Purpose of GCC Stack Padding?

["Andrew Tomazos" <andrew at tomazos dot com>]

I've been studying the x86 compiled form of the following function:

void function()
{
        char buffer[X];
}

where X = 0, 1, 2 .. 100

Naively, I would expect to see:

       pushl   %ebp
       movl    %esp, %ebp
       subl    $X, %esp
       leave
       ret

Instead, the stack appears to be padded:

For a buffer size of  0        the stack size is   0
For a buffer size of  1 to   7 the stack size is  16
For a buffer size of  8 to  12 the stack size is  24
For a buffer size of 13 to  28 the stack size is  40
For a buffer size of 29 to  44 the stack size is  56
For a buffer size of 45 to  60 the stack size is  72
For a buffer size of 61 to  76 the stack size is  88
For a buffer size of 77 to  92 the stack size is 104
For a buffer size of 93 to 100 the stack size is 120

When X >= 8 gcc adds a stack corruption check (__stack_chk_fail),
which accounts for an extra 4 bytes of stack space in these cases.

This does not explain the rest of the padding.  Can anyone explain the
purpose of the rest of the padding?

Thanks,
Andrew.

PS There is a related thread on comp.lang.asm.x86.  It contains the
script I used to compile the function and the full output:

http://groups.google.com/group/comp.lang.asm.x86/browse_thread/thread/2c8c5a2c6050179b/

$ gcc -v
Using built-in specs.
Target: i486-linux-gnu
Configured with: ../src/configure -v
--enable-languages=c,c++,fortran,objc,obj-c++,treelang --prefix=/usr
--enable-shared --with-system-zlib --libexecdir=/usr/lib
--without-included-gettext --enable-threads=posix --enable-nls
--with-gxx-include-dir=/usr/include/c++/4.2 --program-suffix=-4.2
--enable-clocale=gnu --enable-libstdcxx-debug --enable-objc-gc
--enable-mpfr --enable-targets=all --enable-checking=release
--build=i486-linux-gnu --host=i486-linux-gnu --target=i486-linux-gnu
Thread model: posix
gcc version 4.2.4 (Ubuntu 4.2.4-1ubuntu3)

--
Andrew Tomazos <andrew@tomazos.com> <http://www.tomazos.com>