Support for Variadic attributes in ODS


I am working on adding a new op to spv that would require support for variadic attributes. In particular, I am adding support to OpSpecConstantComposite which provides a way to declare a composite specialization constant in a SPIR-V module. The value of a composite specialization constant is derived (directly or indirectly) from other specialization constants with scalar values [1].

Currently, spv only supports scalar specialization constants which are defined as module-level symbols that don’t produce SSA values and instead are materialized inside functions using spv._reference_of.

The way we are trying to extend that support to composites is to add a new op: spv.specConstantComposite which is defined as follows:

def SPV_SpecConstantCompositeOp : SPV_Op<"specConstantComposite", [InModuleScope, Symbol]> {
  let arguments = (ins
    // The following line doesn't actually compile because variadic attributes are not supported.

  let results = (outs);

So the following SPIR-V example:

OpDecorate %x SpecId 1
OpDecorate %y SpecId 2
OpDecorate %z SpecId 3
%x = OpSpecConstant %int 32
%y = OpSpecConstant %int 16
%z = OpSpecConstant %int 1
%composite = OpSpecConstant .. %x %y %z

would be represented in MLIR as:

spv.specConstant @x spec_id(1) = 32 : i32
spv.specConstant @y spec_id(2) = 16 : i32
spv.specConstant @z spec_id(3) = 1  : i32
// 1. We can omit the () around the constituents below.
// 2. specConstantComposite doesn't provide a spec_id element.
spv.specConstantComposite @composite, (@x, @y, @z) : !spv.struct<i32, i32, i32>

As far as I can learn from the docs, there is no current support for such a variadic attributes feature. Can we extend the infrastructure in such a way to support that feature? If so, I can gladly pick up that task.

cc: @antiagainst

[1] SPIR-V Specification

In MLIR we have array attributes that can allow a flexible number of elements not known statically. Have you looked at SymbolRefArrayAttr to see whether that serves the purpose here?

I think it serves the purpose. Thanks a lot!