1. Implementing Subprograms
Acknowledgement:
Mehdi Emadi
Dr. Abhik Roychoudhury
And …

2. Objectives To understand the implementation of subprogram calls
basic procedure of subprogram linkage
activation records
static and dynamic chaining

3. General issues When calling: Save state of calling procedure
Determine parameter-passing mechanism for each parameter and arrange access appropriately
Allocate storage for local variables
Initialize local variable values
Arrange transfer of control to procedure
Arrange return of control to calling procedure
Arrange access to visible non-local variables

4. General issues (cont.) When Returning:
Move final parameter values back to calling arguments (when pass by result)
Deallocate storage used for local variables
Undo access to visible non-local variables
Return control to calling procedure

5. Activation Records Programs consist of Code, Data, and book-keeping
Noncode part is organized in an Activation Record ( Code does not change by each call)
Activation record instances can include
Parameter definitions
Local variables
Functional value
Return address
Other data to be discussed

6. Fortran 77 A simple situation: Actual code is fixed at compile time
Local variables/data are also of fixed size
Subprograms cannot be recursive
A simple solution:
Local variables statically allocated (static activation records)
All referencing of non-local variables is through COMMON: a globally accessible static memory space

7. Fortran 77 Procedure call:
Save the execution status of current procedure
Carry out parameter-passing
Pass return address to callee
Transfer control to callee

8. Fortran 77 (cont.) Procedure return:
If using pass-by-value-result, final values of parameters are moved to arguments
If subprogram is a function, final value is moved back
Execution status of the caller is restored
Control is transferred back to caller

9. Fortran77 Activation Records
No more than one can exist for a procedure
AR Instances can be statically allocated
Either separate or interleave code and data
Code
Return address
Local variables
Main
Procedure A
Parameters
Common

10. Algol-like Languages New complexities over Fortran 77
Parameters can be passed in different ways (value or reference)
Subprogram variables may be dynamically allocated
Recursion legal: multiple activation records may exist simultaneously
Scope rules (usually static) determine visibility of non-local variables

11. Algol-like Languages Activation records
Format and size is known at compile time since all local variables and their size is statically fixed
Instances of activation records are dynamically generated at run-time

12. Contents of Activation Records
Return address
Pointer to code segment of caller and offset address of instruction
Static link:
A pointer to the activation record of static parent
Used to support non-local variable access
Dynamic link:
A pointer to activation record of caller
Used in static scoped languages to support procedure return (indicates how much of stack to remove)
Used in dynamic scoped languages for non-local variable access

13. Activation Records (continued)
Parameters:
Either values or addresses provided by caller
Local variables:
Scalars stored directly within activation record
Non-scalars (structures) stored elsewhere
References to locals can be via offset from beginning of record: known at compile time
Functional Value (if a function)
Place to put the returned value of the function
(In some languages the function name can be treated like a local variable)

14. Activation Record Example
procedure sub (var tot: real); var sum : real; begin : end;
A new activation record is created each time sub is called
Run-time Stack holds activation records
Stack discipline is appropriate for conventional procedures
But not for co-routines
Return Address
Local
Parameter
Dynamic Link
Static Link
sum
tot
Start of record
Offset of sum }

15. Factorial Example Return (fact) Functional: ? Parameter n=1
Dynamic Link
Static Link
Return (main)
Functional: ?
Parameter n=3
Local value = ?
Parameter n=2
Return (main)
Functional: ?
Parameter n=3
Dynamic Link
Static Link
Local value = ?
Return (fact)
Functional: ?
Parameter n=2
Return (main)
Functional: ?
Parameter n=3
Dynamic Link
Static Link
Local value = ?

16. Factorial Example Return (fact) Functional: 1 Parameter n=1
Dynamic Link
Static Link
Return (main)
Functional: ?
Parameter n=3
Local value = ?
Functional: ?
Parameter n=2
Factorial Example
Return (main)
Functional: ?
Parameter n=3
Dynamic Link
Static Link
Local value = ?
Return (fact)
Functional: 2
Parameter n=2
Return (main)
Functional: 6
Parameter n=3
Dynamic Link
Static Link
Local value = ?

