[Bug tree-optimization/85726] New: div C1 to div C2 match.pd suboptimization

[hp at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=85726

            Bug ID: 85726
           Summary: div C1 to div C2 match.pd suboptimization
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hp at gcc dot gnu.org
  Target Milestone: ---

See the attached test-case, ready to drop into testsuite/gcc.dg.

The suboptimization results in separate div and mod sequences
across all architectures where division or modulus by a constant
is implemented as a widened multiplication and right-shift
(actually most; at least x86_64, mips, ppc, aarch64).  The
test-case doesn't fail for e.g. sparc, that doesn't do it that
way.

This is a somewhat long-standing suboptimality, but a regression.
The title hints at the cause and solution; patch to be posted.