[PATCH v3 3/3] PR80791 Consider doloop cmp use in ivopts

["Kewen.Lin" <linkw at linux dot ibm dot com>]

Hi all,

This is the following patch after https://gcc.gnu.org/ml/gcc-patches/2019-06/msg00910.html

Main steps:
  1) Identify the doloop cmp type iv use and record its bind_cand (explain it later).
  2) Set zero cost for pairs between this use and any iv cand.
  3) IV cand set selecting algorithm runs as usual.
  4) Fix up the selected iv cand for doloop use if need.

It only focuses on the targets like Power which has specific count register.
target hook have_count_reg_decr_p is proposed for it.

Some notes:

*) Why we need zero cost?  How about just decrease the cost for the pair
   between doloop use and its original iv cand?  How about just decrease
   the cost for the pair between doloop use and one selected iv cand?

   Since some target supports hardware count register for decrement and
   branch, it doesn't need the general instruction sequence for decr, cmp and
   branch in general registers.  The cost of moving count register to GPR
   is generally high, so it's standalone and can't be shared with other iv 
   uses.  It means IVOPTs can take doloop use as invisible (zero cost).

   Let's take a look at PR80791 for example.

                            original biv (cand 4)  use derived iv (cand 6)
     generic use:                   4                  0
     comp use (doloop use):         0                 infinite
    
    For iv cost, original biv has cost 4 while use derived iv has cost 5.
    When IVOPTs considers doloop use, the optimal cost is 8 (original biv 
    iv cost 4 + use cost 4).  Unfortunately it's not actually optimal, since
    later doloop transformation updates loop closing by count register,
    original biv (and its update) won't be needed in loop closing any more.
    The generic use become the only use for original biv.  That means, if we 
    know the doloop will perform later, we shouldn't consider the doloop use
    when determining IV set.  If we don't consider it, the algorithm will 
    choose iv cand 6 with total cost 5 (iv cost 5 + use cost 0).

    From the above, we can see that to decrease the cost for the pair between 
    doloop use and original biv doesn't work.  Meanwhile it's hard to predict
    one good iv cand in final optimal set here and pre-update the cost
    between it and doloop use.  The analysis would be heavy and imperfect.
   
*) Why we need bind_cand?

    As above, we assign zero cost for pairs between doloop use and each iv 
    cand.  It's possible that doloop use gets assigned one iv cand which is
    invalid to be used during later rewrite.  Then we have to fix it up with iv
    cand originally used for it.  It's fine whatever this iv cand exists in
    final iv cand set or not, even if it's not in the set, it will be 
    eliminated in doloop transformation.

By the way, I was thinking whether we can replace the hook have_count_reg_decr_p
with flag_branch_on_count_reg.  As the description of the "no-" option, "Disable
the optimization pass that scans for opportunities to use 'decrement and branch' 
instructions on a count register instead of instruction sequences that decrement 
a register, compare it against zero, and then branch based upon the result.", it
implicitly says it has count register support.  But I noticed that the gate of 
doloop_optimize checks this flag, as what I got from the previous discussions, some
targets which can perform doloop_optimize don't have specific count register, so it
sounds we can't make use of the flag, is it correct?

Bootstrapped on powerpcle, also ran regression testing on powerpcle, got one failure
which is exposed by this patch and the root cause is duplicate of PR62147.
case is gcc.target/powerpc/20050830-1.c

Is it OK for trunk?  

--------------

gcc/ChangeLog

2019-06-19  Kewen Lin  <linkw@gcc.gnu.org>

	PR middle-end/80791
	* target.def (have_count_reg_decr_p): New hook.
	* doc/tm.texi.in (TARGET_HAVE_COUNT_REG_DECR_P): New hook.
	* doc/tm.texi: Regenerate.
	* config/rs6000/rs6000.c (rs6000_have_count_reg_decr_p): New function.
	(TARGET_HAVE_COUNT_REG_DECR_P): New macro.
	* tree-ssa-loop-ivopts.c (adjust_group_iv_cost_for_doloop): New function.
	(fixup_doloop_groups): Likewise.
	(find_doloop_use_and_its_bind): Likewise.
	(record_group): Init bind_cand.
	(determine_group_iv_cost): Call adjust_group_iv_cost_for_doloop.
	(find_optimal_iv_set): Call fixup_doloop_groups.
	(tree_ssa_iv_optimize_loop): Call function have_count_reg_decr_p, 
	generic_predict_doloop_p and find_doloop_use_and_its_bind.
	(generic_predict_doloop_p): Update attribute.

gcc/testsuite/ChangeLog

2019-06-19  Kewen Lin  <linkw@gcc.gnu.org>

	PR middle-end/80791
	* gcc.dg/tree-ssa/ivopts-lt.c: Adjust.

Attachment: ivopts.diff
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