[llvm-dev] sum elements in the vector

Wed May 11 21:07:21 PDT 2016

> why in order to add this particular instruction (sum elements in a vector) I need to add an insrinsic?
Adding intrinsic is not the only way, it is one of the way and user WILL-NOT be required to invoke
It specifically.

Currently LLVM does not have any instruction to directly represent “sum of elements in a vector” and
generate your particular instruction.However, you can do it without intrinsic by pattern matching the
LLVM-IRs representing “sum of elements in vector” to your particular instruction in DAGCombiner.

Regards,
Shahid

From: Rail Shafigulin [mailto:rail at esenciatech.com]
Sent: Monday, May 09, 2016 11:59 PM
To: Shahid, Asghar-ahmad; llvm-dev
Cc: Das, Dibyendu
Subject: Re: [llvm-dev] sum elements in the vector

I'm a little confused. Here is why.

I was able to add a vector add instruction to my target without using any intrinsics and without adding any new instructions to LLVM. So here is my question: how come I managed to add a new vector instruction without adding an intrinsic and why in order to add this particular instruction (sum elements in a vector) I need to add an insrinsic?

Another question that I have is whether compiler will be able to target this new instruction (sum elements in a vector) if it is implemented as an intrinsic or the user will have to specifically invoke an instrinsic.

Pardon if questions seem dumb, I'm still learning things.

Any help is appreciated.

On Fri, May 6, 2016 at 1:51 PM, Rail Shafigulin <rail at esenciatech.com<mailto:rail at esenciatech.com>> wrote:
Thanks for the reply. These steps will add an instruction as an intrinsic. Is it possible to add an actual new instruction so that a compiler could target it during an optimization? How hard is it to do it? Is that a realistic objective.

Rail

On Mon, Apr 4, 2016 at 9:02 PM, Shahid, Asghar-ahmad <Asghar-ahmad.Shahid at amd.com<mailto:Asghar-ahmad.Shahid at amd.com>> wrote:
Hi Rail,

We had done this for generation of X86 PSAD (sum of absolute difference) instruction through
Llvm intrinsic. Doing this requires following

1.       Define an intrinsic, xyz(),  for the required instruction and corresponding SDNode

2.       Generate the “call xyz() “ IR based the matched pattern

3.       Map “call xyz()” IR to corresponding SDNode in SelectionDagBuilder.cpp

4.       Provide default expansion of the xyz() intrinsic

5.       Legalize type and/or operation

6.       Provide Lowering of intrinsic/SDNode to generate your target instruction

You can visit http://llvm.org/docs/ExtendingLLVM.html for details.

Regards,
Shahid

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Rail Shafigulin via llvm-dev
Sent: Monday, April 04, 2016 11:00 PM
To: Das, Dibyendu
Cc: llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>
Subject: Re: [llvm-dev] sum elements in the vector

Thanks for the pointers. I looked at hadd instructions. They seem to do very similar to what I need. Unfortunately as I said before my LLVM experience is limited. My understanding is that when I create a new type of SDNode I need to specify a pattern for it, so that when LLVM is analyzing the code and is seeing a given pattern it would create this particular node. I'm really struggling to understand how it is done. So here are the problems that I'm having.

1. How do I identify that pattern that should be used?
2. How do I specify a given pattern?

Do you (or someone else) mind helping me out?

Any help is appreciated.

On Mon, Apr 4, 2016 at 9:59 AM, Das, Dibyendu <Dibyendu.Das at amd.com<mailto:Dibyendu.Das at amd.com>> wrote:
This is roughly along the lines of x86 hadd* instructions though the semantics of hadd* may not exactly match what you are looking for. This is probably more in line with x86/ARM SAD-like instructions but I don’t think llvm generates SAD without intrinsics.

From: llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org<mailto:llvm-dev-bounces at lists.llvm.org>] On Behalf Of Rail Shafigulin via llvm-dev
Sent: Monday, April 04, 2016 9:34 AM
To: llvm-dev <llvm-dev at lists.llvm.org<mailto:llvm-dev at lists.llvm.org>>
Subject: [llvm-dev] sum elements in the vector

My target has an instruction that adds up all elements in the vector and stores the result in a register. I'm trying to implement it in my compiler but I'm not sure even where to start.

I did look at other targets, but they don't seem to have anything like it ( I could be wrong. My experience with LLVM is limited, so if I missed it, I'd appreciate if someone could point it out ).

My understanding is that if SDNode for such an instruction doesn't exist I have to define one. Unfortunately, I don't know how to do it. I don't even know where to start looking. Would someone care to point me in the right direction?

Any help is appreciated.

--
Rail Shafigulin
Software Engineer
Esencia Technologies

--
Rail Shafigulin
Software Engineer
Esencia Technologies

--
Rail Shafigulin
Software Engineer
Esencia Technologies

--
Rail Shafigulin
Software Engineer
Esencia Technologies
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