[llvm-dev] Document to understand vectorized code

[Sudakshina Dutta via llvm-dev]

Dear Craig,

Thanks for your reply. It seems the control flow of the vectorized program
is very different from that of the source code. Is there any document
describing difference in control flow, the reason for using shufflevector,
etc ?

Sudakshina

On Mon, 7 Jun 2021, 07:41 Craig Topper, <craig.topper at gmail.com> wrote:

> 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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>
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