Re: About g++ optimization

[address@hidden]

If you're doing more powerful optimizations, such as IPA based, its
likely you will be doing loop unrolling as well. And after loop
unrolling, they should generate code identical code.

> Saves usually a register and replaces a check against an 'arbitrary'
> number with a check for zero (which is faster/smaller on some
> architectures).

Say for a minute, that f(n) returned a constant and IPCP could replace
the f(n) with a constant, say 5.

For example:

 for (int i = 5; i--; )


On x86, gcc would generate for your example:

        movl  $4, %ebx
L1:
        decl    %ebx
        .... ; loop body
        cmpl    $-1, %ebx
        jne     L1


where mine would be:
        xorl    %ebx, %ebx
L1:
        incl    %ebx
        .... ; loop body
        cmpl    $5, %ebx
        jne     L1

'Technically', my code would be faster because zeroing out ebx is
cheaper than a move. :)

Anyways, my point is that I avoid doing 'micro' optimizations because
it does not always make the code faster, and it usually makes the code
slightly harder to read.

Regards,

Ryan Mansfield