S/390: Disable vectorization for loops with few iterations

The following patch defines the PARAM_MIN_VECT_LOOP_BOUND parameter in
the s390 backend.  It helps with the vectorization epilogue problem
described here [1].
I see an overall performance increase of > 1% in SPECfp2006, yet some
cases like cactusADM regress.  This seems to be caused by the vectorizer
creating an epilogue guard for one more iteration than before, which, in
turn, causes e.g. predcom to run on the epilogue that it used to ignore
before ("Loop iterates only 1 time, nothing to do.").  Subsequent,
minor, effects cause an eventual slowdown.

Until the reason for the bad epilogue code is understood, this patch
mitigates the problem.  When investigating the issue, I stumbled across
an attempt to vectorize the epilogue itself as well as combine it with
the vectorized loop in addition to vector masking [2].  A similar
approach might also help here.  My original observation of high register
pressure within the epilogue still stands.  In this specific case, it
would most likely suffice to save all registers once, run the epilogue
and restore the registers.  I'm pretty sure this would be faster than
the "spill fest" that's currently happening.

Regards
 Robin

[1] https://gcc.gnu.org/ml/gcc/2017-01/msg00234.html
[2] https://gcc.gnu.org/ml/gcc-patches/2016-05/msg01562.html

gcc/ChangeLog:

2017-03-06  Robin Dapp  <rdapp@linux.vnet.ibm.com>

	* config/s390/s390.c (s390_option_override_internal): Set
	PARAM_MIN_VECT_LOOP_BOUND

From-SVN: r245918

gcc/ChangeLog

 2017-03-06  Robin Dapp  <rdapp@linux.vnet.ibm.com>
 
+	* config/s390/s390.c (s390_option_override_internal): Set
+	PARAM_MIN_VECT_LOOP_BOUND
+
+2017-03-06  Robin Dapp  <rdapp@linux.vnet.ibm.com>
+
 	* config/s390/s390.c (s390_asm_output_function_label): Use nopr %r0.
 	* config/s390/s390.md: Likewise.
 

gcc/config/s390/s390.c

@@ -14679,6 +14679,10 @@ s390_option_override_internal (bool main_args_p,
                          opts->x_param_values,
                          opts_set->x_param_values);
 
+  maybe_set_param_value (PARAM_MIN_VECT_LOOP_BOUND, 2,
+			 opts->x_param_values,
+			 opts_set->x_param_values);
+
   /* Call target specific restore function to do post-init work.  At the moment,
      this just sets opts->x_s390_cost_pointer.  */
   s390_function_specific_restore (opts, NULL);