[SelectionDAGBuilder] Simplify creation of shufflevector DAG nodes wh…

…ere inputs are larger than the mask

Summary:
The current code loops over all elements to calculate a used range. Then a second short loop looks at the ranges and determines if they can be used in a extract and creates a properly aligned start index for the extract.

This range finding is unnecessary, we can just calculate a properly aligned start index for an extract for each input during the first loop. If we don't find the same start index for each indice we can't use an extract.

Reviewers: zvi, RKSimon

Reviewed By: zvi

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29926

llvm-svn: 295152

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3104,14 +3104,10 @@ void SelectionDAGBuilder::visitShuffleVector(const User &I) {
 
   if (SrcNumElts > MaskNumElts) {
     // Analyze the access pattern of the vector to see if we can extract
-    // two subvectors and do the shuffle. The analysis is done by calculating
-    // the range of elements the mask access on both vectors.
-    int MinRange[2] = { static_cast<int>(SrcNumElts),
-                        static_cast<int>(SrcNumElts)};
-    int MaxRange[2] = {-1, -1};
-
-    for (unsigned i = 0; i != MaskNumElts; ++i) {
-      int Idx = Mask[i];
+    // two subvectors and do the shuffle.
+    int StartIdx[2] = { -1, -1 };  // StartIdx to extract from
+    bool CanExtract = true;
+    for (int Idx : Mask) {
       unsigned Input = 0;
       if (Idx < 0)
         continue;
@@ -3120,41 +3116,28 @@ void SelectionDAGBuilder::visitShuffleVector(const User &I) {
         Input = 1;
         Idx -= SrcNumElts;
       }
-      if (Idx > MaxRange[Input])
-        MaxRange[Input] = Idx;
-      if (Idx < MinRange[Input])
-        MinRange[Input] = Idx;
-    }
 
-    // Check if the access is smaller than the vector size and can we find
-    // a reasonable extract index.
-    int RangeUse[2] = { -1, -1 };  // 0 = Unused, 1 = Extract, -1 = Can not
-                                   // Extract.
-    int StartIdx[2];  // StartIdx to extract from
-    for (unsigned Input = 0; Input < 2; ++Input) {
-      if (MinRange[Input] >= (int)SrcNumElts && MaxRange[Input] < 0) {
-        RangeUse[Input] = 0; // Unused
-        StartIdx[Input] = 0;
-        continue;
-      }
-
-      // Find a good start index that is a multiple of the mask length. Then
-      // see if the rest of the elements are in range.
-      StartIdx[Input] = (MinRange[Input]/MaskNumElts)*MaskNumElts;
-      if (MaxRange[Input] - StartIdx[Input] < (int)MaskNumElts &&
-          StartIdx[Input] + MaskNumElts <= SrcNumElts)
-        RangeUse[Input] = 1; // Extract from a multiple of the mask length.
+      // If all the indices come from the same MaskNumElts sized portion of
+      // the sources we can use extract. Also make sure the extract wouldn't
+      // extract past the end of the source.
+      int NewStartIdx = alignDown(Idx, MaskNumElts);
+      if (NewStartIdx + MaskNumElts > SrcNumElts ||
+          (StartIdx[Input] >= 0 && StartIdx[Input] != NewStartIdx))
+        CanExtract = false;
+      // Make sure we always update StartIdx as we use it to track if all
+      // elements are undef.
+      StartIdx[Input] = NewStartIdx;
     }
 
-    if (RangeUse[0] == 0 && RangeUse[1] == 0) {
+    if (StartIdx[0] < 0 && StartIdx[1] < 0) {
       setValue(&I, DAG.getUNDEF(VT)); // Vectors are not used.
       return;
     }
-    if (RangeUse[0] >= 0 && RangeUse[1] >= 0) {
+    if (CanExtract) {
       // Extract appropriate subvector and generate a vector shuffle
       for (unsigned Input = 0; Input < 2; ++Input) {
         SDValue &Src = Input == 0 ? Src1 : Src2;
-        if (RangeUse[Input] == 0)
+        if (StartIdx[Input] < 0)
           Src = DAG.getUNDEF(VT);
         else {
           Src = DAG.getNode(
@@ -3165,16 +3148,12 @@ void SelectionDAGBuilder::visitShuffleVector(const User &I) {
       }
 
       // Calculate new mask.
-      SmallVector<int, 8> MappedOps;
-      for (unsigned i = 0; i != MaskNumElts; ++i) {
-        int Idx = Mask[i];
-        if (Idx >= 0) {
-          if (Idx < (int)SrcNumElts)
-            Idx -= StartIdx[0];
-          else
-            Idx -= SrcNumElts + StartIdx[1] - MaskNumElts;
-        }
-        MappedOps.push_back(Idx);
+      SmallVector<int, 8> MappedOps(Mask.begin(), Mask.end());
+      for (int &Idx : MappedOps) {
+        if (Idx >= (int)SrcNumElts)
+          Idx -= SrcNumElts + StartIdx[1] - MaskNumElts;
+        else if (Idx >= 0)
+          Idx -= StartIdx[0];
       }
 
       setValue(&I, DAG.getVectorShuffle(VT, DL, Src1, Src2, MappedOps));
@@ -3188,8 +3167,7 @@ void SelectionDAGBuilder::visitShuffleVector(const User &I) {
   EVT EltVT = VT.getVectorElementType();
   EVT IdxVT = TLI.getVectorIdxTy(DAG.getDataLayout());
   SmallVector<SDValue,8> Ops;
-  for (unsigned i = 0; i != MaskNumElts; ++i) {
-    int Idx = Mask[i];
+  for (int Idx : Mask) {
     SDValue Res;
 
     if (Idx < 0) {