diff --git a/llvm/include/llvm/IR/Instructions.h b/llvm/include/llvm/IR/Instructions.h
--- a/llvm/include/llvm/IR/Instructions.h
+++ b/llvm/include/llvm/IR/Instructions.h
@@ -2448,6 +2448,10 @@
   /// StartIndexes are the first indexes of each vector being interleaved,
   /// substituting any indexes that were undef
   /// E.g. <4, -1, 2, 5, 1, 3> (Factor=3): StartIndexes=<4, 0, 2>
+  ///
+  /// Note that this does not check if the input vectors are consecutive:
+  /// It will return true for masks such as
+  /// <0, 4, 6, 1, 5, 7> (Factor=3, LaneLen=2)
   static bool isInterleaveMask(ArrayRef<int> Mask, unsigned Factor,
                                unsigned NumInputElts,
                                SmallVectorImpl<unsigned> &StartIndexes);
diff --git a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
--- a/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
+++ b/llvm/lib/Target/RISCV/RISCVTargetTransformInfo.cpp
@@ -264,13 +264,12 @@
         // deinterleaves of 2 vectors can be lowered into the following
         // sequences
         if (EltTp.getScalarSizeInBits() < ST->getELEN()) {
-          auto InterleaveMask = createInterleaveMask(Mask.size() / 2, 2);
           // Example sequence:
           //   vsetivli     zero, 4, e8, mf4, ta, ma (ignored)
           //   vwaddu.vv    v10, v8, v9
           //   li       a0, -1                   (ignored)
           //   vwmaccu.vx   v10, a0, v9
-          if (ShuffleVectorInst::isInterleaveMask(Mask, 2, Mask.size() * 2))
+          if (ShuffleVectorInst::isInterleaveMask(Mask, 2, Mask.size()))
             return 2 * LT.first * getLMULCost(LT.second);
 
           if (Mask[0] == 0 || Mask[0] == 1) {