func f(m map[int][]*byte, p *byte) {
    m[3] = append(m[3], p)
}

This function under SSA generates two write barriers, where the old compiler only generated one.
The required write barrier is the store of p to the backing array of the slice.
The spurious write barrier is the write of the slice pointer to an autotmp_ in preparation for the address of that autotmp_ to be passed to mapassign1.

See #14869 , this is decomposeBuiltIn case.

@aclements