[llvm-dev] sum elements in the vector

Mon May 16 15:50:20 PDT 2016

On Mon, May 16, 2016 at 3:11 AM, Martin J. O'Riordan via llvm-dev <
llvm-dev at lists.llvm.org> wrote:

> This would be really cool.  We have several instructions that perform
> horizontal vector operations, and have to use built-ins to select them as
> there is no easy way of expressing them in a TD file.  Some like SUM for a ‘
> v4i32’ are easy enough to express with a pattern fragment,
>
Do you mind sharing how to do it with a pattern fragment? I'm not new to TD
files but all the work I've done was very simple.

> SUM ‘v8i16’ takes TableGen a long time to compute, but SUM ‘v16i8’
> resulted in TableGen disappearing into itself for hours trying to reduce
> the patterns before I gave up and cancelled it.
>
>
>
> If there were ISD nodes for these, then it would be far simpler to express
> in TableGen, and also, the pattern fragments only match a very specific
> form of IR to the desired instruction.
>
>
>
> The horizontal operations are particularly useful for finalising a
> vectorised operation - for example I may want to compute the scalar MAX,
> MIN or SUM of a large number of items.  If the number of items is divisible
> by the vector lanes (e.g. 4, 8, or 16 in our case), then 4, 8 or 16 at a
> time can be computed using normal vector operation, and then the final
> scalar value can be computed using a single horizontal operation.
>
>
>
>             MartinO
>
>
>
> *From:* llvm-dev [mailto:llvm-dev-bounces at lists.llvm.org] *On Behalf Of *Chandler
> Carruth via llvm-dev
> *Sent:* 16 May 2016 2:16
> *To:* Shahid, Asghar-ahmad; Rail Shafigulin; llvm-dev; Hal Finkel
>
> *Subject:* Re: [llvm-dev] sum elements in the vector
>
>
>
> I'm starting to think we should directly implement horizontal operations
> on vector types.
>
>
>
> My suspicion is that coming up with a nice model for this would help us a
> lot with things like:
>
> - Idiom recognition of reduction patterns that use horizontal arithmetic
>
> - Ability to use horizontal operations in SLPVectorizer
>
> - Significantly easier cost modeling of vectorizing loops with reductions
> in LoopVectorize
>
> - Other things I've not thought of?
>
>
>  Curious what others think?
>
>
>
> -Chandler
>
>
>
> On Wed, May 11, 2016 at 10:07 PM Shahid, Asghar-ahmad via llvm-dev <
> llvm-dev at lists.llvm.org> wrote:
>
> > 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>
> 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> 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] *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
> *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>
> 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] *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>
> *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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>
>

-- 
Rail Shafigulin
Software Engineer
Esencia Technologies
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