Date: Fri, 05 Mar 2004 23:01:09 +0100 From: ISAAC GELADO FERNANDEZ <igf@tid.es> To: Chungwei Hsiung <skuma17@yahoo.com> Cc: freebsd-hackers@freebsd.org Subject: Re: A simple question Message-ID: <2c8b6a2ce3cf.2ce3cf2c8b6a@tid.es>
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> ISAAC GELADO FERNANDEZ wrote: > > >De: Chungwei Hsiung <skuma17@yahoo.com> > >Fecha: Viernes, Marzo 5, 2004 7:43 pm > > > > > >>I have a simple test program. I compile it, > >>and > >>gdb to disassemble main. I got the following.. > >> > >>0x80481f8 <main>: push %ebp > >>0x80481f9 <main+1>: mov %esp,%ebp > >>0x80481fb <main+3>: sub $0x8,%esp > >>0x80481fe <main+6>: and $0xfffffff0,%esp > >>0x8048201 <main+9>: mov $0x0,%eax > >>0x8048206 <main+14>: sub %eax,%esp > >> > >>I don't know if at line 5, we move zero to %eax. why do we need > to sub %eax, %esp? why do we need to subtract 0 from the stack > pointer??>> > >> > > > > I am no really sure, but it maybe be because you don't have any > local variable, so it is no necessary to allocate memory in the > stack for them. This seems a pattern from the compiler, it subtract > the size of local variables from the stack pointer, so when there > is none it subtracts zero. But this is just a supposition > > > > > > > thank you for the reply.. > I try it again to call a function with local variables, but it does > that > as well. I think that line is not a part of a function prologue > because > when I try it with or without local variables, it is always there. > any > ideas?? > btw.. the compiler I use is gcc 3.2.2 I have performed some tests and the "strange" code apperars always. At first I thought the compiler puts the code to align the stack, but if you write a function it doesn't appear. Maybe the compiler puts the subl to set system flags to a known value, but I don't really know. I will try to obtain a good answer, I am really intrigate ;) Regards, Isaac > > best regards > Chungwei >
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