fwprop patch testing

[Paolo Bonzini]

The only big regression for fwprop on PPC is bzip2.  I've distilled it 
to this small testcase:

    int f(int *);
    int verbosity;
    int *arr;
    int last;

    void g ()
    {
       int i;
       if (last < 4000) {
          if (verbosity >= 4) f(&verbosity);
          for (i = 0; i <= last; i++) arr[i] = i;
       }
    }

where CSE previously was performing load PRE.  So, actually this 
regression is caused by -fno-cse-skip-blocks rather than deleting parts 
of fwprop.  Note that this can be performed on the GIMPLE-level only to 
some extent.  What we are looking for is the following pseudo-C:

    void g ()
    {
       int i;
       int *plast = &last;
       int mylast = *plast;
       if (mylast < 4000) {
         if (verbosity >= 4) f(&verbosity);
         i = 0;
         mylast = *plast;
         while (i <= mylast)
           {
              arr[i] = i;
              /* Note the unfortunate aliasing between arr and plast */
              mylast = *plast;
           }
       }
    }

And tree-based load PRE can only get to

    void g ()
    {
       int i;
       int *plast = &last;
       int mylast = *plast;
       if (mylast < 4000) {
         if (verbosity >= 4) f(&verbosity);
         i = 0;
         plast = &last;      /* redundant */
         mylast = *plast;
         while (i <= mylast)
           {
              arr[i] = i;
              plast = &last;    /* redundant, and very expensive! */
              mylast = *plast;
           }
       }
}

since the "plast" lowering is obviously happening during expand.

I have not yet looked at whether it is possible to teach fwprop about 
this case, but I am not sure it is possible (I'm more inclined to think 
that it is simply not fwprop's job).  Right now, I am thinking more 
about shuffling the pass order.  One possibility that comes to mind is 
GCSE+fwprop+CSE, where GCSE could work out the common code for loading 
the address, and fwprop/CSE could do the addressing mode selection properly.

On one hand, this may mean increasing the number of GCSE passes from 1 
to 2.  On the other hand, it may mean that the CSE pass at the hand of 
rest_of_handle_gcse could become redundant.

Paolo