> 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?
The best thing I can tell you is to ask on the gcc newsgroup or
comp.compilers.
However, remember that when you compile with -O0 you are getting no
optimizations whatsoever. What is likely going on is that it is
calculating some sort of stack offset on piece at a time. When you
compile with -O3 you get something much more sensible:
pushl %eax
pushl %eax
andl $-16, %esp
subl $16, %esp
While still not really needed for an empty function, it makes sense
nonetheless. It aligns the stack on a 16-byte boundary. Which, in fact,
the computation for %eax above results in 16.
If you compile with -O0 -mpreferred-stack-boundary=2 you will get the
results you expect.
Jon
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