[PATCH, i386]: Improve cost function for STV transform of shifts with constant count operand

[Uros Bizjak]

Hello!

Attached patch improves cost function for STV transform of shifts with
constant count operand. The new cost function reduces the gain by
COST_N_INSN(1) for shifts larger than or equal to 32.

2016-12-11  Uros Bizjak  <ubiz...@gmail.com>

    PR target/70799
    * config/i386/i386.c (dimode_scalar_to_vector_candidate_p)
    <case ASHIFT, case LSHIFTRT>: Consider all constant shifts.
    Add FIXME comment.
    (dimode_scalar_chain::compute_convert_gain): Reduce gain for
    constant shifts larger or equal than 32.

testsuite/ChangeLog:

2016-12-11  Uros Bizjak  <ubiz...@gmail.com>

    PR target/70799
    * gcc.target/i386/pr70799-3.c: New test.

Bootstrapped and regression tested on x86_64-linux-gnu, committed to
mainline SVN.

Uros.

Index: config/i386/i386.c
===================================================================
--- config/i386/i386.c  (revision 243523)
+++ config/i386/i386.c  (working copy)
@@ -2809,10 +2809,9 @@ dimode_scalar_to_vector_candidate_p (rtx_insn *ins
     {
     case ASHIFT:
     case LSHIFTRT:
-      /* Consider only non-variable shifts narrower
-        than general register width.  */
-      if (!(CONST_INT_P (XEXP (src, 1))
-           && IN_RANGE (INTVAL (XEXP (src, 1)), 0, 31)))
+      /* FIXME: consider also variable shifts.  */
+      if (!CONST_INT_P (XEXP (src, 1))
+         || !IN_RANGE (INTVAL (XEXP (src, 1)), 0, 63))
        return false;
       break;
 
@@ -3409,6 +3408,9 @@ dimode_scalar_chain::compute_convert_gain ()
          gain += ix86_cost->add;
          if (CONST_INT_P (XEXP (src, 0)))
            gain -= vector_const_cost (XEXP (src, 0));
+         if (CONST_INT_P (XEXP (src, 1))
+             && INTVAL (XEXP (src, 1)) >= 32)
+           gain -= COSTS_N_INSNS (1);
        }
       else if (GET_CODE (src) == PLUS
               || GET_CODE (src) == MINUS
Index: testsuite/gcc.target/i386/pr70799-3.c
===================================================================
--- testsuite/gcc.target/i386/pr70799-3.c       (nonexistent)
+++ testsuite/gcc.target/i386/pr70799-3.c       (working copy)
@@ -0,0 +1,17 @@
+/* PR target/pr70799 */
+/* { dg-do compile { target { ia32 } } } */
+/* { dg-options "-O2 -march=slm -fno-split-wide-types -mno-stackrealign" } */
+/* { dg-final { scan-assembler "psllq" } } */
+/* { dg-final { scan-assembler "psrlq" } } */
+
+unsigned long long a, b, c;
+
+void test1 (void)
+{
+  a = (b << 55) | c;
+}
+
+void test2 (void)
+{
+  a = (b >> 55) | c;
+}