llvm dev - May 2021 - Auto-vectorization option

Dear all,

Thanks to all of you. I have executed the following commands on the code
given above.

clang -O3 -S -c find_max.c -Rpass=vector -Rpass-analysis=vector -o
find_max.ll

However, the generated code is an assembly code (attached). Is there any
way to generate a vectorized IR (.ll) file ?

Thanks and regards,
Sudakshina

On Fri, May 14, 2021 at 2:49 PM Nigel Perks <nigelp at xmos.com> wrote:
> Try   opt -S   to get text IR instead of bitcode.
>
>
>
> The second group of commands use   clang -c   so produce a compiled object
> file. For text IR you need   -S -emit-llvm   like in the first group.
>
>
>
> Nigel
>
>
>
>
>
> *From:* llvm-dev <llvm-dev-bounces at lists.llvm.org> *On Behalf Of
*Sudakshina
> Dutta via llvm-dev
> *Sent:* 14 May 2021 04:17
> *To:* LLVM Development List <llvm-dev at lists.llvm.org>
> *Subject:* [llvm-dev] Auto-vectorization option
>
>
>
> Dear all,
>
>
>
> Greetings. I want to generate the intermediate code (.ll file) after
> having LLVM auto-vectorization transformation on the source C file. I have
> the following code  (find_max.c) on which I want to apply
> auto-vectorization.
>
>
>
> int main()
> {
>     int i, n, max, arr[50];
>     for(i = 0; i < 35; i++)
>     {
>         printf("arr[%d] = ", i);
>         scanf("%d", &arr[i]);
>     }
>     max = arr[0];
>     for(i = 1; i < 35; i++)
>     {
>         if(arr[i] > max)
>         {
>             max = arr[i];
>         }
>     }
>
> }
>
>
>
> I have tried the following.
>
>
>
>    1. clang  -S -emit-llvm find_max.c -o find_max.ll
>    2. opt -loop-vectorize -force-vector-width=8 find_max.ll -o
>    find_max_opt.ll
>
> and
>
>    1. clang -O3 -c find_max.c -Rpass=vector -Rpass-analysis=vector
>    2. clang -O3 -c find_max.c -Rpass=vector -Rpass-analysis=vector -o
>    find_max.ll
>
>
>
> All the above options generate binary files. I request you to please help.
>
>
>
> Thanks and regards,
>
> Sudakshina
>
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On 5/14/2021 13:30, Sudakshina Dutta via llvm-dev wrote:
> Dear all,
> 
> Thanks to all of you. I have executed the following commands on the code 
> given above.
> 
> clang -O3 -S -c find_max.c -Rpass=vector -Rpass-analysis=vector -o 
> find_max.ll
> 
> However, the generated code is an assembly code (attached). Is there any 
> way to generate a vectorized IR (.ll) file ?
To get LLVM IR from the frontend (Clang) use -emit-llvm 
-fno-discard-value-names, e.g., https://godbolt.org/z/aWz37qYdW

If you don't need debugger intrinsics (llvm.dbg.*) add -g0, e.g., 
https://godbolt.org/z/aWz37qYdW

As Sjoerd has mentioned, passing -mllvm -print-before-all to Clang is 
usedful to get pre-vectorized LLVM IR (as well as observe the effects of 
consecutive transformations); Example: https://godbolt.org/z/4za6h6fqo

You can then extract the unoptimized LLVM IR and play with it in "opt"
(the middle-end optimizer tool) to get the LLVM IR optimized by the 
middle-end passes (including loop vectorizer); note that now you can 
just pass -print-before-all directly: https://llvm.godbolt.org/z/P7E3PGE61

In particular, the LLVM IR displayed under "*** IR Dump Before 
LoopVectorizePass on _Z1fPim ***" is a good baseline for comparisons.

Add "-mllvm -print-module-scope" to get the LLVM IR for the full
module
(translation unit): https://godbolt.org/z/Go7zK8vsW

Then, pass this LLVM (right before LoopVectorizePass) to "opt" using 
options "-loop-vectorize -debug-only=loop-vectorize" to observe the
loop
vectorization pass in action:
https://llvm.godbolt.org/z/WMa1qosoq

Note that you need a binary built with assertions enabled to use -debug 
options.

Last but not least you can give the optimized LLVM IR to "llc" (the 
backend tool) to get the final assembly: 
https://llvm.godbolt.org/z/hxevcqKEG

Best,
Matt