[LLVMdev] Vectorization factor limitation in Loop Vectorizer

[Shahid, Asghar-ahmad]

Hi James,

Thanks for the clarification. I hope to come back once I have a fix.
Pls let me know if you have any suggestion.

Thanks,
Shahid

From: James Molloy [mailto:james at jamesmolloy.co.uk]
Sent: Sunday, December 14, 2014 3:48 AM
To: Shahid, Asghar-ahmad; Nadav Rotem
Cc: llvmdev at cs.uiuc.edu
Subject: Re: [LLVMdev] Vectorization factor limitation in Loop Vectorizer

Hi shahid,

Having looked into this in the past, we're not taking into account the fact that integer promotions and truncations will disappear during code generation. The canonicalisation phase likes to create i32 and greater type arithmetic, which creates extensions and promotions . However of course the vectoriser can use i8 arithmetic I vectors, so those should go away.

Often this is the cause of insufficient vectorisation factors being picked, and we should fix it.

James
On Sat, 13 Dec 2014 at 16:09, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid at amd.com<mailto:Asghar-ahmad.Shahid at amd.com>> wrote:
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<mailto:nrotem at apple.com>]
Sent: Saturday, December 13, 2014 2:47 AM
To: Shahid, Asghar-ahmad
Cc: llvmdev at cs.uiuc.edu<mailto: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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