I think it's just the type of the accumulator[1] that influences this, GCC seems to vectorize it fine for any type other than signed char/signed short. [1] s i

I think it's just the type of the accumulator[1] that influences this, GCC seems to vectorize it fine for any type other than signed char/signed short. [1] s in your code, result in the author's Edit: The reason for the failure appears to be this: test.c:7:2: note: reduction: not commutative/associative: s_11 = (int8_t) _10; Edit: GCC vectorizes this fine when compiling with -fwrapv, which gives you the semantics the author probably expected.