As was correctly pointed out by @azteca1998, the element type for a
`llvm.getelementptr` was only read when using an attribute and not when
using a type. As pointed out in
https://github.com/llvm/llvm-project/issues/63832#issuecomment-1643751039,
the translation to LLVM would work for
```mlir
llvm.func @main(%0 : !llvm.ptr) -> !llvm.ptr {
%1 = llvm.getelementptr %0[0] { elem_type = !llvm.ptr } : (!llvm.ptr) -> !llvm.ptr
llvm.return %1 : !llvm.ptr
}
```
but not for
```mlir
llvm.func @main(%0 : !llvm.ptr) -> !llvm.ptr<ptr> {
%1 = llvm.getelementptr %0[0] : (!llvm.ptr) -> !llvm.ptr<ptr>
llvm.return %1 : !llvm.ptr<ptr>
}
```
This was caused by the `LLVM_GEPOp` builder only reading the type from
the attribute (`{ elem_type = !llvm.ptr }`), but not from the pointer
type (`!llvm.ptr<ptr>`).
Fixes#63832.
EDIT: During review Markus Böck pointed out that this bugfix adds new functionality for typed pointers, but this functionality shouldn't be there in the first place. In response, Oleksandr "Alex" Zinenko pointed out that this is okay for now since the typed pointers will be removed in an upcoming release anyway, so it's best to merge this PR and spend time on the removal instead.
LLVM IR has introduced and is moving forward with the concept of opaque
pointers, i.e. pointer types that are not carrying around the pointee type.
Instead, memory-related operations indicate the type of the data being accessed
through the opaque pointer. Introduce the initial support for opaque pointers
in the LLVM dialect:
- `LLVMPointerType` to support omitting the element type;
- alloca/load/store/gep to support opaque pointers in their operands and
results; this requires alloca and gep to store the element type as an
attribute;
- memory-related intrinsics to support opaque pointers in their operands;
- translation to LLVM IR for the ops above is no longer using methods
deprecated in LLVM API due to the introduction of opaque pointers.
Unlike LLVM IR, MLIR can afford to support both opaque and non-opaque pointers
at the same time and simplify the transition. Translation to LLVM IR of MLIR
that involves opaque pointers requires the LLVMContext to be configured to
always use opaque pointers.
Reviewed By: wsmoses
Differential Revision: https://reviews.llvm.org/D123310
