[CostModel][X86] Fix constant vector XOP rights shifts

We'll constant fold these cases so they are as cheap as vector left shift cases.

Noticed while improving funnel shift costs.

llvm-svn: 346760

Author: RKSimon

llvm/lib/Target/X86/X86TargetTransformInfo.cpp

@@ -612,9 +612,18 @@ int X86TTIImpl::getArithmeticInstrCost(
   };
 
   // Look for XOP lowering tricks.
-  if (ST->hasXOP())
-    if (const auto *Entry = CostTableLookup(XOPShiftCostTable, ISD, LT.second))
+  if (ST->hasXOP()) {
+    // If the right shift is constant then we'll fold the negation so
+    // it's as cheap as a left shift.
+    int ShiftISD = ISD;
+    if ((ShiftISD == ISD::SRL || ShiftISD == ISD::SRA) &&
+        (Op2Info == TargetTransformInfo::OK_UniformConstantValue ||
+         Op2Info == TargetTransformInfo::OK_NonUniformConstantValue))
+      ShiftISD = ISD::SHL;
+    if (const auto *Entry =
+            CostTableLookup(XOPShiftCostTable, ShiftISD, LT.second))
       return LT.first * Entry->Cost;
+  }
 
   static const CostTblEntry SSE2UniformShiftCostTable[] = {
     // Uniform splats are cheaper for the following instructions.