llvm dev - Feb 2013 - [LLVMdev] Is there any llvm neon intrinsic that maps to vmla.f32 instruction ?

Hi all,

Everything is in the tile, I would like to enforce generation of vmla.f32
instruction for scalar operations on cortex-a9, so is there a LLMV neon
intrinsic available for that  ?
Thanks for your answers

Best Regards
Seb
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On 8 February 2013 10:40, Sebastien DELDON-GNB <sebastien.deldon at
st.com>wrote:
> Hi all,****
>
> ** **
>
> Everything is in the tile, I would like to enforce generation of vmla.f32
> instruction for scalar operations on cortex-a9, so is there a LLMV neon
> intrinsic available for that  ?****
>
>
Hi Sebastien,

LLVM doesn't use intrinsics when there is a clear way of representing the
same thing on standard IR. In the case of VMLA, it is generated from a
pattern:

%mul = mul <N x type> %a, %b
%sum = add <N x type> %mul, %c

So, if you generate FAdd(FMull(a, b), c), you'll probably get an FMLA.

It's not common, but also not impossible that the two instructions will be
reordered, or even removed, so you need to make sure the intermediate
result is not used  (or it'll probably use VMUL/VADD) and the final result
is used (or it'll be removed) and keep the body of the function/basic block
small (to avoid reordering).

cheers,
--renato
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Hi Renato,

Thanks for the answer, it confirms what I was suspecting. My problem is that
this behavior is controlled by vmlx forwarding on cortex-a9 for which despite
asking on this list, I couldn't get a clear understanding what this option
is meant for.
So here are my new questions:
Why for cortex-a9 vmlx-forwarding is enabled by default ? Is it to guarantee
correctness or for performance purpose ? I've made some experiments and
DISABLING vmlx-forwarding for cortex-a9 leads to generation of more vmla/vmls
.f32 and  significantly improve some benchmarks. I've not enter into a case
where it significantly  degrades performance or give incorrect answers.
Thus my goal is to use my front-end to generate llvm neon intrinsics that maps
to LLVM vmla/vmls f32 when I think it is appropriate and not to rely on
disabling/enabling vmlx-forwarding.

Best Regards
Seb

From: Renato Golin [mailto:renato.golin at linaro.org]
Sent: Friday, February 08, 2013 11:54 AM
To: Sebastien DELDON-GNB
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Is there any llvm neon intrinsic that maps to vmla.f32
instruction ?

On 8 February 2013 10:40, Sebastien DELDON-GNB <sebastien.deldon at
st.com<mailto:sebastien.deldon at st.com>> wrote:
Hi all,

Everything is in the tile, I would like to enforce generation of vmla.f32
instruction for scalar operations on cortex-a9, so is there a LLMV neon
intrinsic available for that  ?

Hi Sebastien,

LLVM doesn't use intrinsics when there is a clear way of representing the
same thing on standard IR. In the case of VMLA, it is generated from a pattern:

%mul = mul <N x type> %a, %b
%sum = add <N x type> %mul, %c

So, if you generate FAdd(FMull(a, b), c), you'll probably get an FMLA.

It's not common, but also not impossible that the two instructions will be
reordered, or even removed, so you need to make sure the intermediate result is
not used  (or it'll probably use VMUL/VADD) and the final result is used (or
it'll be removed) and keep the body of the function/basic block small (to
avoid reordering).

cheers,
--renato
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