[Bug middle-end/37221] Missed early loop-unroll optimization - causes 40% degradation on SPU

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

------- Comment #6 from rguenth at gcc dot gnu dot org  2008-09-02 09:04 -------
It looks like SCEV does not see that for

(number_of_iterations_in_loop
(analyze_scalar_evolution
  (loop_nb = 4)
  (scalar = j_3)
(get_scalar_evolution
  (scalar = j_3)
  (scalar_evolution = ))
(analyze_initial_condition
  (loop_phi_node =
j_3 = PHI <j_26(4), j_31(7)>
)
  (init_cond = j_26))
(analyze_evolution_in_loop
  (loop_phi_node = j_3 = PHI <j_26(4), j_31(7)>
)
(add_to_evolution
  (loop_nb = 4)
  (chrec_before = j_26)
  (to_add = 1)
  (res = {j_26, +, 1}_4))
  (evolution_function = {j_26, +, 1}_4))
(set_scalar_evolution
  (scalar = j_3)
  (scalar_evolution = {j_26, +, 1}_4))
)

j_26 is defined by

<bb 4>:
  # j_26 = PHI <0(3), 0(5), 0(12), 0(6)>

so it is all zeros.  This is because the "iterative" unrolling only does
a cfg cleanup between iterations and not a full constant-prop.

We can "fix" this special case in analyze_initial_condition, but a real
fix would be to run a (partial) CCP pass after each set of unrolling.


-- 


http://gcc.gnu.org/bugzilla/show_bug.cgi?id=37221