[Bug tree-optimization/108757] New: We do not simplify (a - (N*M)) / N + M -> a / N

[bergner at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=108757

            Bug ID: 108757
           Summary: We do not simplify (a - (N*M)) / N + M -> a / N
           Product: gcc
           Version: 13.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: bergner at gcc dot gnu.org
  Target Milestone: ---

The Eigen project code has a missed optimization that basically simplifies down
to the following test case:

linux$ cat bug.c 
#define N 32
#define M 2

unsigned long int
foo (unsigned long int a)
{
  return (a - (N*M)) / N + M;
}

linux$ gcc -O2 -S bug.c 

linux$ cat bug.s 
foo:
        addi 3,3,-64
        srdi 3,3,5
        addi 3,3,2
        blr

We should be able to simplify this down to just 'a / N', which for power-of-2
N, results in just the srdi...although, I don't think N is required to be a
power-of-2 to fold this down to just 'a / N'.  Can we also simplify this for
non-constant N & M?  Maybe under -fast-math or something similar?