[Bug tree-optimization/82853] New: Optimize x % 3 == 0 without modulo

[andi-gcc at firstfloor dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=82853

            Bug ID: 82853
           Summary: Optimize x % 3 == 0 without modulo
           Product: gcc
           Version: 8.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: andi-gcc at firstfloor dot org
  Target Milestone: ---

Ralph Levien pointed out as part of FizzBuzz optimization:

Turns out you can compute x%3 == 0 with even fewer steps, it's (x*0xaaaaaaaab)
< 0x55555556 (assuming wrapping unsigned 32 bit arithmetic).

gcc currently generates the full modulo and then checks.

Could be done in match.pd I suppose.

Test case

unsigned mod3(unsigned a) { return 0==(a%3); }