In or out of the spec doesn't really matter. You can use __restrict__ in C++ and any compiler will diligently accept it, if not for the occasional warning. The

In or out of the spec doesn't really matter. You can use __restrict__ in C++ and any compiler will diligently accept it, if not for the occasional warning. The larger issue is that C and C++ have convoluted aliasing rules which makes reasoning and flagging non-aliased regions hard. The most innocent of cast can break strict aliasing rules and send you in horrible debugging scenarios. The sad side effect being people adding -fno-strict-aliasing to -O2/3.