CODEWITHSUNDEEP
cassemblymips
Melvin Molina
Melvin Molina

Reputation: 21

incorrect output in MIPs program

I'm implementing the following code from c to mips assembly 

/*
  Given an array arr terminated by entry -1, transform the array by adding
  to each entry the sum of the remainder of the array after that entry.
  That is, if initially arr == [a_0, a_1, ..., a_n, -1], then finally
  arr == [b_0, b_1, ..., b_n, -1], where b_j = sum(a_i for i in j..n-1).
 */

uint32_t reverse_prefix_sum(uint32_t *arr) {
  uint32_t r;

  if(*arr == -1) return 0;
  r = reverse_prefix_sum(arr+1) + (uint32_t)*arr;
  *arr = r;        /* may discard MSB */
  return(r);
}

My current program is the following: 

reverse_prefix_sum: # Let the address of array arr be in $a0
    # adjust the stack pointer to save $a0 and $ra
    addi $sp,$sp,-8
    sw $a0,0($sp)
    sw $ra,4($sp)
    # Load *arr
    lw $t0,0($a0)
    beq $t0,-1,exit1 # r is in $v0

    # *arr = *arr + 1
    addu $a0,$a0,4 # recursive call
    jal reverse_prefix_sum
    addu $v0,$v0,$t0
    # restore $a0
    lw $a0,0($sp)
    #restore $ra
    lw $ra,4($sp)
    add $sp,$sp,8
    sw $v0,0($a0)
    jr $ra
exit1:
    # restore $a0
    lw $a0,0($sp)
    #restore $ra
    lw $ra,4($sp)
    addi $sp,$sp,8
    # return 0
    li $v0,0
    jr $ra

With input 1 2 3 4 -1, I'm getting output -4 -3 -2. I'm wondering where this could be improved to fix the output being incorrect.

Upvotes: 1

Views: 321

Answers (1)

prl
prl

Reputation: 12455

The problem is that t0 isn’t preserved across the function call. Reload it from 0(a0) after restoring a0 after the function returns.

Upvotes: 2

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