[llvm-dev] RFC: Interface user provided vector functions with the vectorizer.

[Francesco Petrogalli via llvm-dev]

> On Jun 24, 2019, at 12:20 PM, Saito, Hideki <hideki.saito at intel.com> wrote:
> 
>  
> For example, Type 2 case, scalar-foo used call by value while vector-foo used call by ref.


Yes, the call-by-ref is an important feature that can be used with linear modifiers.

> The question Johannes is asking is whether we can decipher that after the fact, only by looking at the two function signatures, or need some more info (what kind, what’s minimal)?

In the draft implementation we have been working on, we came up with a ParamKind enum that holds the following information:

enum class ParamKind 
{
    Vector,
    OMP_Linear,
    OMP_LinearRef,
    OMP_LinearVal,
    OMP_LinearUVal,
    OMP_LinearPos,
    OMP_LinearValPos,
    OMP_LinearRefPos,
    OMP_LinearUValPos,
    OMP_Uniform
};

The enum is used to classify the `ParameterType`, a class that is attached to each parameter and describes things like uniformity, linearity (with and without modifiers). The list of parameter types is then stored in the VectorFunctionShape:

struct VectorFunctionShape {
            unsigned VF; // Vectorization factor 
            bool IsMasked;
            bool IsScalable;
            ISAKind ISA;
            std::vector<ParamType> Parameters;
};

Here OpenMP is used to classify the parameter types (OMP_*), but nothing prevents the ParamKind and the VectorFunctionShape to be extended to be able to handle other vector paradigms.

I think that we have to handle all 9 different linear and uniform cases in the ParamKind separately, because there is no way to get such information from the vector function shape.

> I think we need to list up cases of interest, and for each vector ABI of interest, we need to work on the requirements and determine whether deciphering after the fact is feasible.
>  


I will provide the cases for AArch64.


> I think we can make further progress on trivial cases (where FE doesn’t “change” type) while we continue working out the details on non-trivial cases.
>  


I think we can agree to proceed these way. The SVFS and the VectorShapeInfo are (I believe) designed with extendibility as a requirements. If we need more metadata to represent some specific cases, we will extend the SVFS internal to handle such metadata.

I will update the RFC by moving the vector function signature generation in the FE. 

Thank you everybody for their input, and for your patience. This is proving harder than expected! :)

Francesco

> Thanks,
> Hideki
>  
> From: Doerfert, Johannes [mailto:jdoerfert at anl.gov] 
> Sent: Monday, June 24, 2019 9:21 AM
> To: Francesco Petrogalli <Francesco.Petrogalli at arm.com>; Tian, Xinmin <xinmin.tian at intel.com>
> Cc: Saito, Hideki <hideki.saito at intel.com>; Simon Moll <moll at cs.uni-saarland.de>; LLVM Development List <llvm-dev at lists.llvm.org>; Clang Dev <cfe-dev at lists.llvm.org>; Renato Golin <rengolin at gmail.com>; Finkel, Hal J. <hfinkel at anl.gov>; Andrea Bocci <andrea.bocci at cern.ch>; Elovikov, Andrei <andrei.elovikov at intel.com>; Alexey Bataev <a.bataev at hotmail.com>; nd <nd at arm.com>; Roman Lebedev <lebedev.ri at gmail.com>; Philip Reames <listmail at philipreames.com>; Shawn Landden <slandden at gmail.com>
> Subject: Re: RFC: Interface user provided vector functions with the vectorizer.
>  
> I mean, the FE will create only one of the 3 vector versions matching the one we want for a given vector length, wouldn't it? The question now is: can we with the scalar and one vector version correctly vectorize the call. If the answer is no, what is the minimal amount of information, in addition to the two version, we would need?
> 
> Get Outlook for Android
>  
> From: Francesco Petrogalli <Francesco.Petrogalli at arm.com>
> Sent: Monday, June 24, 2019 6:06:12 PM
> To: Tian, Xinmin
> Cc: Doerfert, Johannes; Saito, Hideki; Simon Moll; LLVM Development List; Clang Dev; Renato Golin; Finkel, Hal J.; Andrea Bocci; Elovikov, Andrei; Alexey Bataev; nd; Roman Lebedev; Philip Reames; Shawn Landden
> Subject: Re: RFC: Interface user provided vector functions with the vectorizer.
>  
> 
> 
> > On Jun 24, 2019, at 10:53 AM, Tian, Xinmin <xinmin.tian at intel.com> wrote:
> > 
> > To me, it is also an issue related to SIMD signature matching when the vectorizer kicks in. Losing info from FE to BE is not good in general.
> >  
> 
> Yes, we cannot loose such information. In particular, the three examples I reported are all generating i64 in the scalar function signature:
> 
> // Type 1
> typedef _Complex int S;
> 
> // Type 2 
> typedef struct x{
> int a;
> int b;
> } S;
> 
> // Type 3
> typedef uint64_t S;
> 
> S foo(S a, S b) {
> return ...;
> }
> 
> 
> On AArch64, the correspondent vector function signature in the three cases would be (for 2-lane unmasked vectorization):
> 
> // Type 1:
> 
> <4 x int> vectorized_foo(<4 x int>, <4 x int>)
> 
> // Type 2:
> 
> %a = type struct {I 32, i32}
> 
> <2 x %a* > vectorized_foo(<2 x %a*> , <2 x %a*>)
> 
> // Type 3:
> 
> <2 x i64> vectorized_foo(<2 x i64>, <2 x i64)
> 
> To make sure that the vectorizer knows how to map the scalar function parameters to the vector ones, we have to make sure that the original signature  information is stored somewhere.
> 
> I will work on this, and provide examples.
> 
> Suggestions are welcome.
> 
> Thank you
> 
> Francesco