[llvm-dev] [RFC] Expose user provided vector function for auto-vectorization.

[Francesco Petrogalli via llvm-dev]

Hi Andrei,

> On Jun 7, 2019, at 5:46 PM, Elovikov, Andrei <andrei.elovikov at intel.com> wrote:
>
> Hi All,
>
> [I'm only subscribed to digest, so the reply doesn't look great, sorry about that]
>
>> The second component is a tool that other parts of LLVM (for example, the loop vectorizer) can use to query the availability of the vector function, the SVFS I have described in the original post of the RFC, which is based on interpreting the `vector-variant` attribute.
>> The final component is the one that seems to have generated most of the controversies discussed in the thread, and for which I decided to move away from `declare variant`.
>
> Where will the mapping between parameters positions be stored? Using the example from https://software.intel.com/en-us/cpp-compiler-developer-guide-and-reference-vector-variant:
>
> float MyAdd(float* a, int b) { return *a + b; }
> __declspec(vector_variant(implements(MyAdd(float *a, int b)),
>                          linear(a), vectorlength(8),
>                          nomask, processor(core_2nd_gen_avx)))
> __m256 __regcall MyAddVec(float* v_a, __m128i v_b1, __m128i v_b2)
>
> We need somehow communicate which lanes of widened "b" would map for the b1 parameter and which would go to the b2. If we only care about single ABI (like the one mandated by the OMP) than such things could be put to TTI, but what about other ABIs? Should we encode this explicitly in the annotation too?
>

I think that the mapping between a scalar parameter and the correspondent vector parameter(s - there can be more than one) should be handled by the Vector Function ABI when a vector function ABI is defined.

I am working out on a new proposal, I’ll keep you posted.

I think that the requirements of 1. being a user feature 2. Based on a standard (OpenMP), implies the fact that a contract between the scalar functions and the vector functions must be stipulated in some document, such document being a vector function ABI for the target.

I am crafting the attribute so that it makes it explicit that we are using OpenMP and we are expecting a Vector Function ABI.

Kind regards,

Francesco

> Best Regards,
> Andrei

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