Variable length arrays in C, how are they compiled

I have some given code that uses variable length arrays on the stack in C. I cannot easily change that code to use malloced buffers on the heap, since I am working on some stuff where I have no operating system support for dynamic memory. However when I test my method the stack actually get's smashed instead (i.e. the SP get's set to a completely bogus address). To figure out what is going on I had a look at the assembly and I was completely confused by the compiler output.

I am running the code on a pandaboard ES (OMAP4460 ARM processor).

In order to test how variable size arrays on the stack are actually compiled, I produced a simple working test program. However again the code was much too confusing for me to understand.

Here is what I do not understand:

When I compile a simple function:

int test_method(size_t i)
{
   unsigned char buffer[10];
   return -1;
}

I get some very simple assembler code:

80003a68 <test_method>:
80003a68:       b480            push    {r7}        ; store frame pointer
80003a6a:       b087            sub     sp, #28     ; make stack frame (size 28)
80003a6c:       af00            add     r7, sp, #0  ; set frame pointer
80003a6e:       6078            str     r0, [r7, #4]; store parameter on stack
80003a70:       f04f 0300       mov.w   r3, #0      ; load return value
80003a74:       4618            mov     r0, r3      ; put return value in return register
80003a76:       f107 071c       add.w   r7, r7, #28 ; destroy stack frame
80003a7a:       46bd            mov     sp, r7      ; ...
80003a7c:       bc80            pop     {r7}        ; pop stored frame pointer
80003a7e:       4770            bx      lr          ; return

But when I try this with a variable size array using the following code:

int test_method(size_t i)
{
    unsigned char buffer[i];
    return 0;
}

I get t