此代码不能在Qtspim上运行

Question

这是堆排序的mips代码。当我尝试运行它时，我在149行得到以下错误消息语法错误(数组：.word 54,23,56,32,99,7,4,2,88,9,11,21,39,55,100,101,43,1,3,69，-5，-24，-17,0)，并且在第14行得到另一个指令引用未定义的符号。我已经看过它很多次了，仍然没有看到任何错误。如果你找到了，请告诉我。代码如下：

.text
.globl main
main:

  la $a0, array # a0 = &array

  la $t0, size
  lw $a1, 0($t0) # a1 = size(array)

  jal heapsort

  # print the array
  move $t0, $a0
  add $t1, $zero, $zero
printloop:
  li $v0, 1
  lw $a0, 0($t0)
  addi $t0, $t0, 4
  addi $t1, $t1, 1
  syscall
  li $v0, 4
  la $a0, space 
  syscall

  bne $t1, $a1, printloop   # while( t1 != size )
  li $v0, 4
  la $a0, newline
  syscall

done:
  jr $ra

heapsort: # a0 = &array, a1 = size(array)
  addi $sp, $sp, -8
  sw $a1, 0($sp)  # save size
  sw $a2, 4($sp)  # save a2
  sw $ra, 8($sp)  # save return address


  move $a2, $a1  # n will be stored in a2
  addi $a2, $a2, -1    # n = size - 1
  ble $a2,$zero, end_heapsort  # if (n <= 0 ) return;

  jal make_heap  # a0 = arr, a1 = size

  add $a1, $zero, $zero # clear $a1
heapsort_loop:
  # swap(array[0],array[n])
  lw $t0, 0($a0)
  sll $t1, $a2, 2  #t1 = bytes(n)
  add $t1, $t1, $a0
  lw $t2, 0($t1)
  sw $t0, 0($t1)
  sw $t2, 0($a0)

  addi $a2, $a2, -1 # n--
  jal bubble_down  # a0 = &array, a1 = 0, a2 = n

  bnez $a2, heapsort_loop
end_heapsort:
  lw $ra, 8($sp)
  lw $a2, 4($sp)
  lw $a1, 0($sp)
  addi $sp, $sp, 8
  jr $ra

make_heap: # a0 = &array, a1 = size
  addi $sp, $sp, -12
  sw $a1, 0($sp)
  sw $a2, 4($sp)
  sw $ra, 8($sp)

  addi $a2, $a1, -1  # a2 = size - 1

  addi $a1, $a1, -1  # start_index = size - 1
  srl $a1, $a1, 1  # start_index /= 2

  blt $a1, $zero, end_make_heap  # if(start_index < 0) return

make_heap_loop:
  jal bubble_down # a0 = &array, a1 = start_index, a2 = size-1
  addi $a1, $a1, -1
  ble $zero, $a1, make_heap_loop

end_make_heap:
  lw $ra, 8($sp)
  lw $a2, 4($sp)
  lw $a1, 0($sp)
  addi $sp, $sp, 12
  jr $ra

#bubble_down is a leaf in the call graph
bubble_down: # a0 = &array, a1 = s_idx, a2 = end
  move $t0, $a1  # index = s_idx
  sll $t1, $t0, 2  # child = index*2+1
  addi $t1, $t1, 1

  bgt $t1, $a2, end_bubble_down

bubble_down_loop:

  #if ( child < end && arr[child] < arr[child+1] )
  ble $a2, $t1, skipinc
  sll $t3, $t1, 2  # get bytes(child)
  add $t3, $t3, $a0
  lw $t3, 0($t3)  # t3 = arr[child]
  sll $t4, $t1, 2 #get bytes(child)
  addi $t4, $t4, 4 #t4 = bytes(child+1)
  add $t4, $t4, $a0
  lw $t4, 0($t4)  #t4 = arr[child+1]
  ble $t4, $t3, skipinc

  addi $t1, $t1, 1  # child++

skipinc:
  sll $t3, $t0, 2  # get bytes(index)
  add $t3, $t3, $a0
  lw $t4, 0($t3)  #t4 = arr[index], t3 = &arr[index]

  sll $t5, $t1, 2  # get bytes(child)
  add $t5, $t5, $a0
  lw $t6, 0($t5)  #t6 = arr[child], t5 = &arr[child]

  ble $t6, $t4, end_bubble_down

  # swap(arr[index],arr[child]
  # note: t4 = arr[index], t6 = arr[child], t3 = &arr[index], t5 = &arr[child]
  sw $t4, 0($t5)
  sw $t6, 0($t3)

  move $t0, $t1  # index = child

  sll $t1, $t0, 1  # child = index*2+1
  addi $t1, $t1, 1
  ble $t1, $a2, bubble_down_loop

end_bubble_down:
  jr $ra

.data
# the array that holds unordered data
array: .word 54,23,56,32,99,7,4,2,88,9,11,21,39,55,100,101,43,1,3,69,-5,-24,-17,0
size: .word 24
space: .asciiz " "
newline: .asciiz "\n"

Answer 1

语法错误可以通过在数组.word中的每个"，“后面加一个空格来修复:因此"54，23，56，32，99，7，4，2，...”

Answer 2

我无法重现你在Windows9.1.12版QtSpim中描述的错误。因此，您要么使用行为不同的不同版本的QtSpim，和/或没有运行您向我们展示的完全相同的代码，和/或没有在重新加载汇编代码之前重新初始化模拟器。

但是，您的代码中确实有一个bug。嗯，可能不止一个，但至少有一个是显而易见的：

在done:中，您可以尝试以jr $ra结束您的程序。但是，您在jal heapsort中更改了$ra的值，而没有保留它。所以在最后一个jr $ra中发生的事情是，您将跳回打印代码，然后该代码将再次执行。您要做的第一件事是$t0 = $a0，其中$a0应该包含array的地址。但是$a0不再包含数组的地址，因为就在jr $ra之前，您执行了la $a0, newline。

TL;DR:每当你做一个jal，你就是在修改$ra。如果以后需要$ra的前一个值，则需要保留它(在另一个寄存器、内存变量或堆栈中)。