17. Non-local references
Reference to a non-local variable requires a two step process:
Find the activation record instance where the variable was allocated
Use local offset to access the variable within the record
Finding the ARI (under static scoping) is not straightforward

18. Non-local references
program Prog; int i=0; procedure A; int i=1; B(); end;
procedure B; int i=2;
C(); end; procedure C; i=3; end; A();
end;
Show the set of activation records in existence when the statement ‘i=3’ is encountered.
Which declaration of ‘i’ will be updated?
Following the dynamic chain will find the wrong ‘i’!

19. Static Chains
A link from each procedure’s activation record to its parent’s activation record with respect to static scope
The static chain is different from the dynamic chain:
The static parent of C is Prog.
The dynamic parent of C is B.

20. Static Chains (continued)
No need to search for variable: number of static links is known at compile time (why?)
Static Depth: nesting level of scope
Chain Offset (Nesting Depth): difference between static depth of reference environment and static depth of declaration environment
References represented as (chain_offset, local_offset)
Locals can be handled with chain_offset=0

21. Maintaining Static Chains
At subprogram call:
Correct parent known at compile time
Must find most recent activation record of parent
Avoid search of dynamic chain by using existing static chain to find parent (treat procedure reference like variable
At return: just remove record from stack

22. Cost of Static Chains
Static chains allow references to non-local variables to be resolved, but at a cost:
References to non-local variables “above” parent are slow
Time to resolve a reference is difficult to predict, and sensitive to source code changes

23. Summary: MacLennan terminology
State of a procedure:
A fixed program part
A variable activation part
The ep (environment part), the context to be used for this activation of the procedure, local and nonlocal context
The ip (instruction part), the current instruction being (or to be) executed in this activation of the procedure

24. Review of scope rules
Q and T are declarations of procedures within P, so scope of names Q and T is same as scope of declaration a.
R and S are declarations of procedures in Q.
U is a declaration of a procedure in T.
Storage managed by adding activation records, when procedure invoked, on a stack.

25. Activation record stack

26. Scope rules
Scope rules: The scope of a variable are the set of statements where the variable may be accessed (i.e., named) in a program.
Static scope: Scope is dependent on the syntax of the program.
Dynamic scope: Scope is determined by the execution of the program.

27. Scope rules
Static nested scope: A variable is accessible in the procedure it is declared in, and all procedures internal to that procedure, except a new declaration of that variable name in an internal procedure will eliminate the new variable's scope from the scope of the outer variable.
A variable declared in a procedure is local in that procedure; otherwise it is global.

28. Activation record stack

29. Activation record stack
Problem is: How to manage this execution stack?
Two pointers perform this function:
1. Dynamic link pointer points to activation record that called (invoked) the new activation record. It is used for returning from the procedure to the calling procedure.
2. Static link pointer points to the activation record that is global to the current activation record (i.e., points to the activation record of the procedure containing the declaration of this procedure).

30. Program a(…);
var N: integer;
procedure b(sum: real);
var i: integer;
avg: real;
Data: array[1..10] of real;
procedure c(val: real);
begin …
writeln(Data[i]);
end {c};
end {b};
begin …
end {a}

31. Accessing a variable
At run-time skip down the static chain (how many skips? Static distance) to get to the activation record of the variable.
The address of the variable is obtained by adding the fixed offset of the variable to the address obtained in step 1.

32. Static distance(static nesting level:snl) and offset of the variable are computed by the compiler at compile time.
Symbol table
Name Type Snl Offset
N int
Sum real
i int
Avg real
Data real array
Val real

33. Implementation of subprogram storage: more examples
Each subprogram has a block of storage containing such information, called an activation record.
Consider the following C subprogram:
float FN( float X, int Y)
const initval=2;
#define finalval 10
float M(10); int N;
N = initval;
if(N<finalval){ ... }
return (20 * X + M(N)); }
Information about procedure FN is contained in its activation record.

