[Bug middle-end/87203] New: Optimize x % constant ==/!= 0

[jakub at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=87203

            Bug ID: 87203
           Summary: Optimize x % constant ==/!= 0
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: middle-end
          Assignee: unassigned at gcc dot gnu.org
          Reporter: jakub at gcc dot gnu.org
  Target Milestone: ---

As mentioned in
http://duriansoftware.com/joe/Optimizing-is-multiple-checks-with-modular-arithmetic.html
, we could optimize unsigned
x % const == 0
or
x % const != 0
into a single (non-highpart) multiplication plus comparison (at least for odd
const, for even const we can rotate before comparison if target has cheap
rotates, or could test low bits and compare).
Obviously, for power of two const we already emit the best code.