[PATCH] Improve Cost model for SLPVectorizer when we have a vector division by power of 2

[Hal Finkel]

----- Original Message -----
> From: "Karthik Bhat" <kv.bhat at samsung.com>
> To: "kv bhat" <kv.bhat at samsung.com>, nrotem at apple.com, aschwaighofer at apple.com, hfinkel at anl.gov
> Cc: spatel at rotateright.com, llvm-commits at cs.uiuc.edu
> Sent: Wednesday, August 20, 2014 12:18:05 PM
> Subject: Re: [PATCH] Improve Cost model for SLPVectorizer when we have a vector division by power of 2
> 
> Hi Hal,Sanjay,
> Thanks for the inputs. The reason i added support for only uniform
> constant power of 2 is because currently x86 backend successfully
> emits PSRAW/PSRAD (but if i'm not wrong this requires all the shifts
> to be same. i.e. division by same power of 2).
> 
> Non uniform values that are all power of 2 though are currently not
> that profitable as backend doesn't emit/or have instructions that
> can do this profitably.
> 
> Hal i had a doubt, by feature bit do we mean having something like a
> member variable say isPowerof2 and setting this before calling
> getArithmeticInstrCost?
> Please correct me if my understanding is wrong.

No; maybe we want to add something like:

enum OperandValueProperties {
  OP_None = 0,
  OP_PowerOf2 = 1
}
(this is really a bit field).

and then make getArithmeticInstrCost take (OperandValueKind Opd1Info, unsigned Prop1, OperandValueKind Opd2Info, OperandValueProperties Prop2)
instead of just Opd1Info and Opd2Info.

Then in the routines, you check for (Prop1 & OP_PowerOf2), etc.

 -Hal

> 
> Thanks for all the help and taking your time to review the patch.
> I will update the patch shortly based on your i/p's and raise updated
> patch.
> 
> Thanks
> Karthik Bhat
> 
> http://reviews.llvm.org/D4971
> 
> 
> 

-- 
Hal Finkel
Assistant Computational Scientist
Leadership Computing Facility
Argonne National Laboratory