Strange thing with static Array!

[Bob Plantz]

On Sat, 2009-02-28 at 12:06 -0500, me22 wrote:

> 
> GCC allows the Variable Length Arrays feature from C99 in C++ as an extension.
> 
> Consider compiling with -ansi -pedantic
> 
> ~ Scott

You can see what the compiler is doing for you if you look at the
assembly language. Here is the part where the array gets allocated on
the stack (with my comments added):
	call	_ZNSirsERi         # cin >> array_size
	movl	-12(%rbp), %eax    # load array_size
	cltq                       # convert long to quad
	subq	$1, %rax           # make sure the new stack
	addq	$1, %rax           #   pointer meets all the
	salq	$2, %rax           #   alignment specs.
	addq	$15, %rax
	addq	$15, %rax
	shrq	$4, %rax
	salq	$4, %rax
	subq	%rax, %rsp         # allocate the array
	movq	%rsp, -48(%rbp)    # and save pointer to it

I did this on an x86-64 system in 64-bit mode, and I did not worry
through the alignment code to see exactly what's going on. In
particular,
       subq $1, %rax
       addq $1, %rax
is pretty weird. But the real point is where the array gets allocated on
the stack.

- Bob