Large offset memory accesses in stackalloc

[sarranz]

If the stack is big, Stack Allocation will introduce memory accesses with big offsets that are not valid in ARM. For instance,

// Stack allocation needs to split the array set and get operations into several
// instructions to that the immediates fit in one instruction.

// The LDR/STR instructions take an immediate of up to 12 bits.
// Since elements are 4 bytes, we the size limit for an array is 2^12 / 4.
param int LIMIT = 1024;

export
fn large_array() -> reg u32 {
    reg u32 r;
    stack u32[LIMIT + 1] a;

    r = 0;
    a[LIMIT] = r;
    r = a[LIMIT];

    return r;
}

Also, this generates invalid assembly

export
fn main(reg u32 p) -> reg u32 {
    reg u32 x;
    x = #LDR([p + 4097]);
    return x;
}

I will add a new extra_op that gets compiled to several instructions to load the offset and perform the load.