1 files changed, 41 insertions, 0 deletions
diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index b372950d436..61accea84d1 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -7914,6 +7914,47 @@ static SDValue lowerVectorShuffle(SDValue Op, const X86Subtarget *Subtarget,
         return DAG.getVectorShuffle(VT, dl, V1, V2, NewMask);
       }
 
+  // Check for a shuffle of a splat, and return just the splat. While DAG
+  // combining will do a similar transformation, this shows up with the
+  // internally created shuffles and so we handle it specially here as we won't
+  // have another chance to DAG-combine the generic shuffle instructions.
+  if (V2IsUndef) {
+    SDValue V = V1;
+
+    // Look through any bitcasts. These can't change the size, just the number
+    // of elements which we check later.
+    while (V.getOpcode() == ISD::BITCAST)
+      V = V->getOperand(0);
+
+    // A splat should always show up as a build vector node.
+    if (V.getOpcode() == ISD::BUILD_VECTOR) {
+      SDValue Base;
+      bool AllSame = true;
+      for (unsigned i = 0; i != V->getNumOperands(); ++i)
+        if (V->getOperand(i).getOpcode() != ISD::UNDEF) {
+          Base = V->getOperand(i);
+          break;
+        }
+      // Splat of <u, u, ..., u>, return <u, u, ..., u>
+      if (!Base)
+        return V1;
+      for (unsigned i = 0; i != V->getNumOperands(); ++i)
+        if (V->getOperand(i) != Base) {
+          AllSame = false;
+          break;
+        }
+      // Splat of <x, x, ..., x>, return <x, x, ..., x>, provided that the
+      // number of elements match or the value splatted is a zero constant.
+      if (AllSame) {
+        if (V.getValueType().getVectorNumElements() == (unsigned)NumElements)
+          return V1;
+        if (auto *C = dyn_cast<ConstantSDNode>(Base))
+          if (C->isNullValue())
+            return V1;
+      }
+    }
+  }
+
   // For integer vector shuffles, try to collapse them into a shuffle of fewer
   // lanes but wider integers. We cap this to not form integers larger than i64
   // but it might be interesting to form i128 integers to handle flipping the