[llvm-dev] Question about thinLTO

[Teresa Johnson via llvm-dev]

On Thu, Jul 13, 2017 at 2:54 AM, Christudasan D <xander.cd at gmail.com> wrote:

> 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.
>

Unfortunately I don't see any documentation online of llvm-lto. I guess the
best bet for now is to look at llvm-lto.cpp, or some of the ThinLTO tests
in the tree that use it with those options. I assume you plan to use the
internal tools just for testing, and eventually hook up your own code
generator to the compiler directly? Which linker do you use? Both gold and
lld have support for ThinLTO and a newer LTO interface that uses linker
resolution information.

Teresa


>
> 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 <(408)%20460-2413>
>>
>
>


-- 
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/20170713/6995f16f/attachment.html>