ARM cost model: Account for zero cost scalar SROA instructions

By vectorizing a series of srl, or, ... instructions we have obfuscated the
intention so much that the backend does not know how to fold this code away.

radar://15336950

llvm-svn: 193573

2 files changed, 33 insertions, 6 deletions

diff --git a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
index 9dc1cd1feeb..86b6215f09e 100644
--- a/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
+++ b/llvm/lib/Target/ARM/ARMTargetTransformInfo.cpp
@@ -523,8 +523,20 @@ unsigned ARMTTI::getArithmeticInstrCost(unsigned Opcode, Type *Ty, OperandValueK
   if (Idx != -1)
     return LT.first * CostTbl[Idx].Cost;
 
-
-  return TargetTransformInfo::getArithmeticInstrCost(Opcode, Ty, Op1Info,
-                                                     Op2Info);
+  unsigned Cost =
+      TargetTransformInfo::getArithmeticInstrCost(Opcode, Ty, Op1Info, Op2Info);
+
+  // This is somewhat of a hack. The problem that we are facing is that SROA
+  // creates a sequence of shift, and, or instructions to construct values.
+  // These sequences are recognized by the ISel and have zero-cost. Not so for
+  // the vectorized code. Because we have support for v2i64 but not i64 those
+  // sequences look particularily beneficial to vectorize.
+  // To work around this we increase the cost of v2i64 operations to make them
+  // seem less beneficial.
+  if (LT.second == MVT::v2i64 &&
+      Op2Info == TargetTransformInfo::OK_UniformConstantValue)
+    Cost += 4;
+
+  return Cost;
 }
 
diff --git a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
index 012521ad014..2797a214212 100644
--- a/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
+++ b/llvm/lib/Transforms/Vectorize/SLPVectorizer.cpp
@@ -1013,9 +1013,24 @@ int BoUpSLP::getEntryCost(TreeEntry *E) {
         TTI->getCmpSelInstrCost(Opcode, ScalarTy, Builder.getInt1Ty());
         VecCost = TTI->getCmpSelInstrCost(Opcode, VecTy, MaskTy);
       } else {
-        ScalarCost = VecTy->getNumElements() *
-        TTI->getArithmeticInstrCost(Opcode, ScalarTy);
-        VecCost = TTI->getArithmeticInstrCost(Opcode, VecTy);
+        // Certain instructions can be cheaper to vectorize if they have a
+        // constant second vector operand.
+        TargetTransformInfo::OperandValueKind Op1VK =
+            TargetTransformInfo::OK_AnyValue;
+        TargetTransformInfo::OperandValueKind Op2VK =
+            TargetTransformInfo::OK_UniformConstantValue;
+
+        // Check whether all second operands are constant.
+        for (unsigned i = 0; i < VL.size(); ++i)
+          if (!isa<ConstantInt>(cast<Instruction>(VL[i])->getOperand(1))) {
+            Op2VK = TargetTransformInfo::OK_AnyValue;
+            break;
+          }
+
+        ScalarCost =
+            VecTy->getNumElements() *
+            TTI->getArithmeticInstrCost(Opcode, ScalarTy, Op1VK, Op2VK);
+        VecCost = TTI->getArithmeticInstrCost(Opcode, VecTy, Op1VK, Op2VK);
       }
       return VecCost - ScalarCost;
     }