Re: GCC Optimization?

[an0nym@gmx.de]

>> 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.
Alright. Thanks!


>> 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?
xor is imho a little bit faster than mov and xor %eax,%eax does the same
like mov $0x0,%eax. But I still don't know why the accumulator has to be
zeroed.


>> 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?
No I didn't use any optimiziation, I just thought of 'default'
optimiziations without turning -o2 or -o3 on.
So what is this code good for? I subtracts zero from %esp and saves the
result in %esp. So this code doesn't do anything with %esp. But I think
there should be a purpose?!

Best regards

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