[Bug target/106415] loop-ivopts prevents correct usage of dbra with 16-bit loop counters on m68k

["linkw at gcc dot gnu.org" <gcc-bugzilla@gcc.gnu.org>]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=106415

Kewen Lin <linkw at gcc dot gnu.org> changed:

           What    |Removed                     |Added
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                 CC|                            |linkw at gcc dot gnu.org

--- Comment #5 from Kewen Lin <linkw at gcc dot gnu.org> ---
At the top of tree-ssa-loop-ivopts.cc file, there are some useful comments
describing the costs for iv candidate itself and group, it may help.

<Candidate Costs>:
  cand  cost
  0     5
  1     5
  2     5
  3     4
  4     5
  5     5
  6     5
  7     5
  8     5

This part is for "variable costs".

<Group-candidate Costs>:
Group 0:
  cand  cost    compl.  inv.expr.       inv.vars
  0     400     0       NIL;    NIL;
  7     0       0       NIL;    NIL;
  8     400     0       NIL;    NIL;

This part is for "group/use costs".

There would be no dumping for a candidate when it has infinite cost for the
group, so the above means only candidate 0, 7 and 8 are valid to be used for
group 0.

Final cost looks like:

    cost: 7 (complexity 0)
    reg_cost: 2
    cand_cost: 5
    cand_group_cost: 0 (complexity 0)
    candidates: 7
     group:0 --> iv_cand:7, cost=(0,0)
     group:1 --> iv_cand:7, cost=(0,0)
    invariant variables:
    invariant expressions:

----

By quick checking, it looks the inf cost for the pair (cand 3, group 0) is due
to:

  /* Check if we have enough precision to express the values of use.  */
  if (TYPE_PRECISION (utype) > TYPE_PRECISION (ctype))
    return infinite_cost;