[TTI] BasicTTIImpl assumes no vector registers

Summary:
Following the discussion on r241884, it's more reasonable to assume that a
target has no vector registers by default instead of letting every such
target overrides getNumberOfRegisters.

Therefore, this patch modifies BasicTTIImpl::getNumberOfRegisters to
return 0 when Vector is true, and partially reverts r241884 which
modifies NVPTXTTIImpl::getNumberOfRegisters.

It also fixes a performance bug in LoopVectorizer. Even if a target has
no vector registers, vectorization may still help ILP. So, we need both
checks to be false before disabling loop vectorization all together.

Reviewers: hfinkel

Subscribers: llvm-commits, jholewinski

Differential Revision: http://reviews.llvm.org/D11108

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@241942 91177308-0d34-0410-b5e6-96231b3b80d8

include/llvm/CodeGen/BasicTTIImpl.h

@@ -276,7 +276,7 @@ public:
   /// \name Vector TTI Implementations
   /// @{
 
-  unsigned getNumberOfRegisters(bool Vector) { return 1; }
+  unsigned getNumberOfRegisters(bool Vector) { return Vector ? 0 : 1; }
 
   unsigned getRegisterBitWidth(bool Vector) { return 32; }
 

lib/Target/NVPTX/NVPTXTargetTransformInfo.cpp

@@ -117,9 +117,3 @@ unsigned NVPTXTTIImpl::getArithmeticInstrCost(
                                          Opd1PropInfo, Opd2PropInfo);
   }
 }
-
-unsigned NVPTXTTIImpl::getNumberOfRegisters(bool Vector) {
-  if (Vector)
-    return 0;
-  return BaseT::getNumberOfRegisters(Vector);
-}

lib/Target/NVPTX/NVPTXTargetTransformInfo.h

@@ -58,8 +58,6 @@ public:
       TTI::OperandValueKind Opd2Info = TTI::OK_AnyValue,
       TTI::OperandValueProperties Opd1PropInfo = TTI::OP_None,
       TTI::OperandValueProperties Opd2PropInfo = TTI::OP_None);
-
-  unsigned getNumberOfRegisters(bool Vector);
 };
 
 } // end namespace llvm

lib/Transforms/Vectorize/LoopVectorize.cpp

@@ -1456,9 +1456,14 @@ struct LoopVectorize : public FunctionPass {
     const BranchProbability ColdProb(1, 5); // 20%
     ColdEntryFreq = BlockFrequency(BFI->getEntryFreq()) * ColdProb;
 
-    // If the target claims to have no vector registers don't attempt
-    // vectorization.
-    if (!TTI->getNumberOfRegisters(true))
+    // Don't attempt if
+    // 1. the target claims to have no vector registers, and
+    // 2. interleaving won't help ILP.
+    //
+    // The second condition is necessary because, even if the target has no
+    // vector registers, loop vectorization may still enable scalar
+    // interleaving.
+    if (!TTI->getNumberOfRegisters(true) && TTI->getMaxInterleaveFactor(1) < 2)
       return false;
 
     // Build up a worklist of inner-loops to vectorize. This is necessary as