[mlir][EmitC] Expand the MemRefToEmitC pass - Lowering reinterpret_cast

[Jaddyen]

This patch lowers memref.reinterpret_cast.
From:

func.func @casting(%arg0: memref<999xi32>) {
  %reinterpret_cast_5 = memref.reinterpret_cast %arg0 to offset: [0], sizes: [1, 1, 999], strides: [999, 999, 1] : memref<999xi32> to memref<1x1x999xi32>
  return
}

To:

void casting(int32_t v1[999]) {
  std::size_t v2 = 0;
  int32_t* v3 = &v1[v2];
  int32_t(*v4)[1][1][999]  = reinterpret_cast<int32_t(*)[1][1][999]>(v3);
  return;
}

[ilovepi]

In your example, int32_t(*)[1][1][999] v4 is invalid syntax, right? Shouldn't it be: int32_t(*v4)[1][1][999]? Is that a typo in the description? or is that what's generated/translated?

[ilovepi]

I think I've seen a zeroIndex crated in several places now. Its not much code, but that may be a good candidate for a helper function (e.g. in the anonymous namespace and marked static).

[ilovepi]

Suggested change

	llvm::SmallVector<mlir::Value> indices;
	for (int i = 0; i < rank; ++i) {
	indices.push_back(zeroIndex);
	}
	llvm::SmallVector<mlir::Value> indices(rank, zeroIndex);

[ilovepi]

This closure seems familar from your other patches. I'm guessing that means it should probably be a helper function instead, since I don't see anything in the capture list that couldn't be a parameter.

[ilovepi]

an actual reinterpret_cast<T>() is appropriate for C++, but for C, you'd need a C-style cast. given that the C-style cast will work in both languages and what you're doing can't be covered by static or dyn casts, maybe we should use a normal cast operation? Would that work here, do you think?