what is the between in writing a loop by branching statement and a normal loop

Question

what is the between in writing a loop by branching jump statement and a normal loop i.e for example

        main()
        {
        int i=0;
        while(i<9)
        {
          //do something
           i++;
        }

and

        main()
        {
            int i=0;
            label:
             //do something
              i++;
             if(i<9)
              goto label;
         }

is the performance of both of them are equal?

The difference is that if you use the second one you'll get [run over by dinosaurs](http://xkcd.com/292/) — Praetorian, Jun 14 '13 at 12:45
There's not branching here, since the code is not used. Use http://gcc.godbolt.org/ to see the assembler code. — nouney, Jun 14 '13 at 12:46
Your label-Code is the equivalent for a do-while loop, not for a while loop. — Werner Henze, Jun 14 '13 at 12:50
http://stackoverflow.com/questions/5366929/c-c-is-goto-faster-than-while-and-for — Tyler Morrow, Jun 14 '13 at 12:55
I wouldn't be surprised if the `goto` solution is *inferior* because it generates slightly different IR which is missed by some optimization passes. — , Jun 14 '13 at 13:37

score 10 · Accepted Answer · answered Jun 14 '13 at 12:44

These two loops are not equivalent: the second one is similar to a do/while loop, not a while loop:

main() {
    int i=0;
    do {
      //do something
       i++;
    } while(i<9);
}

The two should be the same as far as the performance goes; however, the readability of the solution with the goto suffers considerably, so I would refrain from using it in any of your projects.

score 2 · Answer 2 · edited May 23 '17 at 12:13

The difference is that the first one is a lot easier to read, which is why goto is generally avoided. Also as pointed out in dasblinkenlight's answer the semantics are not the same.

The performance should be about the same, since the CPU has to do jumps in order to implement both. Of course, with a higher-level description of intent (as in the first case) the chance of the compiler being able to optimize something increases.

Perhaps not in this particular case, but in general a more high-level description of what you want to do is to be preferred, and then the looping constructs are better than goto.

score 2 · Answer 3 · answered Jun 14 '13 at 12:46

In this case, compiler should produce an equivalent output. But I don't think these two examples are equivalent - a while first checks the condition and then decides wheter to execute the body. Your example with the goto first executes the body and then checks the conditions - it behaves more like a do while statement.

You shouldn't use goto in this case, as it reduces readability. gotos are valid in only a limited number of scenarios.

what is the between in writing a loop by branching statement and a normal loop

3 Answers3