[Bug rtl-optimization/26847] New: Missed optimization in simplify_plus_minus - regression against 4.1.0

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

copied from:
http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00812.html

Hi Paolo,

with your patch:
http://gcc.gnu.org/ml/gcc-patches/2005-11/msg01718.html

simplify_plus_minus isn't able anymore to simplify things like (r1 + 8) - r1. 
The function is left at:

  if (!canonicalized)
    {
      int n_constants = 0;

      for (i = 0; i < n_ops; i++)
        if (GET_CODE (ops[i].op) == CONST_INT)
          n_constants++;

      if (n_constants <= 1)
        return NULL_RTX;
    }

because only a single constant was found and no canonicalization took place.

Your patch changed:

if (n_ops <= 2 && !force)
  return NULL_RTX;

to:

gcc_assert (n_ops >= 2);
if (!canonicalized)
  return NULL_RTX;

Which was later on changed by Alan Modra with:
http://gcc.gnu.org/ml/gcc-patches/2005-12/msg00015.html
to the version mentioned above. The reason here was that expressions
like reg + c1 + c2 were not simplified anymore.

So the new version exits always if the initial loop couldn't do any
canonicalization work and only 0 or 1 constants are left. What prevents
simplification in all those cases.
...

As Roger Sayle pointed out the patch I've proposed in the email isn't the
proper fix for this issue.

Paolo Bonzini already proposed a solution in:
http://gcc.gnu.org/ml/gcc-patches/2006-02/msg00884.html

Thats why I'll assign the bug to him.


-- 
           Summary: Missed optimization in simplify_plus_minus - regression
                    against 4.1.0
           Product: gcc
           Version: 4.2.0
            Status: UNCONFIRMED
          Keywords: missed-optimization
          Severity: normal
          Priority: P3
         Component: rtl-optimization
        AssignedTo: bonzini at gnu dot org
        ReportedBy: krebbel at gcc dot gnu dot org
 GCC build triplet: s390x-ibm-linux-gnu
  GCC host triplet: s390x-ibm-linux-gnu
GCC target triplet: s390x-ibm-linux-gnu


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=26847