[Bug tree-optimization/23970] loop-invariant-motion is not doing it's work

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

------- Additional Comments From rakdver at gcc dot gnu dot org  2005-09-20 13:33 -------
k*stridev?[2] cannot be moved more at the moment.  The loops looks like

for (k = ...)
  for (j = ...)
    {
      if (i >= i1)
        continue;
      do
        {
          tmp = k * stridevx[2];
          ...
        }  while (i < i1)
    }

Note that if i >= i1, the load of stridevx[2] is never executed.  And it is
possible that it will trap.  Thus, we can only move it to point where we are
sure that we do not make it executed unconditionally.  I.e. it is legal to move
it out of the innermost loop, but not out of the outer ones.

There are two optimization improvements that could help (and as this case is
fairly common, I will leave this PR open as an enhancement request).

The first optimization is predicated invariant motion.  I.e. we cannot move
stridevx[2] out of the second loop, but we can move the expression (i < i1 ?
stridevx[2] : undefined) out of it, and it has the same value in all interesting
cases.

The second optimization is invariant exit motion. I.e. we can replace

do
  {
     code1;
     if (invariant)
       break;
     code2;
  } while (something);

with

if (invariant)
  {
    code1;
  }
else
  {
    do
      {
         code1;
         code2;
      } while (something);
  }

This could be achieved through loop unswitching (for non-innermost loops), or as
a separate optimization.  Note however that for this optimization to be useful
for invariant motion, it would either have to be iterated with it, or, at the
expense of significant complications, to be implemented within invariant motion.

-- 
           What    |Removed                     |Added
----------------------------------------------------------------------------
           Severity|normal                      |enhancement
             Status|UNCONFIRMED                 |NEW
     Ever Confirmed|                            |1
   Last reconfirmed|0000-00-00 00:00:00         |2005-09-20 13:33:54
               date|                            |


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=23970