Setting context variable on kernel builder

[LennyDZ]

Hi,

I'm using the kernel builder, and I'm trying to set, read and edit register. How can I access and set such variable within the kernel builder ? For example, I can do that :
@cudaq.kernel

def kernel():
    q = cudaq.qvector(2)
    reset(q[0])
    h(q[0])
    b1 = False
    b0 = mz(q[0])
    if b0:
        b1 = mz(q[0])
    if b1:
        x(q[1])
    mz(q[0])
    mz(q[1])

With the classical way of writing kernel. But How can I do that using the builder ?
I tried this :

kernel = cudaq.make_kernel()
q0 = kernel.qalloc()
q1 = kernel.qalloc()
kernel.reset(q0)
kernel.h(q0)
b1 = False
b0 = kernel.mz(q0)
def b0_condition():
    #  kernel.set(b1, kernel.mz(q0))
    #   b1 = kernel.mz(q0)
    pass

kernel.c_if(b0, b0_condition)

kernel.c_if(b1, kernel.x(q1))
kernel.mz(q0)
kernel.mz(q1)

print(cudaq.sample(kernel))

My question is, how can I access the register b1 and set its value from b0_condition and then access it again ?

[efratshabtai]

Hi @LennyDZ, the builder mode is limited as you noticed, we recommend using the newer and more expressive kernel mode for your scenario.
Does anything block you using the kernel mode? This is the mode we will invest in going forward so want to make sure it works for your needs.

[LennyDZ]

Hi, I'm working on some module to convert circuits from some homemade representation to cudaq kernel. The instructions are typically processed in an Iterable, so it is much more convenient to have this "just-in-time" creation interface, which allows to construct kernel programmatically.
I typically face the limitation when I want to call a sub-kernel in which I assign a value to a classical register, that is then used in the main kernel. Basically I want to allocate some space for the classical register in the main kernel, and then pass the pointer to it to the sub-kernel (that is a "pure device kernel") and be allowed to assign a value to it.
In the expressive kernel mode, we can pass kernel as argument, as described in section 6.1 of the language specification in the documentation, which allows for passing refs of QuakeValue of the caller (from what I'm guessing of the compilation process, which I'm not too familiar with). I think this feature isn't accessible from the "just-in-time" creation mode.