The goal is to make a requirement for VSIR that signature element
masks are always contiguous. The SPIR-V backend already implicitly
makes that assumption, since it just consider the LSB and popcount
of the mask.
For example, consider this HLSL pixel shader:
float4 main(float4 color : COLOR) : SV_Target
{
return float4(color.x, 10.0f, 11.0f, color.w);
}
Currently the parser describes the input signature element
corresponding to semantic COLOR as having mask .xw, which is
sensible. However, the SPIR-V parser will interpret that as
a mask starting at x and with popcount 2, and assuming it is
contiguous it will implicitly act as if it were .xy. This is
not correct, because the wrong component will be loaded from
the vertex stage.
Here, we implement single inheritance by inserting a field at the
beginning of the derived struct with name "$super".
For the following struct declarations
struct a
{
float4 aa;
float4 bb;
};
struct b : a
{
float4 cc;
};
struct c : b
{
float4 bb;
};
this commit generates the following:
struct a
{
float4 aa;
float4 bb;
};
struct b
{
struct a $super;
float4 cc;
};
struct c
{
struct b $super;
float4 bb;
};
