Re: Memory Allocation On Stack

[Rask Ingemann Lambertsen <rask at sygehus dot dk>]

On Sun, Jul 22, 2007 at 12:32:36AM +0530, Pankaj Kohli wrote:
> What about this one ? Three PUSHes + sub 0x10,%esp. That makes 28 bytes.
> Stack is not aligned on 16-byte boundary in this case.
 
   There is no reason to 16-byte align the stack in this case.
 
> (gdb) list 8
> 3       int main(int argc, char **argv) {
> 4               int a, b, c;
> 5
> 6               a = 1;
> 7               b = 2;
> 8               c = a + b;
> 9
> 10              return 0;
> 11      }

   Compare your example with this significantly modified one:

extern void g (void);

int main (int argc, char **argv) {
	int a, b, c;

	a = 1;
	b = 2;
	c = a + b;
	g ();

	return 0;
}

main:
	leal	4(%esp), %ecx	# 36	*lea_1	[length = 4]
	andl	$-16, %esp	# 37	*andsi_1/1	[length = 3]
	pushl	-4(%ecx)	# 38	*pushsi2	[length = 3]
	pushl	%ebp		# 39	*pushsi2	[length = 1]
	movl	%esp, %ebp	# 40	*movsi_1/1	[length = 2]
	pushl	%ecx		# 41	*pushsi2	[length = 1]
	subl	$20, %esp	# 42	pro_epilogue_adjust_stack_1/1	[length = 3]
	movl	$1, -16(%ebp)	# 8	*movsi_1/2	[length = 7]
	movl	$2, -12(%ebp)	# 10	*movsi_1/2	[length = 7]
	movl	-12(%ebp), %edx	# 12	*movsi_1/1	[length = 3]
	movl	-16(%ebp), %eax	# 13	*movsi_1/1	[length = 3]
	addl	%edx, %eax	# 14	*addsi_1/1	[length = 2]
	movl	%eax, -8(%ebp)	# 15	*movsi_1/2	[length = 3]
	call	g		# 17	*call_0	[length = 5]
	movl	$0, %eax	# 19	*movsi_1/1	[length = 5]
	addl	$20, %esp	# 45	pro_epilogue_adjust_stack_1/1	[length = 3]
	popl	%ecx		# 46	popsi1	[length = 1]
	popl	%ebp		# 47	popsi1	[length = 1]
	leal	-4(%ecx), %esp	# 48	*addsi_1/3	[length = 3]
	ret			# 49	return_internal	[length = 1]

> (gdb) disassemble main

   It is probably a lot easier to compile with -S -dp.

-- 
Rask Ingemann Lambertsen