https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82853
--- Comment #12 from Wilco <wilco at gcc dot gnu.org> --- (In reply to Marc Glisse from comment #11) > (In reply to Wilco from comment #9) > > It works for any C where (divisor*C) MOD 2^32 == 1 (or -1). > > For x%3==0, i.e. z==0 for x==3*y+z with 0<=y<55555556 and 0<=z<3. > Indeed, x*0xaaaaaaab==y+z*0xaaaaaaab is in the right range precisely for > z==0 and the same can be done for any odd number. You can make it work for even numbers as well, eg. (x % 10) == 0 -> (x % 5) == 0 && (x & 1) == 0. If you can avoid branches then it may still be faster. > > You can support any kind of comparison, it doesn't need to be with 0 (but > > zero is the easiest). > > Any ==cst will yield a range test. It is less obvious that inequalities are > transformed to a contiguous range... (try x%7<3 maybe) Indeed, expanding into x % 7 == 0 || x % 7 == 1 || x % 7 == 2 is unlikely to be a good idea. However you can change x % 7 >= 1 into x % 7 != 0 or x % 7 < 6 into x % 7 != 6 which are single ranges.
