C3 avoids a jump instruction by always performing an addition. For a super-tight loop like this, those jumps can make a big difference. I'm not sure that I would expect a C compiler to realize that C4 could be rewritten as C3.

I had a longer answer, but ran it past Claude.AI and it knew a lot more about the topic than I did and my answer would have been wrong. Of course this could be inaccurate/wrong too, but it looks more accurate than what I was going to say. At the risk of offending people, here's is a portion of its answer:

On x86-64, there isn't a direct conditional arithmetic instruction like you describe, but there are several approaches that can eliminate branches:

1. CMOVcc (Conditional Move) - The most common approach

   ; Assuming condition result is in a flag
   mov eax, i
   lea edx, [rax + 1]    ; edx = i + 1
   cmovnz eax, edx       ; if condition != 0, eax = edx
   mov i, eax
   [download]

   or more directly:

   lea edx, [i + 1]
   test condition, condition
   cmovnz i, edx         ; conditionally move the incremented value
   [download]

2. SETcc (Set Byte on Condition) - Convert condition to 0/1

   xor eax, eax
   test condition, condition
   setnz al              ; al = (condition != 0) ? 1 : 0
   add i, eax            ; i += 0 or 1
   [download]

   This is often the most efficient for your exact use case - it turns the condition into a 0 or 1, then unconditionally adds it.
3. ADC/SBB (Add/Subtract with Carry) - If condition is in carry flag

   ; If your condition sets the carry flag:
   adc i, 0              ; i += carry flag
   [download]

The SETcc + ADD pattern is typically what compilers generate for branchless if (cond) i++, and it's usually faster than a branch for unpredictable conditions.

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