[SPIR-V] `vk::BufferPointer<T>().Get().member = undefinedVar` don't generate `OpStore`

[devshgraphicsprogramming]

Description

I ran into this by accident, testing the types that vk::BufferPointer performs loads/stores with.

Steps to Reproduce

struct StaticMeshVertex
{
	float3 Position;
    uint pad;
};


StaticMeshVertex VertexFactory_GetVertex(uint vertexId)
 

{
    StaticMeshVertex retval;
    float3 uninit;
	vk::BufferPointer<StaticMeshVertex,4>(0xdeadbeefull).Get().Position = uninit;
    return retval;
}

https://godbolt.org/z/ee9PEMEsM

Actual Behavior

If the variable on the right hand side of the assignment is an OpUndef the corresponding OpStore won't be generated, and the BDA write gets sliently dropped.

If you perform the store with a constant or something else, it will be preserved

Environment

• DXC version
• Host Operating System <!--- Host operating system and version --->

[s-perron]

I don't see what the problem is? The optimizer removes the store. This is legal because the use of an OpUndef can return any bitpattern, and it does not have to be consistent with other uses. The optimizer pretends that the store will store the same value that is already in the memory location, and removes it.

[devshgraphicsprogramming]

The optimizer pretends that the store will store the same value that is already in the memory location, and removes it.

bold assumption, breaks expectations but ok.

I guess that would be an issue to raise in the SPIR-V Tools repo then?

[s-perron]

The optimizer pretends that the store will store the same value that is already in the memory location, and removes it.

bold assumption, breaks expectations but ok.

I guess that would be an issue to raise in the SPIR-V Tools repo then?

This is a standard compiler optimization. Your sample has an undefined value, so any output is correct. Llvm does the same and more. It will fold branches and replace function calls with unreachable if it observes undefined behavior. The only way we will change Spirv-opt is if you can show this change violates the spirv spec.

[devshgraphicsprogramming]

ok case closed then.