[PATCH, i386] Fix ix86_multiplication_cost for SKX

[Shalnov, Sergey]

Hi,
This patch is one of the set of patches to fix SKX costs.

I think multiplication costs calculation algorithm needs to be adjusted in 
gcc/config/i386/i386.c ix86_multiplication_cost() function.
For TARGET_AVX512DQ emulation is not used and single vpmullq instruction 
emitted.
I think we have to align costs calculation algorithm with sequence of emitted 
instructions.

I don't see visible regressions for this change. This patch helps to avoid a 
regressions with further changes.

Could you please merge the patch to the main trunk?

Thank you
Sergey

2018-02-06  Sergey Shalnov  <sergey.shal...@intel.com>
        gcc/
        * config/i386/i386.c (ix86_multiplication_cost): Fix multiplication
        cost for TARGET_AVX512DQ since it uses vpmullq instead
        emulated sequence


diff --git a/gcc/config/i386/i386.c b/gcc/config/i386/i386.c index 
70b6775..6f4ec5d 100644
--- a/gcc/config/i386/i386.c
+++ b/gcc/config/i386/i386.c
@@ -40393,6 +40393,10 @@ ix86_multiplication_cost (const struct processor_costs 
*cost,
                           ? cost->mulsd : cost->mulss, true);
   else if (GET_MODE_CLASS (mode) == MODE_VECTOR_INT)
     {
+      /* vpmullq is used in this case. No emulation is needed.  */
+      if (TARGET_AVX512DQ)
+       return ix86_vec_cost (mode, cost->mulss, true);
+
       /* V*QImode is emulated with 7-13 insns.  */
       if (mode == V16QImode || mode == V32QImode)
        {