[Pgubook-readers] Question on assembly code generated by gcc

[Yannis Haralambous]

Hi, I have a small question:
when I compile the trivial program main () {} with gcc -O0 I get the  
following lines

        pushl   %ebp
        movl    %esp, %ebp
        subl    $8, %esp

which are very clear. But also the following lines:

        andl    $-16, %esp
        movl    $0, %eax
        addl    $15, %eax
        addl    $15, %eax
        shrl    $4, %eax
        sall    $4, %eax
        subl    %eax, %esp

of which I understand the semantics individually, but I don't see  
what are needed for. Why put 0 into %eax and then add 15 and again  
add 15?
Could you tell me what is going on? Or point me to documentation or  
to a knowledgeable person?

Thanks in advance!!

Yannis

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