1 Run time vs. Compile time The compiler must generate code to handle issues that arise at run time Representation of various data types Procedure linkage Storage organization

2 Data representation Fundamental types are supported directly by machine operations Enumerated types? Booleans? Arrays? Strings? Records? packed unpacked

3 Control abstraction A procedure is a control abstraction it associates a name with a chunk of code that piece of code is regarded in terms of its purpose and not of its implementation. Big issue #1: Allow separate compilation Without it we can't build large systems Saves compile time Saves development time We must establish conventions on memory layout, calling sequences, procedure entries and exits, interfaces, etc.

4 Control abstraction Procedures must have a well defined call mechanism In Algol-like languages: a call creates an instance (activation) of the procedure on exit, control returns to the call site, to the point right after the call. Use a call graph to see set of potential calls

5 Control abstraction Generated code must be able to preserve current state save variables that cannot be saved in registers save specific register values establish procedure environment on entry map actual to formal parameters create storage for locals restore previous state on exit This can be modeled with a stack Allocate a memory block for each activation Maintain a stack of such blocks This mechanism can handle recursion

6 Name space A procedure creates its own name space It can declare local variables Local declarations may hide non-local ones Local names cannot be seen from outside. The scope of a variable is the area in which it is active Scope rules determine how declarations are mapped to names.

7 Storage allocation Static allocation object is allocated address during compile time location is retained during execution Stack allocation objects are allocated in LIFO order Heap allocation objects may be allocated and deallocated at any time.

8 Static allocation Objects that are allocated statically include: globals explicitly declared static variables instructions string literals compiler-generated tables used during run time.

9 Stack allocation Follows stack model for procedure activation A procedure's memory block associated with an activation of the procedure is called an activation record or stack frame The stack frame is pushed on the stack when the procedure is called and popped (and destroyed) when the procedure terminates What can we determine at compile time? We cannot determine the address of the stack frame But we can determine the size of the stack frame and the offsets of various objects within a frame

10 Heap allocation Used for dynamically allocated/resized objects Managed by special algorithms General model maintain list of free blocks allocate block of appropriate size handle fragmentation handle garbage collection

11 Scope rules Static scoping determined at compile time Algol languages: resolve conflicts by using "closest nested scope" rule We must come up with a mechanism to access enclosing scopes (later)

12 Scope rules Dynamic scoping depends on flow of control at run time resolve conflicts using "most recently executed" rule type checking deferred until run time complicates program any advantages?

13 Scope rules Dynamic scoping example procedure main x, a: int; procedure one() begin x := 1; end procedure two() begin x:int; one(); end begin x := 2; read(a); if a>0 then one() else two(); print(x); end. run 1 : a is positive one() is called. The x in one() is bound to the most recent declaration which is the global one. The global x is given the value of 1 and the program prints 1 in the end. run 2 : a is negative two() is called. It declares a local x. Then it calls one(). The x in one() is bound to the most recent declaration which is the local declaration in two(). That local x is given the value of 1. one() exits. two() exits and the local x dies. The global x is still 2. The program prints 2 in the end.

14 Scope rules Dynamic scoping -- why bother? It allows us to customize procedures. Consider a procedure print_integer that can print its argument in base 8, 10 or 16. Furthermore, we usually want to print it in base 10. We can use a global variable (such as x in the example) which is set to 10. Then, if we want a different base, we can declare a local x, set it to 8 or 16, and call print_integer. Once we exit the current scope, the local x dies and we are back to the default value. Is there another way to do this? Yes, we could have passed x as a parameter Yes, we could have made x a statically allocated variable (e.g. global), initialized it to 10 and, if necessary, reset its value right before and after a call to print_integer bad idea: we might forget to restore it after the call.