Question

Is there any way to optimize the following line of C code (to avoid branching)?

if (i < -threshold || i > threshold) { 
    counter++; 
}

All variables are 16-bit signed integers. An optimized version should be highly portable.

Answer 1

How about the following:

counter += (i < -threshold) | (i > threshold);

Assuming the original code was valid, then this should work too, in a portable way. The standard says that relational operators (<, > and so on) return an int equal to 1 on success, or 0 on failure.

Update

To answer Sheen’s comment below, the following code:

int main()
{
    short threshold = 10;
    short i = 20;
    short counter = 0;
    
    counter += (i < -threshold) | (i > threshold);
    
    return 0;
}

results in the following disassembler on x86 using GCC, with no optimisations:

  push   %rbp
  mov    %rsp,%rbp
  movw   $0xa,-6(%rbp)
  movw   $0x14,-4(%rbp)
  movw   $0x0,-2(%rbp)
  movswl -4(%rbp),%edx
  movswl -6(%rbp),%eax
  neg    %eax
  cmp    %eax,%edx
  setl   %dl
  movzwl -4(%rbp),%eax
  cmp    -6(%rbp),%ax
  setg   %al
  or     %edx,%eax
  movzbw %al,%dx
  movzwl -2(%rbp),%eax
  lea    (%rdx,%rax,1),%eax
  mov    %ax,-2(%rbp)
  mov    $0x0,%eax
  leaveq 
  retq