[llvm] [DirectX] add GEP i8 legalization (PR #142475)

[Deric C. via llvm-commits]

Icohedron wrote:




> the bug I was looking at used allocsa, this one uses a globals its an easy fix to add:
> 
> ```
> if (auto *GV = dyn_cast<GlobalVariable>(BasePtr))
>       ElementType = GV->getValueType();
> ```
> 
> However during my testing I found another gap. we still need to figure out how to support constExpr GEPs.
> 
> > > This PR appears to fail to compile this shader: https://godbolt.org/z/9MPc8Wca5
> > 
> > 
> > This shader now compiles, but it currently fails DXIL validation

Incorporating that change still results in a shader that doesn't pass validation due to the same message: `Explicit gep operator type does not match pointee type of pointer operand`

The DXIL Legalize pass now emits:
```llvm
@g = local_unnamed_addr addrspace(3) global [2 x float] zeroinitializer, align 4
...
define void @CSMain() local_unnamed_addr #0 {
entry:
  ...
  %2 = load float, ptr addrspace(3) getelementptr inbounds nuw (float, ptr addrspace(3) @g, i32 1), align 4
```

I would expect the `%2` instruction to be the following to be a valid GEP:
```llvm
  %2 = load float, ptr addrspace(3) getelementptr inbounds nuw ([2 x float], ptr addrspace(3) @g, i32 0, i32 1), align 4
```

This doesn't seem to be something straightforward to fix.



https://github.com/llvm/llvm-project/pull/142475