34. Activation records

35. Dynamic nature of activation records
Each invocation of FN causes a new activation record to be created.
Thus the static code generated by the compiler for FN will be associated with a new activation record, each time FN is called.
A stack structure is used for activation record storage.

36. Dynamic nature of activation records

37. Activation record structure

38. Example of act. record stack
Declarations
Var A in P
B in Q
C in R

39. Activation record example 1
Ex 1. In R: C := B+A;  C local, A,B global
For each variable, get pointer to proper activation record.
Assume AR is current act.record pointer (R).
1. B is one level back:
Follow AR.SL to get AR containing B.
Get R-value of B from fixed offset L-value
2. A is two levels back:
Follow (AR.SL).SL to get activation record containing A.
Add R-value of A from fixed offset L-value
3. C is local. AR points to correct act record.
Store sum of B+A into L-value of C

40. Activation record example 2
Example 2. Execution in procedure Q: A := B
 B is local, A global
Assume AR is current activation record pointer (Q)
1. B is now local. AR points to activation record
Get R-value from local activation record

41. Activation record example 2
2. A is now one level back
AR.SL is activation record of P
Store R-value of B into L-value of A
Compiler knows static structure, so it can generate the number of static link chains it has to access in order to access the correct activation record containing the data object. This is a compile time, not a runtime calculation.

42. Setting the static link…

43. Displays: an alternative
Static links are in a separate array
One element per static depth
Exactly two steps per reference: (display_offset, local_offset)
When a subprogram at a given static depth is called, save and replace the link at that depth
Efficiency compared to Static Chains:
Deeply nested references more efficient
Maintenance of subprogram calls less efficient
Yet most programs are not deeply nested (three in practice)

44. Display Example Static Depth 0 Display Static Depth 3 Static Depth 2
Sub1
Sub2
MAIN
Static Depth 0
Display
Static Depth 3
Static Depth 2
Static Depth 1
Sub1
Sub5
Sub4
Sub3
Sub2
MAIN

45. Dynamic Scoping Deep Access (NOT same as deep binding):
References to non-local variables are be resolved by following dynamic links
Easy to implement
Search down call chain may be slow
Activation records must store variable names

46. Shallow Access (shallow binding method)
Like FORTRAN, each procedure is statically allocated one copy of its activation record.
Containing the most recent activation of that procedure.
Variable access is efficient.
In the case of recursive cal, push the activation record on the stack.
Activation records are allocated statically, their size is fixed (=> cannot be used for languages with dynamic arrays …)

47. Implementing Subprogram as Parameters
Two major problems:
Static type checking
Full specification for formal parameter shall be present: type of the function, the number, order, and type of each parameter.
Nonlocal references (free variables)
what is the nonlocal environment used when a function is invoked?

48. Program Main;
var X: integer;
Procedure Q(var I:integer;function R(J:integer):integer);
begin
X:=4;
I:=R(I);
end;
Procedure P;
var I: integer;
function FN(K:integer):integer;
X:=X+K; FN:=I+K;
I:=2; Q(X,FN);
X:=7; P;
End.

49. What should be the nonlocal environment used when FN is invoked within Q (Formal parameter R in Q)?
The nonlocal environment is the same as the one that would be used if the call R(I,X) were simply replaced by the call FN(I,X) in subprogram Q.
The nonlocal environment is the same as the one that would be used if the subprogram FN were invoked at the point where it appears as an actual parameter in the parameter list (in calling Q(X,FN)).

50. Procedural parameters are represented by closures.
The ip field: entry address of the actual parameter
The ep field: a pointer to the environment of definition of the actual procedure

51. Statement labels as parameters
Which activation should be used?
jump to the proper label in proper activation
How to implement it?
Pop all the activations to get to the right place

52. Nonlocal GOTOs
Nonlocal gotos require environment restoration.