[Bug tree-optimization/88854] New: redundant store after load that would makes aliasing UB

[kretz at kde dot org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=88854

            Bug ID: 88854
           Summary: redundant store after load that would makes aliasing
                    UB
           Product: gcc
           Version: 9.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: kretz at kde dot org
                CC: rguenth at gcc dot gnu.org
  Target Milestone: ---

Test cases:

This is optimized at -O1 and with GCC 5 at -O2. -fdisable-tree-fre1 and
-fno-strict-aliasing also remove the store to a.

void f(int *a, float *b) {
    int x = *a;
    *b = 0;
    x = *a;
    *a = x;
}

The following is an extension that reloads *a after store to b into a different
variable. Still the store to a must be dead, since otherwise the read of a
would be UB.

int g(int *a, float *b) {
    int x = *a;
    *b = 0;
    int r = *a;
    *a = x;
    return r;
}