Procedures I Fall 2010 Lecture 6: Procedures. Procedures I Fall 2010 Function call book-keeping in C...

Procedures I Fall 2010

Lecture 6: Procedures


Function call book-keeping in Cmain() {int i,j,k,m;...i = mult(j,k); ... m = mult(i,i); ...

}

/* really dumb mult function */

int mult (int mcand, int mlier){int product;

product = 0;while (mlier > 0) { product = product + mcand; mlier = mlier -1; }return product;}

What information mustcompiler/programmer keep track of?

What instructions can accomplish this?


Function Call Book-keeping in Assembly

•Registers play a major role in keeping track of information for function calls.

•Register conventions:• Return address $ra

• Arguments $a0, $a1, $a2, $a3

• Return value $v0, $v1

• Local variables $s0, $s1, … , $s7

• The stack is also used; more later.


Instruction Support for Functions (1/6) ... sum(a,b);... /* a,b:$s0,$s1 */}int sum(int x, int y) {return x+y;}

address1000 1004 1008 1012 1016

2000 2004

C

MIPS

In MIPS, • instructions are 4 bytes &• stored in memory just like Here are addresses where the instructions are stored.



address1000 add $a0,$s0,$zero # x = a1004 add $a1,$s1,$zero # y = b 1008 addi $ra,$zero,1016 #$ra=10161012 j sum #jump to sum1016 ...

2000 sum: add $v0,$a0,$a12004 jr $ra # new instruction

C

MIPS



2000 sum: add $v0,$a0,$a12004 jr $ra # new instruction

C

MIPS

• Question: Why use jr instead of using j?

• Answer: sum might be called by many functions, so we can’t return to a fixed location/address. The calling proc to sum must be able to specify “return address”.


Instruction Support for Functions (4/6)• Single instruction to jump and save return

address: jump and link (jal)

• Earlier approach:

1008 addi $ra,$zero,1016 #$ra=10161012 j sum #goto sum

• Faster approach:

1008 jal sum # $ra=1012,goto sum

• Why have a new instruction (jal)? • Make the common case fast, function calls are very

common.

• Also, you don’t need to know the memory address of individual instructions with jal.


Instruction Support for Functions (5/6)• Syntax for jal (jump and link) is same as for j (jump):

jal label

• jal should really be called laj for “link and jump”:

• Step 1 (link): Save address of next instruction into $ra

- Why next instruction? Why not current one?

• Step 2 (jump): Jump to the given label


Instruction Support for Functions (6/6)

• Syntax for jr (jump register):jr register

• Instead of providing a label to jump to, the jr instruction provides a register which contains an address to jump to.

• Only useful if we know exact address to jump to.

• Very useful for function calls:•jal stores return address in register ($ra)•jr $ra jumps back to that address


########################################################### MY FIRST SPIM PROCEDURE## Simple procedure example: not more than 4 arguments, only ## 1 return value, no calls from within the procedure, and ## no local variables!########################################################### In c it would be:## int foo (int ain, int bin) {# int n= 2*ain*bin;# return n;# }#########################################################


# load parameters for test call to foo(4,6) li $a0,4 # set up first parameter li $a1,6 # set up second parameter

# call foo jal foo # call function

# on return from foo, the result is in $v0, save it and print it move $a0,$v0 # move result into argument li $v0,1 # syscall code for print integer syscall

# end program li $v0,10 syscall # terminate execution


## Procedure foofoo:# get arguments move $t0,$a0 # get first argument move $t1,$a1 # get second argument

# perform body of the procedure mul $t2,$t0,2 #compute 2*first argument mul $t2,$t2,$t1 #compute 2*first arg*second arg

# set up return value in register $v0 move $v0,$t2

# return jr $ra #return


Rules for Procedures• Called with a jal instruction, returns with

a jr $ra

• Accepts up to 4 arguments in $a0, $a1, $a2 and $a3

• Return value is always in $v0 (and if necessary in $v1)

• Must follow register conventions (even in functions that only you will call)!

• More details in a few slides


Nested Procedures (1/2)

int sumSquare(int x, int y) {return mult(x,x)+ y;

}

• A procedure called sumSquare, now sumSquare is calling mult.

