On 2018-01-19 19:42, Linus Torvalds wrote: > > I actually asked (long long ago) for an optinal compiler warning for > "pointer subtraction with non-power-of-2 sizes". Not because of it > being undefined, but simply because it's expensive. The > divide->multiply thing doesn't always work,
Huh? If (compile-time constant, positive) d=m*2^k with m odd, and x is known to be a multiple of d, x/d can always be computed as (x>>k)*m' == (x*m')>>k, with m' being the mod 2^N multiplicative inverse of m, right? This works whether x has signed or unsigned type and whether it indeed happens to be negative, AFAICT. Sure, the multiplication by m' may not exactly be cheap, but one only needs the low N bits of the result. Rasmus