[Bug tree-optimization/50955] [4.7 Regression] IVopts incorrectly rewrite the address of a global memory access into a local form.

[rguenth at gcc dot gnu.org]

http://gcc.gnu.org/bugzilla/show_bug.cgi?id=50955

--- Comment #4 from Richard Guenther <rguenth at gcc dot gnu.org> 2011-11-02 
13:09:40 UTC ---
It looks like IVOPTs fails to consider a candidate for the use inquestion
and thus, after choosing the final IV set ends up rewriting that use into
this stupid form (instead of using candidate 0).  The use is

use 3
  generic
  in statement vect_p.60_223 = PHI <vect_p.60_224(16), vect_p.63_222(5)>

  at position
  type vector(8) unsigned char *
  base batmp.61_221 + 1
  step 8
  base object (void *) batmp.61_221
  is a biv
  related candidates

but costs say:

Use 3:
  cand  cost    compl.  depends on
  0     8       1        inv_expr:0
  1     4       1        inv_expr:1
  2     4       1        inv_expr:0
  3     4       1        inv_expr:1
  4     4       1        inv_expr:2

Initial set of candidates:
  cost: 24 (complexity 4)
  cand_cost: 10
  cand_use_cost: 10 (complexity 4)
  candidates: 0, 4
   use:0 --> iv_cand:4, cost=(2,1)
   use:1 --> iv_cand:4, cost=(2,1)
   use:2 --> iv_cand:4, cost=(2,1)
   use:3 --> iv_cand:4, cost=(4,1)
   use:4 --> iv_cand:0, cost=(0,0)
  invariants 7

Selected IV set:
candidate 0 (important)
  var_before ivtmp.107_150
  var_after ivtmp.107_256
  incremented before exit test
  type unsigned int
  base 0
  step 1
candidate 4 (important)
  var_before ivtmp.110_241
  var_after ivtmp.110_146
  incremented before exit test
  type unsigned int
  base (unsigned int) (&p1 + 8)
  step 8
  base object (void *) &p1

so expressing use 3 with candidate 4 is cheaper than with candidate 0...

Now, for address-uses we have

  if (address_p)
    {
      /* Do not try to express address of an object with computation based
         on address of a different object.  This may cause problems in rtl
         level alias analysis (that does not expect this to be happening,
         as this is illegal in C), and would be unlikely to be useful
         anyway.  */
      if (use->iv->base_object
          && cand->iv->base_object
          && !operand_equal_p (use->iv->base_object, cand->iv->base_object, 0))
        return infinite_cost;

in cost calculation, but in this case it's a nonlinear use, and we have

(gdb) call debug_generic_expr (use->iv->base_object)
(void *) batmp.61_221
(gdb) call debug_generic_expr (cand->iv->base_object)
(void *) &p1