llvm dev - Apr 2020 - Codegen of `declare simd`

Hi all,

I have started working on modifying the codegeneration of `declare simd` to
produce the `vector-function-abi-variant` attribute [0].

Given that the `vector-function-abi-variant` attribute is supposed to be used in
multiple codegen cases:

1. `declare simd`
2. `declare variant` for `simd` context, 
3. and in the C attribute that we have agreed in [2], 

I propose, as a first step, to come up with a unified way to handle such
code-generation, so that most of the code will be shared among the three cases.

As a matter of fact, each of 1 2 and 3 will have their own clang attribute in
clang/Basic/Attr.td. We can create a layer that translate each of these
attributes into a shared representation of the scalar-to-vector mappings, so
that we can have a single implementation that perform code generation for such
attributes.

In LLVM there is already a layer that maps scalar functions to vector function,
the VFInfo class defined in llvm/Analysis/VectorUtils.h [3], where the vector
function classification is done by mapping the scalar name to the vector name
using a VFShape (vector function shape)n [4] (VFShape knows about the different
vector  parameter types [5] inferred by the OpenMP clauses).

With this in mind, I propose to modify codegen of `declare simd` as follow:

A. translate the attribute `OMPDeclareSimdDeclAttr` used by
`CGOpenMPRuntime::emitDeclareSimdFunction` VFInfo [3] and modify the codegen to
use such class instead of a vector of `ParamAttrTy`.
B. modify `CGOpenMPRuntime::emitDeclareSimdFunction` to generate the attribute
in [0].

An alternative to A. could be to modify the `ParamAttrTy` to handle all cases
that we need to support (linear, linear ref, uniform, align...), and then use it
for all 1.2.3. cases listed, but it seem quite handy to reuse the classification
of vector functions that is already available in VFShape/VFInfo.

Any thoughts? Does this make sense?

Kind regards,

Francesco

[0] https://llvm.org/docs/LangRef.html#call-site-attributes
[1] https://reviews.llvm.org/D72734
[2] http://lists.llvm.org/pipermail/llvm-dev/2019-June/133484.html
[3] https://llvm.org/doxygen/structllvm_1_1VFInfo.html
[4] https://llvm.org/doxygen/structllvm_1_1VFShape.html
[5] https://llvm.org/doxygen/structllvm_1_1VFParameter.html and
https://llvm.org/doxygen/namespacellvm.html#add480f2ed87faa59c16d4c01ffb4bf39