[gcc r14-3999] [RA]: Improve cost calculation of pseudos with equivalences

[gcc r14-3999] [RA]: Improve cost calculation of pseudos with equivalences

[Vladimir Makarov]

https://gcc.gnu.org/g:3c834d85f2ec42c60995c2b678196a06cb744959

commit r14-3999-g3c834d85f2ec42c60995c2b678196a06cb744959
Author: Vladimir N. Makarov <vmakarov@redhat.com>
Date:   Thu Sep 14 10:26:48 2023 -0400

    [RA]: Improve cost calculation of pseudos with equivalences
    
    RISCV target developers reported that RA can spill pseudo used in a
    loop although there are enough registers to assign.  It happens when
    the pseudo has an equivalence outside the loop and the equivalence is
    not merged into insns using the pseudo.  IRA sets up that memory cost
    to zero when the pseudo has an equivalence and it means that the
    pseudo will be probably spilled.  This approach worked well for i686
    (different approaches were benchmarked long time ago on spec2k).
    Although common sense says that the code is wrong and this was
    confirmed by RISCV developers.
    
    I've tried the following patch on I7-9700k and it improved spec17 fp
    by 1.5% (21.1 vs 20.8) although spec17 int is a bit worse by 0.45%
    (8.54 vs 8.58).  The average generated code size is practically the
    same (0.001% difference).
    
    In the future we probably need to try more sophisticated cost
    calculation which should take into account that the equiv can not be
    combined in usage insns and the costs of reloads because of this.
    
    gcc/ChangeLog:
    
            * ira-costs.cc (find_costs_and_classes): Decrease memory cost
            by equiv savings.

Diff:
---
 gcc/ira-costs.cc | 11 ++---------
 1 file changed, 2 insertions(+), 9 deletions(-)

diff --git a/gcc/ira-costs.cc b/gcc/ira-costs.cc
index d9e700e8947..8c93ace5094 100644
--- a/gcc/ira-costs.cc
+++ b/gcc/ira-costs.cc
@@ -1947,15 +1947,8 @@ find_costs_and_classes (FILE *dump_file)
 	    }
 	  if (i >= first_moveable_pseudo && i < last_moveable_pseudo)
 	    i_mem_cost = 0;
-	  else if (equiv_savings < 0)
-	    i_mem_cost = -equiv_savings;
-	  else if (equiv_savings > 0)
-	    {
-	      i_mem_cost = 0;
-	      for (k = cost_classes_ptr->num - 1; k >= 0; k--)
-		i_costs[k] += equiv_savings;
-	    }
-
+	  else
+	    i_mem_cost -= equiv_savings;
 	  best_cost = (1 << (HOST_BITS_PER_INT - 2)) - 1;
 	  best = ALL_REGS;
 	  alt_class = NO_REGS;