Efficient C for ARM

Looping Structures

Loop termination condition can have a significant overhead.

  • You should write loops to count down.

Examples

Here’s a routine to calculate a factorial.

int fact1(int N)
{
  int i, fact = 1;
  for (i = 1; i <= N; i++)
    fact *= i;
  return fact;
}

Compiled by ARM ADS:

fact1 MOV   r2,#1    ; fact
      MOV   r1,#1    ; i
loop  CMP   r1,r0    ; i <= N
      MULLE r2,r1,r2
      ADDLE r1,r1,#1
      BLE   loop
      MOV   r0,r2
      MOV   pc,lr

The revised routine counts down.

int fact2(int N)
{
  int fact = 1;
  do
    fact *= N;
  while (--N != 0);
  return fact;
}

Compiled by ARM ADS:

fact2 MOV  r1,#1
loop  MUL  r1,r0,r1
      SUBS r0,r0,#1
      BNE  loop
      MOV  r0,r1
      MOV  pc,lr
  • Allows ADD+CMP to become SUBS.
  • The inner loop is now an instruction shorter.

But do note the subtly different behaviour:

  • Iterates at least once (ie. N == 0 won’t work, causing an infinite loop.)
  • Uses a pre-decrement combined with test.

Remarks

Inner loop is 4 instructions.

Writing loops to count down is beneficial because:

  • The SUBS instruction (subtract, setting flags) which is generated gives us a compare with zero for free.
  • Replacing the ADD+CMP with SUBS saves a cycle per iteration.
after changes:

i has been removed reducing register pressure.

The counting up for loop was changed to a counting down do .. while loop.

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