[SPIR-V] Counter is emitted when RWStructuredBuffer is passed into a function

[Raikiri]

Description

[[vk::binding(0, 0)]]
StructuredBuffer<int> buf;

void Func(StructuredBuffer<int> b){}

void main()
{
  Func(buf);
}

Compiling with

dxc -spirv -T ps_6_0 -E main `"$srcPath`" -fspv-preserve-interface -fspv-preserve-bindings -Fo `"$dstPath`"

This code emits a binding for buf, but also for counter.var.buf. If I comment out the function call Func(buf), the second binding disappears.

Seems possibly related to #3287 ?

Environment

• DXC version libdxcompiler.so: 1.8(dev;0-00000000); libdxil.so: 1.8
• Host Operating System NixOS (not sure how this is relevant)

[s-perron]

This is expected. When passing the buffer to a function, we cannot know what the function will do with the buffer. The counter has to be created and passed along in case. Typically it would be removed during optimization if it is not actually needed.

However, you have the option to preserve the variable. The optimizer will leave it at your request.

[Raikiri]

I use the flag "preserve inteface" to preserve the interface that I defined, not the interface that the compiler decided to generate for its internal purposes. In fact, what the compiler does in this case is the opposite of preserving the interface that I defined.

[s-perron]

My mistake. StructuredBuffers do not have counter. They should never show up. I look at things too quickly sometimes. I thought it was a RWStructruedBuffer. We'll look into what is going on.

[Raikiri]

Actually, I'm not sure which one I was testing on. It might have been a RWStructuredBuffer (I don't have access to the compiler rn).

[s-perron]

There is no counter for a StructuredBuffer: https://godbolt.org/z/dbjT7v44f.

There is a counter for a RWStructuredBuffer: https://godbolt.org/z/99h4ceqqj.

As I said before this is expected. When you declare a RWStructuredBuffer it is declaring two Vulkan buffers. One for the main data and another for the counter. If the counter is not used, we often optimize the counter. In DXC, we never generate the counter unless it is possibly used. It is not generated for "internal purposes". You can see that this doc that there is a counter associated with the RWStructuredBuffer.

If anything, we should be generating it even if the resource is not called when -fspv-preserve-bindings is used. However, I am hesitatant to change that at this point. In the Clang implementation, the resource will always be generated and only removed if optimization finds that it is unused.

Can you explain your use case in more detail, so I can try to find a workaround for you?