A C compiler is not required to know what the alloca() call does, but it is allowed to do so. If your call to alloca() resolves to the one in your system's standard C library, the compiler is allowed to make use of any knowledge it may have about that call, how it works, and what its semantics are. It is then perfectly legal for it to make optimizations based on those semantics.

I know that NULL isn't necessarily 0x0. My example simply assumes that it is, which is usually the case. Note that even if NULL is not 0x0, and the address that is 0x0 is valid, it's still not legal to do * (int * )0 = 0, as (int *)0 is NULL, regardless of the actual underlying value of NULL.