[Bug tree-optimization/40744] New: SRA scalarizes dead objects, single-use objects

["rguenth at gcc dot gnu dot org" <gcc-bugzilla at gcc dot gnu dot org>]

struct X { int i; int j; };
void foo(void)
{
  struct X x;
  x.i = 1;
  x.j = 2;
}

early SRA produces

foo ()
{
  int x$j;
  int x$i;
  struct X x;

<bb 2>:
  x$i_3 = 1;
  x$j_2 = 2;
  return;


which is unnecessary work as DCE will end up removing the stores anyway.
We should avoid doing useless work here (and thus not skew statistics
compared to when somebody inserts DCE before ESRA).  At least if it is
easy to do.

Likewise for

struct X { int i; int j; };
int foo(struct X x)
{
  return x.i;
}

early SRA produces an extra register copy with no benefit.

foo (struct X x)
{
  int x$i;
  int D.1606;

<bb 2>:
  x$i_3 = x.i;
  D.1606_1 = x$i_3;
  return D.1606_1;

}

both cases, only loads from a structure or only stores to a structure
probably should be simply skipped.


-- 
           Summary: SRA scalarizes dead objects, single-use objects
           Product: gcc
           Version: 4.5.0
            Status: UNCONFIRMED
          Keywords: compile-time-hog
          Severity: normal
          Priority: P3
         Component: tree-optimization
        AssignedTo: unassigned at gcc dot gnu dot org
        ReportedBy: rguenth at gcc dot gnu dot org


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=40744