[Bug tree-optimization/70497] New: Missed CSE of subregs on GIMPLE

[rguenth at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=70497

            Bug ID: 70497
           Summary: Missed CSE of subregs on GIMPLE
           Product: gcc
           Version: 6.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: tree-optimization
          Assignee: unassigned at gcc dot gnu.org
          Reporter: rguenth at gcc dot gnu.org
  Target Milestone: ---

RTL DSE handles removing the redundand load (and thus 'u' itself) in

union U { int i[16]; char c; };

char foo(int i)
{
  union U u;
  u.i[0] = i;
  return u.c;
}

but nothing on GIMPLE does that.  FRE could do this, replacing
u.c with proper sub-setting of i (via a cast or a BIT_FIELD_REF).