[Bug tree-optimization/32226] Missed optimization caused by copy loop header (yes a weird case)

[Bug tree-optimization/32226] Missed optimization caused by copy loop header (yes a weird case)

[pinskia at gcc dot gnu.org]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=32226

--- Comment #3 from Andrew Pinski <pinskia at gcc dot gnu.org> ---
To do this optimization (the reduced testcase works right now), you have to
simulate each statement until the end with "width_5 == 0" (the opposite range
of the initial condition) to see if get the other phi operand.

  if (width_5(D) != 0)
    goto <bb 3>; [89.00%]
  else
    goto <bb 4>; [11.00%]

  <bb 3> [local count: 105119325]:
  _1 = (long unsigned int) dir_8(D);
  _3 = width_5(D) + 4294967295;
  _14 = (sizetype) _3;
  _6 = _14 + 1;
  _17 = _1 * _6;
  _18 = _17 * 2;
  errorptr_4 = errorptr_7(D) + _18;

  <bb 4> [local count: 118111601]:
  # errorptr_16 = PHI <errorptr_4(3), errorptr_7(D)(2)>


I don't know if this optimization is that important, even clang does not do it.
It should most likely be only done if the branch is highly predicted taken down
the route of the longer path.

[Bug tree-optimization/32226] Missed optimization caused by copy loop header (yes a weird case)

[pinskia at gcc dot gnu dot org]

------- Comment #2 from pinskia at gcc dot gnu dot org  2009-04-22 21:32 -------
Actually this comes down to:
int f(int a, int c)
{
  if (a) return a*c;
  return 0;
}

It might be faster to use the branch rather than the multiple, I don't know and
I think it depends on the target.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32226

[Bug tree-optimization/32226] Missed optimization caused by copy loop header (yes a weird case)

[rguenth at gcc dot gnu dot org]

------- Comment #1 from rguenth at gcc dot gnu dot org  2007-06-05 22:01 -------
Confirmed.


-- 

rguenth at gcc dot gnu dot org changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|0                           |1
   Last reconfirmed|0000-00-00 00:00:00         |2007-06-05 22:01:22
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=32226