• Problem: there’s a return address in $ra that sumSquare wants to jump but it will be overwritten by the call to mult.

• Need to save sumSquare return address before call to mult.


Nested Procedures (2/2)

• In general, may need to save some other info in addition to $ra.

• When a C program is run, there are 3 important memory areas allocated:

• Static: Variables declared once per program, cease to exist only after execution completes. e.g., C globals

• Heap: Variables declared dynamically

• Stack: Space to be used by procedure during execution; this is where we can save register values


C Memory Allocation

0

fff fffAddress

Code Program

Static Variables declaredonce per program

HeapExplicitly created space, e.g., malloc(); C pointers

StackSpace for saved procedure information$sp

stackpointer


Using the Stack (1/3)

• register $sp always points to the last used space in the stack.

• To use stack, we decrement this pointer by the amount of space we need and then fill it with info.

addi $sp,$sp,-8

# space on stack

# for two words



• So, how do we compile this?int sumSquare(int x, int y) {

return mult(x,x)+ y;}

•$a0 has x; $a1 has ysumSquare: addi $sp,$sp,-8 # space on stack sw $ra, 4($sp)# save ret addr sw $a1, 0($sp) # save y



sumSquare: # $a0 has x; $a1 has y. addi $sp,$sp,-8 # space on stack sw $ra, 4($sp) # save ret addr sw $a1, 0($sp) # save y

add $a1,$a0,$zero # mult(x,x) jal mult # call mult

lw $a1, 0($sp) # restore y add $v0,$v0,$a1 # mult()+y lw $ra, 4($sp) # get ret addr addi $sp,$sp,8 # update sp jr $ra

int sumSquare(int x, int y) {return mult(x,x)+ y; }

“push”

“pop”


Register Conventions (1/4)

• Caller: the calling function

• Callee: the function being called

• When callee returns from executing, the caller needs to know:

• which registers may have changed, and

• which registers are guaranteed to be unchanged.

• Register Conventions: A set of generally accepted rules as to which registers will be unchanged after a procedure call (jal) and which registers may be changed.


Register Conventions (2/4) - saved

•$0: No Change. Always 0.

•$s0-$s7: Save and restore if you change. Very important, that’s why they’re called saved registers. If the callee changes these in any way, it must restore the original values before returning.

•$sp: Save and restore if you change. The stack pointer must point to the same place before and after the jal call, or else the caller won’t be able to restore values from the stack.

• HINT -- All saved registers start with S!


Register Conventions (3/4) - volatile

•$ra: Can Change. The jal call itself will change this register. Caller needs to save on stack if nested call.

•$v0-$v1: Can Change. These will contain the new returned values.

•$a0-$a3: Can change. These are volatile argument registers. Caller needs to save if they’ll need them after the call.

•$t0-$t9: Can change. That’s why they’re called temporary: any procedure may change them at any time. Caller needs to save if they’ll need them afterwards.


Register Conventions (4/4)

• What do these conventions mean?• If function R calls function E, then function R/caller must save any t (temporary) registers that it may be using onto the stack before making a jal call.

• Function E/callee must save any S (saved) registers it intends to use before garbling up their values

• Remember: Caller/callee need to save only temporary/saved registers they are using, not all registers.


Beta: Save Saved Registers

Restore Saved Registers

Return Results

…

Send Parameters

Receive Results

Call Procedure

…

Save Unsaved Registers$t’s, $v’s, $a’s, $ra

Restore Unsaved Regs.$t’s, $v’s, $a’s, $ra

Return

…Retrieve Parameters

Alpha: Save Saved Registers

Restore Saved Registers

Return Results

…

Send Parameters

Receive Results

Call Procedure

…

Retrieve Parameters

Save Unsaved Registers$t’s, $v’s, $a’s, $ra

Restore Unsaved Regs.$t’s, $v’s, $a’s, $ra

Return

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MIPS Registers

The constant 0 $0 $zeroReserved for Assembler $1 $atReturn Values $2-$3 $v0-$v1Arguments $4-$7 $a0-$a3Temporary $8-$15 $t0-$t7Saved $16-$23 $s0-$s7More Temporary $24-$25 $t8-$t9Used by Kernel $26-27 $k0-$k1Global Pointer $28 $gpStack Pointer $29 $spFrame Pointer $30 $fpReturn Address $31 $ra

Use names for registers -- code is clearer!


