Question

Consider:

    addi t0, zero, 3
    addi a6, zero, 3

fast_multiply:
   ADD  a0, zero, zero

next_digit:
   ANDI a1, a6, 1
   SRAI a6, a6, 1

   BEQ  a1, zero, skip
   ADD  a0, a0, t0

skip:
   SLLI t0, t0, 1
   BNE  a6, zero, next_digit
   ADD  t0, zero, a0
   sw a0, 0x0(x0)



    addi t1, zero, 5
    addi a7, zero, 5

fast_multiply2:
   ADD  a2, zero, zero

next_digit2:
   ANDI a3, a7, 1
   SRAI a7, a7, 1

   BEQ  a3, zero, skip
   ADD  a2, a2, t1

skip2:
   SLLI t1, t1, 1
   BNE  a7, zero, next_digit
   ADD  t1, zero, a2
   sw a2, 0x4(x0)

When I try to move onto the next integer, it continues to run.

Answer 1

The second copy of fast multiply has a typo in the label.

BEQ  a3, zero, skip

should be jumping to skip2.

If you can t find the problem by reading the code, you can observe the problem by single step debugging — you can notice the control flow that inside the second look it is jumping back into the first loop.

The same problem for

skip2:
   SLLI t1, t1, 1        
   BNE  a7, zero, next_digit

Should be jumping to next_digit2.

We can also think of this as copy & paste error common to assembly language and due to the use of labels and the fact that labels have to be renamed in order to deconflict them from those in the first copy. Some assembly languages have a feature called local label to reduce this type of error.

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