Index: llvm/include/llvm/Analysis/VectorUtils.h
===================================================================
--- llvm/include/llvm/Analysis/VectorUtils.h
+++ llvm/include/llvm/Analysis/VectorUtils.h
@@ -301,6 +301,11 @@
 /// from the vector.
 Value *findScalarElement(Value *V, unsigned EltNo);
 
+/// If all non-negative \p Mask elements are the same value, return that value.
+/// If all elements are negative (undefined) or \p Mask contains different
+/// non-negative values, return -1.
+int getSplatIndex(ArrayRef<int> Mask);
+
 /// Get splat value if the input is a splat vector or return nullptr.
 /// The value may be extracted from a splat constants vector or from
 /// a sequence of instructions that broadcast a single value into a vector.
Index: llvm/lib/Analysis/VectorUtils.cpp
===================================================================
--- llvm/lib/Analysis/VectorUtils.cpp
+++ llvm/lib/Analysis/VectorUtils.cpp
@@ -307,6 +307,24 @@
   return nullptr;
 }
 
+int llvm::getSplatIndex(ArrayRef<int> Mask) {
+  int SplatIndex = -1;
+  for (unsigned i = 0, e = Mask.size(); i != e; ++i) {
+    // Ignore invalid (undefined) mask elements.
+    int M = Mask[i];
+    if (M < 0)
+      continue;
+
+    // There can be only 1 non-negative mask element value if this is a splat.
+    if (SplatIndex >= 0 && SplatIndex != M)
+      return -1;
+
+    // Initialize the splat index to the 1st non-negative mask element.
+    SplatIndex = M;
+  }
+  return SplatIndex;
+}
+
 /// Get splat value if the input is a splat vector or return nullptr.
 /// This function is not fully general. It checks only 2 cases:
 /// the input value is (1) a splat constant vector or (2) a sequence
Index: llvm/lib/Target/X86/X86ISelLowering.cpp
===================================================================
--- llvm/lib/Target/X86/X86ISelLowering.cpp
+++ llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -28,6 +28,7 @@
 #include "llvm/Analysis/BlockFrequencyInfo.h"
 #include "llvm/Analysis/EHPersonalities.h"
 #include "llvm/Analysis/ProfileSummaryInfo.h"
+#include "llvm/Analysis/VectorUtils.h"
 #include "llvm/CodeGen/IntrinsicLowering.h"
 #include "llvm/CodeGen/MachineFrameInfo.h"
 #include "llvm/CodeGen/MachineFunction.h"
@@ -12715,23 +12716,13 @@
 
   // With MOVDDUP (v2f64) we can broadcast from a register or a load, otherwise
   // we can only broadcast from a register with AVX2.
-  unsigned NumElts = Mask.size();
   unsigned NumEltBits = VT.getScalarSizeInBits();
   unsigned Opcode = (VT == MVT::v2f64 && !Subtarget.hasAVX2())
                         ? X86ISD::MOVDDUP
                         : X86ISD::VBROADCAST;
   bool BroadcastFromReg = (Opcode == X86ISD::MOVDDUP) || Subtarget.hasAVX2();
 
-  // Check that the mask is a broadcast.
-  int BroadcastIdx = -1;
-  for (int i = 0; i != (int)NumElts; ++i) {
-    SmallVector<int, 8> BroadcastMask(NumElts, i);
-    if (isShuffleEquivalent(V1, V2, Mask, BroadcastMask)) {
-      BroadcastIdx = i;
-      break;
-    }
-  }
-
+  int BroadcastIdx = getSplatIndex(Mask);
   if (BroadcastIdx < 0)
     return SDValue();
   assert(BroadcastIdx < (int)Mask.size() && "We only expect to be called with "
Index: llvm/unittests/Analysis/VectorUtilsTest.cpp
===================================================================
--- llvm/unittests/Analysis/VectorUtilsTest.cpp
+++ llvm/unittests/Analysis/VectorUtilsTest.cpp
@@ -98,6 +98,17 @@
   EXPECT_FALSE(isSplatValue(SplatWithUndefC));
 }
 
+TEST_F(BasicTest, getSplatIndex) {
+  EXPECT_EQ(getSplatIndex({0,0,0}), 0);
+  EXPECT_EQ(getSplatIndex({1,0,0}), -1);     // no splat
+  EXPECT_EQ(getSplatIndex({0,1,1}), -1);     // no splat
+  EXPECT_EQ(getSplatIndex({42,42,42}), 42);  // array size is independent of splat index
+  EXPECT_EQ(getSplatIndex({42,42,-1}), 42);  // ignore negative
+  EXPECT_EQ(getSplatIndex({-1,42,-1}), 42);  // ignore negatives
+  EXPECT_EQ(getSplatIndex({-4,42,-42}), 42); // ignore all negatives
+  EXPECT_EQ(getSplatIndex({-4,-1,-42}), -1); // all negative values map to -1
+}
+
 TEST_F(VectorUtilsTest, isSplatValue_00) {
   parseAssembly(
       "define <2 x i8> @test(<2 x i8> %x) {\n"