Recursive Insertion Sort in Assembly - c

Question

I am trying to implement insertion sort in Assembly ARMV-8A. More specifically i tried to translate the following code in C:
void insertionSortRecursive(int arr[], int n)
{
   
    if (n <= 1)
        return;
 
    insertionSortRecursive( arr, n-1 );
    int last = arr[n-1];
    int j = n-2;
 
    while (j >= 0 && arr[j] > last)
    {
        arr[j+1] = arr[j];
        j--;
    }
    arr[j+1] = last;
}
 
I have tried to translate it as it is but my implementation gets into infinite loop in function loop_insertion as debugger shows:
.data
my_Table: .space 16
size: .word 4
FileOpenMode: .string "r"
FileName: .string "test1.txt"
fscanf_str: .string "%d"
printf_str: .string "%d "
out_message_str: .string "%d "
.text
.global main
main:
stp x29,x30,[sp,-32]!
add x29,sp,0
adr x1,FileOpenMode
adr x0,FileName
bl fopen
mov x20,x0
adr x0,my_Table
mov x22,x0 //x22 adr of table
mov x21,4
mov x23,0
//**************** READ FROM FILE ******************
loop:
add x2,x29,28
adr x1,fscanf_str
mov x0,x20
bl fscanf
ldr w1,[x29,28]
mov x0,x22
str w1,[x0,x23]
add x23,x23,4
add w21,w21,-1
cbnz w21,loop
//********************************************
mov x0,x22 //adr of table
mov x1,4
bl insertion_sort
//**************** PRINT TO SCREEN FROM TABLE *****************
mov x21,0
mov x23,4
loop_print:
adr x0, out_message_str
ldr w1,[x22,x21]
bl printf
add x21,x21,4
sub x23,x23,1
cbnz x23,loop_print
//***********************************************************
ldp x29, x30, [sp], 32
ret
insertion_sort:
stp x29,x30,[sp,-64]!
add x29,sp,0
str x19,[x29,32] //str the save register
str x0,[x29,16] //str the address of table
str w1,[x29,24] //str the n
mov x19,4
cmp w1,1
b.le exit_ins
sub w1,w1,1
bl insertion_sort
ldr w9,[x29,24] //load the n for the suitable function
sub w9,w9,1 //n-1
mul w9,w9,w19
ldr x10,[x29,16] //adr table
ldr w11,[x10,x9] //last
udiv w9,w9,w19
sub w12,w9,1 //j=n-2
loop_insertion:
ldr w12,[x29,24]
cmp w12,0
b.lt label1
mul w12,w12,w19
ldr w13,[x10,x12] // w13=arr[j]
cmp w13,w11
b.le label1
add w12,w12,w19
str w13,[x10,x12] //arr[j+1]=w13
udiv w12,w12,w19
sub w12,w12,1
str w12,[x29,24]
b loop_insertion
label1:
add w12,w12,1
mul w12,w12,w19
str w11,[x10,x12]
exit_ins:
ldr x19,[x29,32] //ldr the value of x19 back to the x19
ldp x29, x30, [sp], 64
ret
I did some modifications such as loading and storing the value of n-2 inside the insertion_loop function but that does not make any change.

Answer 1

You need to comment your code better, especially if you want others to help.
I am guessing that instead of preserving j in w12 you are doing calculations with it and then try to get the original value back but fail. Since you have done add w12,w12,w19 to get arr[j+1], the value of w12 after udiv w12,w12,w19 will be j+1 and when you subtract one from that you end up with j again, hence the infinite loop. You have tons of registers, just use a different one for the j+1.
You should have been able to see this in your debugger.