[llvm-dev] [RFC] A new multidimensional array indexing intrinsic

[Michael Kruse via llvm-dev]

Hi,

I think I understand what the problem is. The return type will also be
an untyped pointer, just like GEP. However, GEP needs to know what the
size of one element is. Like it is now with overloadable intrinsics,
we might derive it from the suffix. Another solution is that we pass
the element and array sizes in bytes, instead of multiples of the
element size. As a third options, we might change and auto-upgrade the
intrinsic to a proper instruction when the change comes.

Michael


Am Mo., 22. Juli 2019 um 20:14 Uhr schrieb Kaylor, Andrew via llvm-dev
<llvm-dev at lists.llvm.org>:
>
> I don't think the return type has anything to do with what I was asking about. Perhaps seeing a concrete, fully-legal IR representation of what's being proposed would help.
>
> Here's a best guess, switching the indexed base type to a non-integer for illustration purposes:
>
> %arrayidx = call i64 @llvm.multidim.array.index.i64.p0f64.i64.i64.i64.i64 double* %A, i64 %str_1, i64 %idx_1, i64 %str_2, i64 %idx_2
>
> According to the RFC, that would get lowered to this:
>
>   %mul1 = mul nsw i64 %str_1, %idx_1
>   %mul2 = mul1 nsw i64 %str_2, %idx_2
>   %total = add nsw i64 %mul2, %mul1
>   %arrayidx = getelementptr inbounds double, double* %A, i64 %total, !multidim !1
>
> The problem I'm having is that the source element type in the GEP instruction in the lowering can only be inferred from the pointer type, which is going away at some point. When pointers become typeless, the call will look something like this:
>
> %arrayidx = call i64 @llvm.multidim.array.index.i64.p0.i64.i64.i64.i64 void* %A, i64 %str_1, i64 %idx_1, i64 %str_2, i64 %idx_2
>
> Do you see what I'm saying? Have I misunderstood something?