used llvm-link/opt, and yes now we do whole program optimizations during
the thin link, beyond just linking in additional IR for inlining etc.


>
> Before I post my question, kudos to the entire team behind thinLTO
> optimization - Teresa Johnson, Mehdi Amni, Xinliang David Li, other
> developers and test engineers across the globe.
>
>
>
> I am working on the compiler for a target wherein the code size
> improvement is a critical factor. We are still using LLVM 3.5 code base. We
> assume, by moving to LLVM4.0.0 (with thinLTO and other recent target
> independent optimizations) we would be able to improve the codegen to a
> considerable number.
>
>
>
> With thinLTO in LLVM4.0.0 compiler, when we build an application with
> multiple compilation units, is it possible to achieve any benefit purely
> with LLVM IR passes (without really involving the compiler backend)?
>
> If yes, can anyone provide me the information about the command-line
> options and the sequence to call the llvm components (clang, opt, etc.) to
> achieve it.
>

Do you just want the bitcode out after all optimization passes and before
codegen? It is doable with llvm-lto -thinlto-action=run -save-temps I
believe (although you will get more bitcode output files than you want, but
one of the output bitcode files per input file should be the post-opt
pre-codegen bitcode file. The input files to llvm-lto would be bitcode
files created by "clang -flto=thin -c -O2 ..."

Teresa


>
> I truly value any input in this regard.
>
>
>
> Regards,
>
> Christu
>
> _______________________________________________
> LLVM Developers mailing list
> llvm-dev at lists.llvm.org
> http://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>
>


-- 
Teresa Johnson |  Software Engineer |  tejohnson at google.com |  408-460-2413
-------------- next part --------------
An HTML attachment was scrubbed...
URL: <http://lists.llvm.org/pipermail/llvm-dev/attachments/20170712/34b6b5eb/attachment.html>