C 到 MIPS 代码翻译问题与嵌套过程

Question

I need to tranlate piece of C code

int main(){
int a, b, result;
if(a == b)
result = a*b;
else
result = assess(a, b);
return result;
}
int assess(int a, int b){
if(b<a)
return upgrade(a, b);
else
return demote(a, b);
}
int upgrade(int a, int b)
{return 4*(a+b);}
int demote(int a, int b)
{return 4*(b-a);}

a and b will be tested for a=8 b=8 a=3 b=5 a=5 b=3 here is what i tried

.text
main:
    add $s0,$s0,5
    add $s1,$s1,3
    add $s3,$s3,0
    beq $s0,$s1,Resultmul
    bne $s0,$s1,assess
    li $v0, 10
    syscall
assess:
    addi $sp,$sp,-8
    sw $s3,0($sp)
    sw $ra,4($sp)
    jal upgrade
    lw $ra,4($sp)
    add $sp,$sp,4
    jr $ra
Resultmul :
    mul $s3,$s1,$s0
    li $v0, 10
    syscall

upgrade:
    add $s3,$s0,$s1
    mul $s3,$s3,4
    jr $ra

demote:
    sub $v0,$s1,$s0
    mul $v0,$v0,4
    jr $ra

But it gets stuck in jr $ra in the assess procedure can someone fix this issue that would be great.

Answer 1

You are branching to assess instead of calling it like a function via jal . Thus, there is no proper value in $ra upon the entry to assess for it use upon completion to return to main .

You are (almost) properly saving $ra and restoring it later, but it never had a good value in the first place, so the save & restore (which will be needed) doesn't help yet.

You should pop as many bytes off the stack as you push — you're pushing 8 but popping only 4.

You are also not restoring $s3 though you do save it.

You might consider $ra as a parameter passed to a function, and inspect its value upon function entry and during function execution to see where becomes incorrect. The value passed to the callee should be the address of the return point in the caller — a code address.