[llvm-dev] Question about thinLTO

[Christudasan D via llvm-dev]

Thank you Teresa.

Yes, I would like to save the IR (*.bc and/or *.ll) after all optimizations
(especially thinLTO) are done and call *llc* separately.
Is there any specific document available online to see more about this
feature and various command-line switches that a compiler developer can
take advantage of?
It would help us to enable this feature for a custom architecture.

Regards,
Christu

On Wed, Jul 12, 2017 at 10:55 PM, Teresa Johnson <tejohnson at google.com>
wrote:

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