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

Question

This might be a silly question to ask, but this kind of optimization is sometimes boost performance of your application. Here I am asking specifically for C++, because the way C++ compile code is a lot different that c# or Java.

The question is which one performs better, if variable i is int.

1. i > -1
2. i >= 0

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

Thanks in advance.

Answer 1

In assembly language, both are on the same structure:

1. i > -1

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

2. i >= 0

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

According to used jump flags, the instruction jg make two flags comparaisons (ZF = 0 and SF = OF) but jge does only one (SF = OF).

So I'm tempted to say that both use almost same registers and CPU cycles, with maybe a very little quicker comparaison for i >= 0.

Answer 2

Well, according to logic > operation may be "cheaper" than >=, but I guess you are compiling with Optimization option enabled, so probably the compiler do whatever he wants to optimize your code, so I would say that doesn't matter, even if one is really faster, probably the compiler change it to the best option

Answer 3

When writing a for loop it could be beneficial to convert it from for (i = 0; i < 1000; i++) to for (i = 1000; i > 0; i--) because on some architectures the compiler can skip the compare instruction as a flag will be set when i reaches 0. On modern architectures I'm not sure it matters.