Codegen insertelement with constant insertion points as scalar_to_vector

and a shuffle.  For this:

void %test2(<4 x float>* %F, float %f) {
        %tmp = load <4 x float>* %F             ; <<4 x float>> [#uses=2]
        %tmp3 = add <4 x float> %tmp, %tmp              ; <<4 x float>> [#uses=1]
        %tmp2 = insertelement <4 x float> %tmp3, float %f, uint 2               ; <<4 x float>> [#uses=2]
        %tmp6 = add <4 x float> %tmp2, %tmp2            ; <<4 x float>> [#uses=1]
        store <4 x float> %tmp6, <4 x float>* %F
        ret void
}

we now get this on X86 (which will get better):

_test2:
        movl 4(%esp), %eax
        movaps (%eax), %xmm0
        addps %xmm0, %xmm0
        movaps %xmm0, %xmm1
        shufps $3, %xmm1, %xmm1
        movaps %xmm0, %xmm2
        shufps $1, %xmm2, %xmm2
        unpcklps %xmm1, %xmm2
        movss 8(%esp), %xmm1
        unpcklps %xmm1, %xmm0
        unpcklps %xmm2, %xmm0
        addps %xmm0, %xmm0
        movaps %xmm0, (%eax)
        ret

instead of:

_test2:
        subl $28, %esp
        movl 32(%esp), %eax
        movaps (%eax), %xmm0
        addps %xmm0, %xmm0
        movaps %xmm0, (%esp)
        movss 36(%esp), %xmm0
        movss %xmm0, 8(%esp)
        movaps (%esp), %xmm0
        addps %xmm0, %xmm0
        movaps %xmm0, (%eax)
        addl $28, %esp
        ret


git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@27765 91177308-0d34-0410-b5e6-96231b3b80d8

lib/CodeGen/SelectionDAG/LegalizeDAG.cpp

@@ -867,6 +867,34 @@ SDOperand SelectionDAGLegalize::LegalizeOp(SDOperand Op) {
       }
       // FALLTHROUGH
     case TargetLowering::Expand: {
+      // If the insert index is a constant, codegen this as a scalar_to_vector,
+      // then a shuffle that inserts it into the right position in the vector.
+      if (ConstantSDNode *InsertPos = dyn_cast<ConstantSDNode>(Tmp3)) {
+        SDOperand ScVec = DAG.getNode(ISD::SCALAR_TO_VECTOR, 
+                                      Tmp1.getValueType(), Tmp2);
+        
+        unsigned NumElts = MVT::getVectorNumElements(Tmp1.getValueType());
+        MVT::ValueType ShufMaskVT = MVT::getIntVectorWithNumElements(NumElts);
+        MVT::ValueType ShufMaskEltVT = MVT::getVectorBaseType(ShufMaskVT);
+        
+        // We generate a shuffle of InVec and ScVec, so the shuffle mask should
+        // be 0,1,2,3,4,5... with the appropriate element replaced with elt 0 of
+        // the RHS.
+        std::vector<SDOperand> ShufOps;
+        for (unsigned i = 0; i != NumElts; ++i) {
+          if (i != InsertPos->getValue())
+            ShufOps.push_back(DAG.getConstant(i, ShufMaskEltVT));
+          else
+            ShufOps.push_back(DAG.getConstant(NumElts, ShufMaskEltVT));
+        }
+        SDOperand ShufMask = DAG.getNode(ISD::BUILD_VECTOR, ShufMaskVT,ShufOps);
+        
+        Result = DAG.getNode(ISD::VECTOR_SHUFFLE, Tmp1.getValueType(),
+                             Tmp1, ScVec, ShufMask);
+        Result = LegalizeOp(Result);
+        break;
+      }
+      
       // If the target doesn't support this, we have to spill the input vector
       // to a temporary stack slot, update the element, then reload it.  This is
       // badness.  We could also load the value into a vector register (either