Tail Call Elimination

We have discussed (in tail recursion) that a recursive function is tail recursive if the recursive call is the last thing executed by the function. 

We also discussed that a tail-recursivea doestail-recursive

If we take a closer look at the above function, we can remove the last call with goto. Below are examples of tail call elimination.

QuickSort : One more example
QuickSort is also tail recursive (Note that MergeSort is not tail recursive, this is also one of the reasons why QuickSort performs better) 

The above function can be replaced by following after tail call elimination. 

Therefore job for compilers is to identify tail recursion, add a label at the beginning and update parameter(s) at the end followed by adding the last goto statement.

Function stack frame management in Tail Call Elimination :
Recursion uses a stack to keep track of function calls. With every function call, a new frame is pushed onto the stack which contains local variables and data of that call. Let's say one stack frame requires O(1) i.e, constant memory space, then for N recursive call memory required would be O(N). 

Tail call elimination reduces the space complexity of recursion from O(N) to O(1). As function call is eliminated, no new stack frames are created and the function is executed in constant memory space. 

It is possible for the function to execute in constant memory space because, in tail recursive function, there are no statements after call statement so preserving state and frame of parent function is not required. Child function is called and finishes immediately, it doesn't have to return control back to the parent function. 

As no computation is performed on the returned value and no statements are left for execution, the current frame can be modified as per the requirements of the current function call. So there is no need to preserve stack frames of previous function calls and function executes in constant memory space. This makes tail recursion faster and memory-friendly.

Next Article:
QuickSort Tail Call Optimization (Reducing worst case space to Log n )