Re: GCC Optimization?

[Brian Raiter <breadbox@muppetlabs.com>]

> But why do they use and .... I think and does only even %esp
> so that the last 4 Bits are set to Zero. 
> But if %esp was 0xcccccccc before it would now be 0xccccccc0
> what happens to the 4 Bits from 0xccccccc0 to 0xcccccccc?
> And what is this good for?

The x86 push and pop instructions are much, much less efficient if the
stack pointer is not dword-aligned. So if one function (say) pushes a
segment register onto the stack, it can affect the performance of
every function that is called from there.

> Second question:
> What does mov $0x0,%eax? Why dont they use xor %eax and what is it
> good for?

It zeros the accumulator, of course. As for why they don't use xor,
why should they? What would be the advantage?

> Third question: 
> What is sub %eax,%esp good for? We subtract 0 from
> esp... mmmhh... for what?

Did you try building the same code with gcc optimizations turned on?

b