In C ++, which is better than i> -1 or i> = 0

Question

This may be a dumb question, but such optimization sometimes improves the performance of your application. Here I ask specifically for C ++, because the way to compile C ++ code is very different from that of C # or Java.

The question is which one works best if the variable i is int.

• i > -1
• i >= 0

I am looking for performance in terms of the required memory block or registers and processor cycles required for both conditions.

Thanks in advance.

Answer 1

In assembly language, both are in the same structure:

• i > -1

   cmp [register with i value],-1 jg [somewhere] 

• i >= 0

   cmp [register with i value],0 jge [somewhere] 

According to the transition flags used , the jg command does two flag mappings (ZF = 0 and SF = OF), but jge does only one (SF = OF).

So, I am tempted to say that both use almost the same registers and processor cycles, possibly a very quick comparison for i >= 0 .

Answer 2

Well, logically,> an operation may be "cheaper" than> =, but I think you are compiling with the optimization option turned on, so maybe the compiler does everything it wants to optimize your code, so I would say, even if someone it's really faster, maybe the compiler will change it to a better option

Answer 3

When writing a for loop, it can be useful to convert it from for (i = 0; i < 1000; i++) to for (i = 1000; i > 0; i--) , because on some architectures the compiler may skip the comparison statement, since the flag will be set when I reach 0. On modern architectures, I'm not sure if this is important.