[llvm-dev] Auto-vectorization option
Sudakshina Dutta via llvm-dev
{
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];
}
}
}
- clang -S -emit-llvm find_max.c -o find_max.ll
- opt -loop-vectorize -force-vector-width=8 find_max.ll -o find_max_opt.ll
- clang -O3 -c find_max.c -Rpass=vector -Rpass-analysis=vector
- clang -O3 -c find_max.c -Rpass=vector -Rpass-analysis=vector -o find_max.ll
Sjoerd Meijer via llvm-dev
Sent: 14 May 2021 04:16
To: LLVM Development List <llvm...@lists.llvm.org>
Subject: [llvm-dev] Auto-vectorization option
Sjoerd Meijer via llvm-dev
Sent: 14 May 2021 09:08
To: LLVM Development List <llvm...@lists.llvm.org>; Sudakshina Dutta <sudak...@iitgoa.ac.in>
Subject: Re: [llvm-dev] Auto-vectorization option
Nigel Perks via llvm-dev
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...@lists.llvm.org>
On Behalf Of Sudakshina Dutta via llvm-dev
Sent: 14 May 2021 04:17
To: LLVM Development List <llvm...@lists.llvm.org>
Subject: [llvm-dev] Auto-vectorization option
Dear all,
Sudakshina Dutta via llvm-dev
Matt P. Dziubinski via llvm-dev
> 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
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Sudakshina Dutta via llvm-dev
Matt P. Dziubinski via llvm-dev
> Dear Matt P. Dziubinski,
>
> Thanks a lot for your reply. Although the vectorization is clearly
> visible in godbolt, I could not generate it by command line. Does it
> require some specific version of llvm/clang ?
No, only the -debug options require build with asserts; everything else
should be working with a regular release of Clang/LLVM. Chances are you
have to debug it (perhaps a missing side effect causing the entire loop
to be optimized away, etc.).
Best,
Matt
>
> Regards
> Sudakshina
>
> On Fri, May 14, 2021 at 5:53 PM Matt P. Dziubinski <mat...@gmail.com
> <mailto:mat...@gmail.com>> wrote:
>
> 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
> <https://godbolt.org/z/aWz37qYdW>
>
> If you don't need debugger intrinsics (llvm.dbg.*) add -g0, e.g.,
> https://godbolt.org/z/aWz37qYdW <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 <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
> <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
> <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
> <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