Why auto variables NOT overlap on stack?

[Ding Curie]

(I asked this question on the gcc-help mailing list, but no answer
yet. It seems no one there knows or is interested in the internals of
gcc. So, please forgive me for cross-posting the question here again.
)

Hello, everyone!

I'm with the latest MinGW, and following program:

-----------------------------------------------------------------------
char *p1, *p2;

int main(void)
{
   {
       char a[1024];
       p1 = &a[0];
   }

   {
       char a[1024];
       p2 = &a[0];
   }

   printf("p1 - p2 = %d\n", p1 - p2);

   return 0;
}
-----------------------------------------------------------------------

output:
> p1 - p2 = 1024

(Optimize or not makes no difference.)

That means the two arrays don't overlap on the stack. But since that
will save a significant amount of stack space, I wonder, why gcc don't
manage to make the two arrays share one same memory block of 1K bytes
on the stack?

BTW, I checked two embedded C compilers: CCS from TI and VisualDSP
from ADI, do the trick, outputing p1 - p2 = 0.

Thanks in advance.

Ding Curie