[Bug target/90952] New: Costs of moves are used for costs of RTL expressions

[hjl.tools at gmail dot com]

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90952

            Bug ID: 90952
           Summary: Costs of moves are used for costs of RTL expressions
           Product: gcc
           Version: 10.0
            Status: UNCONFIRMED
          Severity: normal
          Priority: P3
         Component: target
          Assignee: unassigned at gcc dot gnu.org
          Reporter: hjl.tools at gmail dot com
                CC: hubicka at ucw dot cz, skpgkp1 at gmail dot com, ubizjak at 
gmail dot com
  Target Milestone: ---
            Target: i386,x86-64

This patch:

https://gcc.gnu.org/ml/gcc-patches/2018-02/msg00405.html

includes:

diff --git a/gcc/config/i386/x86-tune-costs.h
b/gcc/config/i386/x86-tune-costs.h
index e943d13..8409a5f 100644
--- a/gcc/config/i386/x86-tune-costs.h
+++ b/gcc/config/i386/x86-tune-costs.h
@@ -1557,7 +1557,7 @@ struct processor_costs skylake_cost = {
   {4, 4, 4}, /* cost of loading integer registers
     in QImode, HImode and SImode.
     Relative to reg-reg move (2).  */
-  {6, 6, 6}, /* cost of storing integer registers */
+  {6, 6, 3}, /* cost of storing integer registers */
   2, /* cost of reg,reg fld/fst */
   {6, 6, 8}, /* cost of loading fp registers
     in SFmode, DFmode and XFmode */

It lowered the cost for SImode store and made it cheaper than SSE<->integer
register move.  It caused a regression:

https://gcc.gnu.org/bugzilla/show_bug.cgi?id=90878

Since the cost for SImode store is also used to compute costs of
scalar_store RTL expression in ix86_builtin_vectorization_cost, it
changed loop costs in

void
foo (long p2, long *diag, long d, long i)
{
  long k;
  k = p2 < 3 ? p2 + p2 : p2 + 3;
  while (i < k)
    diag[i++] = d;
}

As the result, the loop is unrolled 4 times with -O3 -march=skylake,
instead of 3.