Calling methods on large structs may lead to numerous copies

[dubiousconst282]

Repro:

// slangc -target spirv-asm -emit-spirv-directly -profile glsl_460 -lang slang -O0 test.slang
struct WorkData {
    float A[16 * 16];
    float B[16 * 16];

    float Foo(uint i) { return A[i] * B[i]; }
};
ConstantBuffer<WorkData> input;
RWStructuredBuffer<float> dest;

[numthreads(64, 1, 1)]
void ComputeMain(uint tid: SV_DispatchThreadID)
{
    dest[tid] = input.Foo(tid);
}

The generated SPIRV contains numerous OpCompositeExtract instructions followed by a OpCompositeConstruct, which I'd hope to be redundant. Enabling optimizations makes them go away but I think only because of inlining, so I would prefer not to rely on it.

This is not specific to ConstantBuffer and also happens with pointers.

[csyonghe]

The copy is actually correct by the semantic meaning of the code.
In Slang, if you call a function that takes a struct as parameter, it semantically means the argument is passed by value (copying the value in), and it is expected that the compiler optimization will take place to remove unnecessary copies.

General support of passing by reference is unfortunately not possible in many platforms. For example, HLSL and GLSL does not have a notion of pointers, and pointers is only valid in SPIRV with restrictions.

This code when compiled with DXC is producing similar results with -O0.

It is possible for us to do a better job when targeting SPIRV, but it is not clear to me that will result in more performant code after optimization, since the optimization passes are designed to be good at removing this kind of copies.

[csyonghe]

I am closing this issue for now, since there is not much we can do in the short term. We cannot generate code that makes pervasive use of pointers in parameter passing given the restrictions of many targets just yet.