[Bug tree-optimization/42108] [4.4/4.5 Regression] 50% performance regression

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

------- Comment #34 from rguenth at gcc dot gnu dot org  2009-12-14 12:57 -------
"Another possibility is to artificially grow SCCs and
their dependencies by honoring dominating virtual operand uses, not only
defs (ugh)."

what I mean with this is that when finding SCCs we process all uses of a
stmt.  In case you have a memory load

  # VUSE <.MEM_3>
  x_2 = *p_1(D);

then you have to consider all dominating DEFs that use .MEM_3 uses of
that stmt, not only the single def of .MEM_3 (well, you only have to
consider aliasing uses of course).  This isn't easyly retrofitted into
the non-recursive DFS walk though.

Thus, for

  # .MEM_3 = VDEF <.MEM_5(D)>
  *p_1(D) = 0;
  # VUSE <.MEM_3>
  y_4 = *p_1(D);
  # VUSE <.MEM_3>
  x_2 = *p_1(D);

when visiting x_2 = *p_1(D); the uses are p_1, y_4 and .MEM_3.  y_4
is new because its the DEF in the dominating stmt that uses .MEM_3.

This would possibly increase SCC sizes and compile-time a lot.


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http://gcc.gnu.org/bugzilla/show_bug.cgi?id=42108