What is the benefit of automatic variables?

Question

I'm looking for benefits of "automatic" in Systemverilog. I have been seeing the "automatic" factorial example. But I can't get though them. Does anyone know why we use "automatic"?

Answer 1

Traditionally, Verilog has been used for modelling hardware at RTL and at Gate level abstractions. Since both RTL and Gate level abstraction are static/fixed (non-dynamic), Verilog supported only static variables. So for example, any reg or wire in Verilog would be instantiated/mapped at the beginning of simulation and would remain mapped in the simulation memory till the end of simulation. As a result, you can take dump of any wire/reg as a waveform, and the reg/wire would have a value from the beginning till the end, since it is always mapped. In a programmers perspective, such variables are termed static. In C/C++ world, to declare such a variable, you will have to use storage class specifier static. In Verilog every variable is implicitly static.

Note that until the advent of SystemVerilog, Verilog supported only static variables. Even though Verilog also supported some constructs for modelling at behavioural abstraction, the support was limited by absence of automatic storage class.

automatic (called auto in software world) storage class variables are mapped on the stack. When a function is called, all the local (non-static) variables declared in the function are mapped to individual locations in the stack. Since such variables exist only on the stack, they cease to exist as soon as the execution of the function is complete and the stack correspondingly shrinks.

Amongst other advantages, one possibility that this storage class enables is recursive functions. In Verilog world, a function can not be re-entrant. Recursive (or re-entrant) functions do not serve any useful purpose in a world where automatic storage class is not available. To understand this, you can imagine a re-entrant function as a function which dynamically makes multiple recursive instantiations of itself. Each instance gets its automatic variables mapped on the stack. As we progress into the recursion, the stack grows and each function gets to make its computations using its own set of variables. When the function calls return the computed values are collated and a final result made available. With only static variables, each function call will store the variable values at the same common locations thus erasing any benefit of having multiple calls (instantiations).

Coming to the factorial algorithm, it is relatively easy to conceptualize factorial as a recursive algorithm. In maths we write factorial(n) = n(factial(n-1))*. So you need to calculate factorial(n-1) in order to know factorial(n). Note that recursion can not be completed without a terminating case, which in case of factorial is n=1.

function automatic int factorial;
   input int n;
   if (n > 1)
     factorial = factorial (n - 1) * n;
   else
     factorial = 1;
endfunction

Without automatic storage class, since all variables in a function would be mapped to a fixed location, when we call factorial(n-1) from inside factorial(n), the recursive call would overwrite any variable inside the caller context. In the factorial function as defined in the above code snippet, if we do not specify the storage class as automatic, both n and the result factorial would be overwritten by the recursive call to factorial(n-1). As a result the variable n would consecutively be overwritten as n-1, n-2, n-3 and so on till we reach the terminating condition of n = 1. The terminating recursive call to factorial would have a value of 1 assigned to n and when the recursion unwinds, factorial(n-1) * n would evaluate to 1 in each stage.

With automatic storage class, each recursive function call would have its own place in the memory (actually on the stack) to store variable n. As a result consecutive calls to factorial will not overwrite variable n of the caller. As a result when the recursion unwinds, we shall have the right value for factorial(n) as n*(n-1)(n-2) .. *1.

Note that it is possible to define factorial using iteration as well. And that can be done without use of automatic storage class. But in many cases, recursion makes it possible for the user to code algorithms in a more intuitive fashion.