llvm dev - Dec 2014 - [LLVMdev] Vectorization factor limitation in Loop Vectorizer

Hi Nadav/Devs

I am exploring Loop Vectorizer to vectorize i8 scalar operations into 8xi8
vector operation.

I was expecting the Loop Vectorizer to analyze the profitability for
vectorization factor(VF) of 8,
However it is not doing so due to the widest type calculation done for the
blocks inside the loop.

May be I am missing something, however, I am curious to know why Loop Vectorizer
limits the
profitability check to widest type and not allowing for other narrower type?

Regards,
Shahid
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Hi Shahid, 
> On Dec 10, 2014, at 10:48 PM, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid
at amd.com> wrote:
> 
> Hi Nadav/Devs
>  
> I am exploring Loop Vectorizer to vectorize i8 scalar operations into 8xi8
vector operation.
>  
> I was expecting the Loop Vectorizer to analyze the profitability for
vectorization factor(VF) of 8,
> However it is not doing so due to the widest type calculation done for the
blocks inside the loop.
>  
> May be I am missing something, however, I am curious to know why Loop
Vectorizer limits the
> profitability check to widest type and not allowing for other narrower
type?
The vectorizer stops the search of profitable vectorization factors at the
widest type because higher vectorization factors would require the compiler to
split the vectorized value into multiple registers. The vectorizer’s cost model
first tries to optimize for SIMD instruction utilization. Later, we optimize for
ILP by doubling the vectorization factor (we call it “interleave”) and exposing
ILP.

Thanks, 
Nadav 
>  
> Regards,
> Shahid
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So IMO, if we modify the VF calculation for targets/subtargets using TTI where
higher VF is supported
The vectorizer’s scope will become wider.

Did/do you foresee any issue with this?

Thanks,
Shahid

From: Nadav Rotem [mailto:nrotem at apple.com]
Sent: Saturday, December 13, 2014 2:47 AM
To: Shahid, Asghar-ahmad
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Vectorization factor limitation in Loop Vectorizer

Hi Shahid,

On Dec 10, 2014, at 10:48 PM, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid at
amd.com<mailto:Asghar-ahmad.Shahid at amd.com>> wrote:

Hi Nadav/Devs

I am exploring Loop Vectorizer to vectorize i8 scalar operations into 8xi8
vector operation.

I was expecting the Loop Vectorizer to analyze the profitability for
vectorization factor(VF) of 8,
However it is not doing so due to the widest type calculation done for the
blocks inside the loop.

May be I am missing something, however, I am curious to know why Loop Vectorizer
limits the
profitability check to widest type and not allowing for other narrower type?

The vectorizer stops the search of profitable vectorization factors at the
widest type because higher vectorization factors would require the compiler to
split the vectorized value into multiple registers. The vectorizer’s cost model
first tries to optimize for SIMD instruction utilization. Later, we optimize for
ILP by doubling the vectorization factor (we call it “interleave”) and exposing
ILP.

Thanks,
Nadav



Regards,
Shahid
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