[Bug tree-optimization/109746] New: Fails removing redundant comparison in for loop over multiple variables, unless members of struct

[magnus.hegdahl at gmail dot com via Gcc-bugs]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=109746

            Bug ID: 109746
           Summary: Fails removing redundant comparison in for loop over
                    multiple variables, unless members of struct
           Product: gcc
           Version: 13.1.1
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: magnus.hegdahl at gmail dot com
  Target Milestone: ---

Compiled with g++ (GCC) 13.1.1 20230429 on x86-64 Linux, with -O2 or higher.

In the code below, the comparisons with i are redundant because j is increasing
much faster.
GCC manages to optimize this away when i and j are members of some struct, but
not when they are just integers.

struct S {
  unsigned x;
};

unsigned f() {
  unsigned N = 1e9, x = 0;
  for (unsigned i = 3, j = 1; i < N && j < N; i += 2, j += 3)
    x ^= i * j;
  return x;
}

unsigned g() {
  unsigned N = 1e9, x = 0;
  for (S i {3}, j {1}; i.x < N && j.x < N; i.x += 2, j.x += 3)
    x ^= i.x * j.x;
  return x;
}

int main() {
  return f();
}