Other Registers•$at: may be used by the assembler at any time; unsafe to use

•$k0-$k1: may be used by the OS at any time; unsafe to use

•$gp, $fp: don’t worry about them

• Note: Feel free to read up on $gp and $fp in Appendix A, but you can write perfectly good MIPS code without them.


#------------- Function fibo ---------------------------------

## int fibo(int n) {## if (n <= 2)## return 1;## else return(fib(n-2) + fib(n-1));## }

## fibo is a recursive procedure to find a given Fibbonacci numberfibo: sub $sp, $sp, 12 # push stack frame sw $ra, 0($sp) # save return address sw $s0, 4($sp) # and s registers used here sw $s1, 8($sp)

bgt $a0, 2, f0 # check for input greater than 1 li $v0, 1 # if not, just return a 1 b fret


# input OK, set up recursive callsf0: move $s0, $a0 # copy a0 to s0 so it's saved across calls

sub $a0, $s0, 2 # set param to n-2 jal fibo # and make recursive call move $s1, $v0 # save fib(n-2)

sub $a0, $s0, 1 # param = n-1 jal fibo # compute fib(n-1) add $v0, $v0, $s1 # add fib(n-2)

# return value from this call is already in $v0 as needed

fret: lw $ra, 0($sp) # restore return address, lw $s0, 4($sp) # s registers, lw $s1, 8($sp) add $sp, $sp, 12 # pop stack frame, jr $ra # and return....


# RECURSIVE PROCEDURE TO COMPUTE FACTORIALS

FACT: #save the registers that FACT uses sub $sp,$sp,8 #modify stack top sw $s0,0($sp) #save registers sw $ra,4($sp)

move $s0,$a0 #get parameter out of a0

#For n=1, return 1

bne $s0,1,MORE li $v0,1 #return a 1 j RET


#For n>1, return n*FACT(n-1)MORE:## move arguments in for call sub $a0,$s0,1 #compute n-1

## call FACT on n-1 jal FACT #recursive call

## use returned value and set up return value for this call mul $v0,$v0,$s0

RET: lw $s0,0($sp) #restore registers lw $ra,4($sp) add $sp,$sp,8 #reposition stack top

jr $ra #return


Quickie Quiz (True or False?)

When translating this to MIPS…

A. We COULD copy $a0 to $a1 (not store $a0 or $a1 on the stack) to preserve n across recursive calls.

B. We MUST save $a0 on the stack since it gets changed.

C. We MUST save $ra on the stack since we need to know where to return to…

int fact(int n){ if(n == 0) return 1; else return(n*fact(n-1));}

Tail Recursion

F

F

T


What does r have to push on the stack before “jal e”?

1: Nothing 2: 1 of ($s0,$sp,$v0,$t0,$a0,$ra) 3: 2 of ($s0,$sp,$v0,$t0,$a0,$ra) 4: 3 of ($s0,$sp,$v0,$t0,$a0,$ra) 5: 4 of ($s0,$sp,$v0,$t0,$a0,$ra) 6: 5 of ($s0,$sp,$v0,$t0,$a0,$ra)

r: ... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem ... ### PUSH REGISTER(S) TO STACK? jal e # Call e ... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to caller of r e: ... # R/W $s0,$v0,$t0,$a0,$sp,$ra,mem jr $ra # Return to r

Volatile! -- need to pushSaved

e can’t return changed, no need to push

e can return changed

Quickie Quiz


“And in Conclusion…”

• Functions called with jal, return with jr $ra.

• Use stack to save anything you need. Just be sure to leave it the way you found it.

• Instructions we know so farArithmetic: add, addi, sub, addu, addiu, subu

Memory: lw, sw, lh, sh, lb, sb

Decision: beq, bne, (blt, ble, bgt, bge)

slt, slti, sltu, sltiu

Unconditional Branches (Jumps): j, jal, jr

• Registers we know so far• All of them!

Procedures I Fall 2010 Lecture 6: Procedures. Procedures I Fall 2010 Function call book-keeping in C...

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