Question on unexpected asm output

[Tim Oertel <toertel at rpa dot net>]

Hi,

I'm examining the output from a compile and don't understand
why the assembly is being generated the way it is.  A simple
example:

extern int other_foobar0( void );
extern "C" void
foobar0(void)
{
   (void) other_foobar0();
}

Generates an object file, which disassembles to:
00000000 <foobar0>:
   0: 83 ec 0c                sub    $0xc,%esp
   3: e8 fc ff ff ff          call   4 <foobar0+0x4>
   8: 83 c4 0c                add    $0xc,%esp
   b: c3                      ret    

My question is why is there the extra sub and add?  I'd
expect them if I had any locals, but I don't, and the
extra stack allocated isn't even used.

I've found similar cases with more complex code where there
are unusual adds and subtracts sprinkled throughout a function.
(Including a strange 'add $0xfffffffc,%esp', rather than a sub.)

This looks strange, but the stack adjustments are all consistent,
so nothing is showing up buggy. I want to understand what is
going on, and if possible, can I turn it off.

rand# g++ -v
Reading specs from /usr/local/lib/gcc-lib/i686-pc-linux-gnu/2.95.2/specs
gcc version 2.95.2 19991024 (release)

rand# make foo.S
g++ -mpentium -O6 -fomit-frame-pointer -c foo.c -o foo.o
objdump -CD foo.o > foo.S

Please CC: me.

Thanks for any help.


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