[llvm-dev] Question about thinLTO

[Teresa Johnson via llvm-dev]

On Wed, Jul 12, 2017 at 10:19 AM, Teresa Johnson <tejohnson at google.com>
wrote:

> 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
>

Sorry, that last option should be -thinlto-save-temps=foo, where "foo" will
be the prefix of the generate temp files (can include a path). You want the
foo*.opt.bc files for the output of the opt pipeline in the backends.

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
>>
>> _______________________________________________
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>> llvm-dev at lists.llvm.org
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>>
>>
>
>
> --
> Teresa Johnson |  Software Engineer |  tejohnson at google.com |
> 408-460-2413 <(408)%20460-2413>
>



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