[SPIRV] Invalid SSA and missing OpPhi?

[sylware]

version 450

layout(location = 0) in vec4 in_position;

void main()
{
float y;

    if(in_position[0]>50.0)
            y=1.0;
    else
            y=2.0;

    gl_Position[0] = y;

}

It seems to generate invalid SSA because it is assigning 2 times a variable, probably because it misses a phi SSA function.
Or do I miss something?

[mknejp]

There are really two models that SPIR-V supports. The one chosen by glslang seems to be a load/store model without OpPhi. The alternative is to use only OpPhi and do without loads/stores where possible. Both are equally valid (if side effects are handled properly) and one can transform the former into the latter.

[sylware]

Then why keeping phi? Is phi that efficient at removing excessive load/store? (Because phi is kind of complex to implement on SIMD machines)

[sylware]

It seems since spirv is based on ssa, but not formal ssa,
phi may be more an extra implementation burden than anything else.
There is a bug an khronos bugzilla for the spirv product.