[Bug tree-optimization/14295] [tree-ssa] copy propagation for aggregates

[rguenth at gcc dot gnu dot org]

------- Comment #8 from rguenth at gcc dot gnu dot org  2008-06-22 15:05 -------
If we would disallow struct copies in the gimple IL and instead require a
register temporary that we would re-write into SSA form like

  struct s temp_struct3;
  struct s temp_struct2;
  struct s temp_struct1;
  struct s temp_struct3.3;
  struct s temp_struct2.2;
  struct s temp_struct1.1;
  struct s r.0;

<bb 2>:
  r.0_1 = r;
  temp_struct1 ={v} r.0_1;
  temp_struct1.1_2 = temp_struct1;
  temp_struct2 ={v} temp_struct1.1_2;
  temp_struct2.2_3 = temp_struct2;
  temp_struct3 ={v} temp_struct2.2_3;
  temp_struct3.3_4 = temp_struct3;
  <retval> ={v} temp_struct3.3_4;
  return <retval>;

then value-numbering can recognize the redundant copies and we end up
with

  struct s temp_struct3.3;
  struct s r.0;

<bb 2>:
  r.0_1 = r;
  <retval> = r.0_1;
  return <retval>;

(and of course with the possibility of out-of-SSA having to deal with
overlapping life-ranges of struct-typed SSA names)


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=14295