diff --git a/lib/Target/X86/Utils/X86ShuffleDecode.cpp b/lib/Target/X86/Utils/X86ShuffleDecode.cpp
index 51d251e551c..6d42a101b0e 100644
--- a/lib/Target/X86/Utils/X86ShuffleDecode.cpp
+++ b/lib/Target/X86/Utils/X86ShuffleDecode.cpp
@@ -247,9 +247,8 @@ void DecodePSHUFBMask(const ConstantDataSequential *C,
     if (Element & (1 << 7))
       ShuffleMask.push_back(SM_SentinelZero);
     else {
-      int Index = Base + Element;
-      assert((Index >= 0 && Index < NumElements) &&
-             "Out of bounds shuffle index for pshub instruction!");
+      // Only the least significant 4 bits of the byte are used.
+      int Index = Base + (Element & 0xf);
       ShuffleMask.push_back(Index);
     }
   }
@@ -266,9 +265,8 @@ void DecodePSHUFBMask(ArrayRef<uint64_t> RawMask,
     if (M & (1 << 7))
       ShuffleMask.push_back(SM_SentinelZero);
     else {
-      int Index = Base + M;
-      assert((Index >= 0 && (unsigned)Index < RawMask.size()) &&
-             "Out of bounds shuffle index for pshub instruction!");
+      // Only the least significant 4 bits of the byte are used.
+      int Index = Base + (M & 0xf);
       ShuffleMask.push_back(Index);
     }
   }
diff --git a/test/CodeGen/X86/pshufb-mask-comments.ll b/test/CodeGen/X86/pshufb-mask-comments.ll
new file mode 100644
index 00000000000..3c5d6d6864a
--- /dev/null
+++ b/test/CodeGen/X86/pshufb-mask-comments.ll
@@ -0,0 +1,30 @@
+; RUN: llc < %s -march=x86-64 -mattr=+ssse3 | FileCheck %s
+
+; Test that the pshufb mask comment is correct.
+
+define <16 x i8> @test1(<16 x i8> %V) {
+; CHECK-LABEL: test1:
+; CHECK: pshufb {{.*}} # xmm0 = xmm0[1,0,0,0,0,2,0,0,0,0,3,0,0,0,0,4]
+  %1 = tail call <16 x i8> @llvm.x86.ssse3.pshuf.b.128(<16 x i8> %V, <16 x i8> <i8 1, i8 0, i8 0, i8 0, i8 0, i8 2, i8 0, i8 0, i8 0, i8 0, i8 3, i8 0, i8 0, i8 0, i8 0, i8 4>)
+  ret <16 x i8> %1
+}
+
+; Test that indexes larger than the size of the vector are shown masked (bottom 4 bits).
+
+define <16 x i8> @test2(<16 x i8> %V) {
+; CHECK-LABEL: test2:
+; CHECK: pshufb {{.*}} # xmm0 = xmm0[15,0,0,0,0,0,0,0,0,0,1,0,0,0,0,2]
+  %1 = tail call <16 x i8> @llvm.x86.ssse3.pshuf.b.128(<16 x i8> %V, <16 x i8> <i8 15, i8 0, i8 0, i8 0, i8 0, i8 16, i8 0, i8 0, i8 0, i8 0, i8 17, i8 0, i8 0, i8 0, i8 0, i8 50>)
+  ret <16 x i8> %1
+}
+
+; Test that indexes with bit seven set are shown as zero.
+
+define <16 x i8> @test3(<16 x i8> %V) {
+; CHECK-LABEL: test3:
+; CHECK: pshufb {{.*}} # xmm0 = xmm0[1,0,0,15,0,2,0,0],zero,xmm0[0,3,0,0],zero,xmm0[0,4]
+  %1 = tail call <16 x i8> @llvm.x86.ssse3.pshuf.b.128(<16 x i8> %V, <16 x i8> <i8 1, i8 0, i8 0, i8 127, i8 0, i8 2, i8 0, i8 0, i8 128, i8 0, i8 3, i8 0, i8 0, i8 255, i8 0, i8 4>)
+  ret <16 x i8> %1
+}
+
+declare <16 x i8> @llvm.x86.ssse3.pshuf.b.128(<16 x i8>, <16 x i8>) nounwind readnone