1. Activation Records and Function Calls

2. Today’s Objectives Explain why runtime stack needed for C
Draw logical division of memory into stack, heap
Compare and contrast stack and heap
List variables stored in heap; stack; neither
Describe 2 different function calling conventions
Implement function calls and activation records in your CTran compiler

3. Fortran Calling Requirements
Where should variables be stored?
Globals (common blocks) ?
Locals ?
Basically two possible areas
Data
Code

4. What about Recursion? Not a problem for FORTRAN
What makes recursion a problem?
Need to have MULTIPLE copies of local variables, one set per function in execution at any one time.
Dynamic memory?

5. Logical Memory Layout Stack Heap Static Space D Y N A M I C
Currently not in use
Currently not in use
Heap
Static Space

6. Dice.c int NUM = 100000; main() { int i, roll, *counts;
counts = (int *) malloc (13 * sizeof(int));
for (i = 0; i < 13; i++) counts[i] = 0;
for ( i = 0; i < NUM; i++)
roll = rand() % 6 + rand() %6 + 2
counts[roll]++;
for (i = 2; i < 13; i++)
printf(“There were %d rolls of %d\n”,
counts[i],i); }

7. Where Are the Variables?
Stack D Y N A M I C
i; roll; counts
Currently not in use
Currently not in use
Space for 13 ints – pointed to by counts
Heap
Num
“There were %d rolls of %d\n”
Static Space

8. Runtime Stack Activation record for Hanoi; 1 2 3 1
Activation Record for Main
Activation record for main

9. Support for Function Calls
Call usually simple, but setup not
call printf
what comes before and after call?
Support recursion?
No --- simple, use static space
Yes --- runtime stack of activation record(s)
Return value(s)

10. Solution? A contract between “architecture” and “compiler/OS”
One we’ll (ok I’ll) choose particularly useful for C

11. Activation Record Information
Parameters
Locals
Compiler Temporaries
Register Save/Restore
Previous Activation Record
Previous Scope
Anything else?

12. A Function Call Convention
fp and sp defined by assembler
Caller function (in order)
push parameters on stack, in reverse
push fp
fp = sp - 4
call callee
fp = 0 (fp)
sp = sp – (size of arguments + 4)
Callee function (in order)
sp = sp + size of function
execute function
sp = sp – size of function
return

13. An Activation Record sp fp Next stack location – beyond the stack
Reg save …
Reg save …
Reg save …
Temps …
8(fp) – local 2
4(fp) – local 1 fp
0(fp) – old fp
-4(fp) – parameter 1
-8(fp) – parameter 2
-12(fp) – parameter 3

14. Function Call Convention (2)
Alpha conventions
Parameters passed in registers, r16 - r24 or d16 - d24
Return in r0 (or d0)
Registers r1 - r8, d1 - d8 are NOT saved across function call invocations (caller save)
Registers r25 - r31 and d25 - d31 ARE saved across function call invocations (callee save)