[llvm-dev] Question about thinLTO

Thu Jul 13 08:37:13 PDT 2017

Hi Teresa,

Yes, we plan to have our code at CG directly.
We use our own linker. That's the pain. We might only get a partial benefit
of thinLTO which occurs at compile time.
I will have a close look at the LTO-tests for any useful command-line
option.

Thanks for the quick response.

Regards,
Christu

On Thu, Jul 13, 2017 at 7:49 PM, Teresa Johnson <tejohnson at google.com>
wrote:

>
>
> 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
>
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