X86 cost model: Differentiate cost for vector shifts of constants

SSE2 has efficient support for shifts by a scalar. My previous change of making
shifts expensive did not take this into account marking all shifts as expensive.
This would prevent vectorization from happening where it is actually beneficial.

With this change we differentiate between shifts of constants and other shifts.

radar://13576547

llvm-svn: 178808

1 files changed, 29 insertions, 0 deletions

diff --git a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
index e8940bfc230..a98c6991192 100644
--- a/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
+++ b/llvm/lib/Target/X86/X86TargetTransformInfo.cpp
@@ -206,6 +206,35 @@ unsigned X86TTI::getArithmeticInstrCost(unsigned Opcode, Type *Ty,
       return LT.first * AVX2CostTable[Idx].Cost;
   }
 
+  static const CostTblEntry<MVT> SSE2UniformConstCostTable[] = {
+    // We don't correctly identify costs of casts because they are marked as
+    // custom.
+    // Constant splats are cheaper for the following instructions.
+    { ISD::SHL,  MVT::v16i8,  1 }, // psllw.
+    { ISD::SHL,  MVT::v8i16,  1 }, // psllw.
+    { ISD::SHL,  MVT::v4i32,  1 }, // pslld
+    { ISD::SHL,  MVT::v2i64,  1 }, // psllq.
+
+    { ISD::SRL,  MVT::v16i8,  1 }, // psrlw.
+    { ISD::SRL,  MVT::v8i16,  1 }, // psrlw.
+    { ISD::SRL,  MVT::v4i32,  1 }, // psrld.
+    { ISD::SRL,  MVT::v2i64,  1 }, // psrlq.
+
+    { ISD::SRA,  MVT::v16i8,  4 }, // psrlw, pand, pxor, psubb.
+    { ISD::SRA,  MVT::v8i16,  1 }, // psraw.
+    { ISD::SRA,  MVT::v4i32,  1 }, // psrad.
+  };
+
+  if (Op2Info == TargetTransformInfo::OK_UniformConstantValue &&
+      ST->hasSSE2()) {
+    int Idx = CostTableLookup<MVT>(SSE2UniformConstCostTable,
+                                   array_lengthof(SSE2UniformConstCostTable),
+                                   ISD, LT.second);
+    if (Idx != -1)
+      return LT.first * SSE2UniformConstCostTable[Idx].Cost;
+  }
+
+
   static const CostTblEntry<MVT> SSE2CostTable[] = {
     // We don't correctly identify costs of casts because they are marked as
     // custom.