"External declaration [decl name] is unused" after resource assignment

[jeremyong]

Description
Given a global extern resource with an auto-assigned or user assigned resource register, reassigning the variable to a different resource (e.g. from ResourceDescriptorHeap) results in the "external declaration is unused" validation error, even if the resource is used prior to assignment.

Steps to Reproduce

CE link

RWByteAddressBuffer buffer : register(u0);

[numthreads(8, 8, 1)]
void CSMain(uint3 id : SV_DispatchThreadID)
{
        buffer.Store(id.x, 0);
        buffer = ResourceDescriptorHeap[0];
        buffer.Store(id.x, 0);
}

Actual Behavior

error: validation errors
<source>:1: error: External declaration '\01?buffer@@3URWByteAddressBuffer@@A' is unused.
Validation failed.

Note that this may be intended behavior (understandably), however, I think in this case the error message is misleading and should directly indicate that this sort of reassignment is forbidden if so.

Environment

• DXC version 1.7.2207
• Host Operating System Windows

[llvm-beanz]

Godbolt link:
https://godbolt.org/z/TqbEa3qMc

The gist is that we seem to add the dynamic handle binding to the top of the basic block so it gets put in front of the store. That could potentially cause a number of bugs if resources get re-assigned and used throughout a block things will go sideways.

That said, we also shouldn't allow assigning to global resource declarations. That just seems wrong.