Use MemRef to represent pointer

[jackalcooper]

Inspired by triton-shared, to represent a pointer with a memref of dynamic offset, it makes it possible to implement the pointer offset addition. For instance, memref<1xf32, strided<[1], offset: ?>>.

How to work with enif

Calling enif functions, like enif_get_int which has a signature int enif_get_int(ErlNifEnv* env, ERL_NIF_TERM term, int* ip) requires raw pointer. So we need a mechanism to make the align_ptr accessible at runtime.

Here are some options:

1. differentiate a llvm pointer and charms pointer?

i_ptr = Pointer.allocate(int32())
ip = llvm.allocate
enif_get_int(env, i_term, ip)
i = llvm.load(ip)
Pointer.store(i，i_ptr)

2. add memref_to_ptr C function and op. But it is ugly that memref descriptor struct has different layout at different rank.

i_ptr = Pointer.allocate(int32())
ip = memref_to_ptr(i_ptr)
enif_get_int(env, i_term, ip)
Pointer.load(i_ptr)

3. use llvm.allocate and ptr_to_memref, to cast the memref to one with dynamic offset. The downside is that this can't reuse MLIR's buffer infrastructure.

ip = Pointer.allocate(int32())
enif_get_int(env, i_term, ip)

# is compiled to 
ip= llvm.allocate(int32())
i_ptr  = ptr_to_memref(i_ptr)
enif_get_int(env, i_term, ip)
Pointer.load(i_ptr)

4. use a dialect and resolve unrealized_conversion_cast

%22 = builtin.unrealized_conversion_cast %21 : !llvm.struct<(ptr, ptr, i64, array<1 x i64>, array<1 x i64>)> to memref<?xi32, strided<[1], offset: ?>>
%23 = "ex.aligned_ptr"(%22) : (memref<?xi32, strided<[1], offset: ?>>) -> !llvm.ptr

builtin.unrealized_conversion_cast and ex.aligned_ptr will be replaced with

%0 = llvm.load %arg0 : !llvm.ptr -> !llvm.struct<(ptr, ptr, i64, array<3 x i64>, array<3 x i64>)>
%1 = llvm.extractvalue %0[0] : !llvm.struct<(ptr, ptr, i64, array<3 x i64>, array<3 x i64>)> 

Tensor of pointers

One highlight of Triton which makes it super easy to use is tensor of pointers. With that it is possible to write succinct code with operations like arrange to do "batch" indexing, instead of using a loop form. It is import for Charms to also support tensor of pointers and makes it really first-class.

Add Pointer

Adding an integer to an index should be syntax sugar for Pointer.element_ptr.