how to modify gcc to compile a[b] differently

[Gry Gunvor]

I want to modify gcc 4.9.2 so that array subscripting expressions a[b]
generate a new instruction/syscall foo(a, b) (that is, taking a and b
as arguments) rather than just being turned into *(a+b).

Further, I want accesses into multi-dimensional arrays a[b][c] to turn
into foo(a, (b * row_size + c)) rather than *(a + b * row_size + c).

Any help finding a simple and robust way to do this would be greatly
appreciated.

Gry