Performance, for(int i...) vs for(i...)

[Tim Prince]

On 08/26/2012 11:30 AM, lab@kayohamid.net wrote:
> Hello, I like performance, and until last month i follow what I see on 
> the web to do better coding for performance, like:
>
> - Avoid i++, use ++i and so on.
>
> But now i'm using the assembly (-S) code generated to see if 
> somethings is correct, and today i'm investigating this:
>
> int i;
> for(i = 0;....);
>
> vs
>
> for(int i = 0;...);
>
> I make 2 codes:
>
> ----
> int main()
> {
> int i;
> for(i = 0; i < 10; i++) {}
> }
>
> ----
>
> int main()
> {
> for(int i=0;i<10;i++) {}
> }
>
> ----
>
> the result of forin.c and forout.c is the same:
>
>         .file   "forin.c"
>         .text
>         .globl  main
>         .type   main, @function
> main:
> .LFB0:
>         .cfi_startproc
>         pushl   %ebp
>         .cfi_def_cfa_offset 8
>         .cfi_offset 5, -8
>         movl    %esp, %ebp
>         .cfi_def_cfa_register 5
>         subl    $16, %esp
>         movl    $0, -4(%ebp)
>         jmp     .L2
> .L3:
>         addl    $1, -4(%ebp)
> .L2:
>         cmpl    $9, -4(%ebp)
>         jle     .L3
>         movl    $0, %eax
>         leave
>         .cfi_restore 5
>         .cfi_def_cfa 4, 4
>         ret
>         .cfi_endproc
> .LFE0:
>         .size   main, .-main
>         .ident  "GCC: (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3"
>         .section        .note.GNU-stack,"",@progbits
>
> Seeing this, can I say that using int inside the for or outside for is 
> the same? Have no difference on performance?
I expect the distinction to be important only where local scope is 
wanted, for example in parallel constructs such as OpenMP and cilk_for.
It's probably important to continue to handle the declaration outside 
the scope well, as that version is required for C source code by 
Microsoft compilers, even VS2012.
I have had occasion to wish that people didn't rely on non-standard 
usage for the case of a break; maybe Microsoft is right in forbidding 
both incorrect and correct C++ usage in C code.

-- 
Tim Prince