[llvm-dev] Document to understand vectorized code

[Craig Topper via llvm-dev]

There's some notes here https://llvm.org/docs/Vectorizers.html but I didn't
look too closely at it.  If you compile with -fno-discard-values names or
use a debug build, some of the basic blocks will be named similar to the
CFG diagram in this section
https://llvm.org/docs/Vectorizers.html#epilogue-vectorization

~Craig


On Sun, Jun 6, 2021 at 7:04 PM Sudakshina Dutta via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> Dear Stefanos,
>
> I want to understand how the generated code works. For example, the code
> that I generated using -Rpass=loop-vectorize -Rpass-analysis=loop-vectorize
> has many shufflevector and other instructions. I wanted to have a document
> (if there exists any) where an example on loop-vectorization is done with
> some explanation on the workflow of the code.
>
> Thanks,
> Sudakshina
>
> On Mon, Jun 7, 2021 at 7:03 AM Stefanos Baziotis <
> stefanos.baziotis at gmail.com> wrote:
>
>> Hi Sudakshina,
>>
>> First, it helps if you can put your code in a godbolt snippet, like this
>> [1]. It helps people in multiple ways (e.g., they don't have to download
>> files, they can see exactly what cmd arguments you used, they can
>> tweak the cmd arguments without having LLVM on their machine etc.).
>>
>> Is there any comprehensive tutorial/document to understand generated
>>> instructions or the semantics of the vectorized code ?
>>
>>
>> This is quite generic, what is more specifically that you want to
>> understand? Do you want to understand what each individual instruction
>> does? Do you maybe understand that but
>> you don't know what is the general method to generate, let's say by hand,
>> vectorized code (or more specifically, branching vectorized code). Or
>> maybe, you want to understand
>> how _LLVM_ generates this code, i.e., the inner workings of the
>> vectorization passes.
>>
>> Best,
>> Stefanos
>>
>> [1] https://godbolt.org/z/8eKqnrMPn
>>
>> Στις Κυρ, 6 Ιουν 2021 στις 6:17 π.μ., ο/η Sudakshina Dutta via llvm-dev <
>> llvm-dev at lists.llvm.org> έγραψε:
>>
>>> Dear all,
>>>
>>> Greetings. I have generated a vectorized code from a C source file
>>> (attached). Is there any comprehensive tutorial/document to understand
>>> generated instructions or the semantics of the vectorized code ?
>>>
>>> Thanks,
>>> Sudakshina
>>> _______________________________________________
>>> LLVM Developers mailing list
>>> llvm-dev at lists.llvm.org
>>> https://lists.llvm.org/cgi-bin/mailman/listinfo/llvm-dev
>>>
>> _______________________________________________
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> llvm-dev at lists.llvm.org
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