ANNAJAH NATIONAL UNIVERCITY COMPUTER DEPARTMENT COMPILERS DONE BY : AHMED KHALED AL_MOSLEH

1. ANNAJAH NATIONAL UNIVERCITY COMPUTER DEPARTMENT COMPILERS DONE BY : AHMED KHALED AL_MOSLEH SUPJECT : COMPILERS LEC#(19). DATE : 3 / 3 /2004

2. __Program Variable Allocation__ ◘ Dragon:7,1-7.5 , fisher 9.1 , 9.2 ,9.6 ◘ where are variables kept?? □ Depends on language’s scope rules. □ Also depends on compilers optimization. ◘ In general prefer to keep frequently used variables in registers. □ Faster to access registers than memory □ many computers are load/store without operations on memory. ◘ Can always keep variables in memory □ Unbounded size , unlike limited registers. □ Easier to find variables from surrounding scope if kept in memory

3. ____C Storage Allocation_____ ◘ Global variables in C programs are allocated in data segment □ Compiler knows size of variables and its name ● Generate a static allocation count : .word ● Linker resolves references to this Word ◘ Local ( aka automatic ) variables are located on stack. □ Each subroutine invocation create an activation record to hold its local variables ●e.g. P calls U calls R calls S ●compiler may keep variables from currently- executing routine in registers

4. _______Active Records________ ◘ information stored on the stack for a subroutine invocation is called a stack frame or active record. ◘ Designed for particular language and computer □ Want minimum cost to crate , use ,and discard ◘ Has 4 parts □ Incoming arguments (actual for routine). □ Local variables □ Dynamically allocated data □ Outgoing arguments

5. _Creating an activation Record_ ◘ Arguments and variables are stored in a fixed offset from beginning from stack frame ◘ Create activation record when routine is called. □ Set $fp to point to first argument ● addiu $sp , $sp , Asize □ Make room for local variables ● addiu $sp , $sp , -Vsize ◘ Discard record when leaving function □ Pop frame of stack ● move $sp , $fp □ Restore $fp’s value in previous frame ● Register must have been saved during call ● Caller or callee could save

6. ____Simple Subroutine Call___ ◘ A simple call by value routine invocation □ Push the arguments onto the stack ● addiu $sp , $sp , -4 sw <arg 1> 4($sp) addiu $sp , $sp, -4 sw <arg 2> 4($sp) □ Save registers (see next slide) □ Call the routine ● jal routine □ When routine returns (callee save) ● $sp points after arguments ● $fp is unchanged

7. _______Register Saving______ ◘ Contents of registers must be saved on stack during a call and restored upon return ◘ Tow conventions □ Function executing the call saves/restores registers (caller-saved). □ Invoked functions saves /restores registers (callee-saved) ◘ Both have advantages and disadvantages □ MIPS uses combination of the two

8. _______Caller-Saved________ ◘ Function executing call saves registers before invoking the routine and restores them after □ Only have to save registers that contain useful (live) values □ But don’t know if registers will be used in callee □ Adds many instructions at each call site ◘ MIPS registers St0 - st8 are caller-saved □ Calee can write them without saving values ◘ Caller must have space in its stack frame to save registers

9. ________Callee-Saved_______ ◘ Upon invocation , subroutine saves the registers that it will use □ Only saves registers that will be used □ But don’t know if registers have live values □ Registers save/store code not duplicated at call sites ◘ MIPS registers $s0 - $s7 are callee-saved □ Routine cannot use them without saving values ◘ Also $sp and $fp are callee-saved on MIPS ◘ Callee must have space in its stack frame to save registers

10. __Subroutine Calls on MIPS__ ◘ Caller □ Push arguments on stack ● MIPS passes first four arguments in $a0 - $a3 □ Save caller registers ( $t0 - $t8) □ Execute a jal instruction ● Stores return address in $ ra ($31) ◘ Callee □ Set up stack frame ● $sp = $sp – frame_size ● Save $ fp ● $fp = $sp + 4* n_args + frame size □ Save callee-saved registers ● $fp already saved and $sp’s value is $fp ● $ ra and $ s0 - $ s7

11. _Procedure returns on MIPS__ ◘ Callee □ Result returned in register $v0 □ Return to address in $ra ● jr $ra □ Restore callee-saved registers ● $ra and $s0 - $s7 ● $sp = $ sp – frame_size ● $sp ◘ Caller □ Restore caller-saved registers

12. ________EXAMPLE________ ◘ Main() { fact(2); } int fact (in x) { if ( x==1) return 1; else return ( x* fact(x-1)); ◘ main : addiu $sp , $sp ,-8 sw $fp , 8( $sp) addiu $fp , $sp , 8 sw $ra , -4 ($fp) li $a0 , 2 jal fact lw $ra , -4($fp) lw $fp , 0($fp) addiu $sp , $sp ,8 jr $ra

13. _____EXAMPLE ,cont’d_____ ◘ fact : addiu $sp , $ sp , -12 sw $fp , 0($sp). addiu $sp , $ sp , 16 sw $ra , -8($fp) sw $s0 , -12($fp) bneq $a0 , 1 , L1 li $v0 , 1 bneq L2 L1: move $s0 , $a0 sub $a0 , $a0 , 1 jal fact mult $v0 , $v0,$s0 L2: lw $ra , -8($fp) lw $s0 , -12($fp) lw $fp , 4($fp) addiu $sp , $sp, 12 jr $ra

14. ____Local Variables in C____ ◘ Variables in C are associated with begin-end block □ Created upon entry and destroyed upon exit ◘ Two ways to allocate □ Allocate the maximum amount of storage upon subroutine entry ● single push is cheap □ allocate appropriate space upon block entry ● keep stack frame smaller ,but requires more work ◘ e.g. for ( int a , int b) { Int x; If ( a > x) { int y = b + x ; } else { int z= a + b ; } }

15. _____Register Allocation_____ ◘ Compilers try to keep variables in computers registers □ Faster to access and more flexibility ◘ Limited number of registers □ Process of fitting variables to registers is called register allocation ● Goal is to reduce program’s execution cost by making effective use of registers ◘ CS536 compiler will keep variables in memory □ Later , we will discuss how to register allocate