[LLVMdev] [llvm-commits] Vectors of Pointers and Vector-GEP

[Rotem, Nadav]

Hi Hal!

This is great!   Vector-selects are always profitable compared to scalar selects. They are even emulated using a few Xor/And/Or instructions on platforms that don't have a native 'blend' support.

Vector-geps on the other hand are only useful in very specific cases.  Currently we do not support Load/Store instructions with a pointer-vector operand, so you need to extract each pointer element prior to loading and storing. 
X86 supports a complex addressing modes which means that most GEPs are included in the load/store instruction and they cost nothing. So, performing vector-gep on these instructions would actually be slower. Vector-geps are only useful if your hardware supports scatter/gather using a vector of pointers/indices.  For example AVX2 has support for the 'gather' instruction which receives a vector of indices.  In many cases it is possible to lower vector-gep instructions into a gather instruction, but this is currently not supported by LLVM. 

Thanks,
Nadav


 
-----Original Message-----
From: Hal Finkel [mailto:hfinkel at anl.gov] 
Sent: Saturday, April 14, 2012 11:02
To: Rotem, Nadav
Cc: David A. Greene; Jose Fonseca; LLVM Developers Mailing List
Subject: Re: [LLVMdev] [llvm-commits] Vectors of Pointers and Vector-GEP

On Tue, 6 Dec 2011 09:19:43 +0200
"Rotem, Nadav" <nadav.rotem at intel.com> wrote:

> Hi,
> 
> I just wanted to let you know that I committed the pointer-vector 
> patch.

Nadav,

I just committed a change to BBVectorizer to allow it to generate these (and also vector selects) [>= r154735]. If you have cases where you think these should be generated, and they're now not generated by the vectorizer, please let me know.

Thanks again,
Hal

> 
> Thanks,
> Nadav
> 
> -----Original Message-----
> From: David A. Greene [mailto:greened at obbligato.org]
> Sent: Tuesday, December 06, 2011 00:10
> To: Jose Fonseca
> Cc: David A. Greene; Rotem, Nadav; LLVM Developers Mailing List
> Subject: Re: [LLVMdev] [llvm-commits] Vectors of Pointers and 
> Vector-GEP
> 
> Jose Fonseca <jfonseca at vmware.com> writes:
> 
> > I was referring to gathering a vector of sparse 32bit words, all 
> > relative from a base scalar pointer in a 64bit address space, where 
> > the offsets are in a 32bit integer vector.  My other reply gave a 
> > more detailed and concrete example.
> 
> Yep, I saw that.  I think LLVM IR should support it directly.
> 
> > Anyway, from Nadav's and your other replies on this thread it is now 
> > clear to me that even if the IR doesn't express base scalar pointers 
> > w/ vector indices directly, the backend can always match and emit 
> > the most efficient machine instruction. This addresses my main 
> > concern.
> 
> It can, but it would have to go through varying amounts of pain to do 
> so.  This is such a common operation that the IR should really support 
> it directly.
> 
>                            -Dave
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--
Hal Finkel
Postdoctoral Appointee
Leadership Computing Facility
Argonne National Laboratory
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