Why can we not specialize based on loop variable states?

Suppose we had the following:

Var idx;
Func out;

out(idx) = select(idx == 0, VAL_1, select(idx == 1, VAL_2, VAL3));

It would be nice to be able to force Halide to use if/then/else constructs for this in the loop body, instead of selects. I was assuming that this is done using the specialize() scheduling command:

out.specialize(idx == 0);
out.specialize(idx == 1);

However, this seems to be forbidden:

"Error at (...): Specialization condition (...) depends on Var or RVar idx. 
Specialization conditions may not depend on any Vars or RVars.

Is there a particular reason why this limitation exists? Or maybe some other way to reach the intended behavior, other than unrolling?

Thanks and kind regards,

Sander

Solution

Specialization puts an if statement outside of the outermost loop, which means that other Funcs that are compute_at the Func being specialized get included in the specialization.

The only way to get an if statement with a condition that depends on a loop variable is RDom::where, and there's no way to give it an else clause, so this may not be possible. Could you explain more why an if statement is better than unrolling in your case? Is idx parallel?