I am going through the book Compiler Construction by Niklaus Wirth. The following is what I feel to be stated summarily by his lesson on procedure calls:
The compilation of a procedure call is the allocation of a block of stack memory. It starts with the base address which is stored in the frame pointer and the local variables are allocated addresses with respect to it with negative offsets.
For every procedure the block of stack memory is termed as its stack frame or the activation frame and a DYNAMIC LIST is maintained of the base pointers of the stack frame
Then an illustration of an situation is made where nested procedure calls are made with the following code snippet:
PROCEDURE P; P 0 VAR x: INTEGER; x 1 PROCEDURE Q; Q 1 VAR y: INTEGER; y 2 PROCEDURE R; R 2 VAR z: INTEGER; z 3 BEGIN x := y + z END R; BEGIN R END Q ; PROCEDURE S; S 1 BEGIN Q END S; BEGIN Q; S END P;
Now it is said that assuming that access of the local variables of R by traversing the dynamic list, it is possible that call to R can be made through the sequence P → S → Q → R. As such a second list of activation records is necessary, denoting the nestedness of the procedures.
Why is it necessary to have a static list of the activation frames of the procedures and the procedures nested in them?
At the end of a procedure, we can restore the value of the frame pointer by taking the previous value in the dynamic list and the return address would be restored from the register where it was saved. I don't see why a static list linking the activation frames of the nested procedure is required.
It has been suggested that the following question :
might be having the answer to my question and the questions are co-duplicate to each other but I would beg to differ as :
The suggested question is more about evaluation strategy of a function , more focused on the implementation of argument passing to functions , however my question can be said to be about "returning the control to the calling procedure" or "linking of nested procedures ". My question basically asks the reasons for which Wirth considers maintaining of one static list as a necessity . The following is the link to With's book and my question is centered around the topic "addressing of variables" in pages 72 and 73 .
ON WHY I SAID THAT MY QUESTION IS MORE ABOUT "returning the control to the calling procedure"
I know Wirth starts up the topic with the problem of accessing the variables of an internal procedure or nested procedure but since the dynamic list of frame pointers is maintained to restore the registers from activation frames on return of control to the calling procedure that is why I said that my question is more about the "returning the control to the calling procedure".