[DAG] Teach how to combine a pair of shuffles into a single shuffle if the resulting mask is legal.

This patch teaches how to fold a shuffle according to rule:
  shuffle (shuffle (x, undef, M0), undef, M1) -> shuffle(x, undef, M2)

We do this only if the resulting mask M2 is legal; this is to avoid introducing
illegal shuffles that are potentially expanded into a sub-optimal sequence
of target specific dag nodes.

This patch has the advantage of being target independent, since it works on ISD
nodes. Therefore, all targets (not only x86) can take advantage of this rule.
The idea behind this patch is that most shuffle pairs can be safely combined
before we run the legalizer on vector operations. This allows us to
combine/simplify dag nodes earlier in the process and not only immediately
before instruction selection stage.

That said. This patch is not meant to replace any existing target specific
combine rules; backends might still introduce new shuffles during legalization
stage. Also, this rule is very simple and avoids to aggressively optimize
shuffles.

llvm-svn: 212539

3 files changed, 627 insertions, 3 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 5d2b02b4bff..d58824a92d6 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -10670,6 +10670,8 @@ SDValue DAGCombiner::visitVECTOR_SHUFFLE(SDNode *N) {
     assert(OtherSV->getOperand(0).getValueType() == VT &&
            "Shuffle types don't match");
 
+    SmallVector<int, 4> Mask;
+    // Compute the combined shuffle mask.
     for (unsigned i = 0; i != NumElts; ++i) {
       int Idx = SVN->getMaskElt(i);
       assert(Idx < (int)NumElts && "Index references undef operand");
@@ -10677,13 +10679,29 @@ SDValue DAGCombiner::visitVECTOR_SHUFFLE(SDNode *N) {
       // shuffle. Adopt the incoming index.
       if (Idx >= 0)
         Idx = OtherSV->getMaskElt(Idx);
+      Mask.push_back(Idx);
+    }
+
+    bool IsIdentityMask = true;
+    for (unsigned i = 0; i != NumElts && IsIdentityMask; ++i) {
+      // Skip Undefs.
+      if (Mask[i] < 0)
+        continue;
 
       // The combined shuffle must map each index to itself.
-      if (Idx >= 0 && (unsigned)Idx != i)
-        return SDValue();
+      IsIdentityMask = (unsigned)Mask[i] == i;
     }
 
-    return OtherSV->getOperand(0);
+    if (IsIdentityMask)
+      // optimize shuffle(shuffle(x, undef), undef) -> x.
+      return OtherSV->getOperand(0);
+
+    // It may still be beneficial to combine the two shuffles if the
+    // resulting shuffle is legal.
+    //   shuffle(shuffle(x, undef, M1), undef, M2) -> shuffle(x, undef, M3).
+    if (TLI.isShuffleMaskLegal(Mask, VT))
+      return DAG.getVectorShuffle(VT, SDLoc(N), N0->getOperand(0), N1,
+                                  &Mask[0]);
   }
 
   return SDValue();
diff --git a/llvm/test/CodeGen/X86/swizzle-2.ll b/llvm/test/CodeGen/X86/swizzle-2.ll
new file mode 100644
index 00000000000..4b1f903c444
--- /dev/null
+++ b/llvm/test/CodeGen/X86/swizzle-2.ll
@@ -0,0 +1,515 @@
+; RUN: llc < %s -mtriple=x86_64-unknown-linux-gnu -mcpu=corei7 | FileCheck %s
+
+; Test that we correctly fold a shuffle that performs a swizzle of another
+; shuffle node according to the rule
+;  shuffle (shuffle (x, undef, M0), undef, M1) -> shuffle(x, undef, M2)
+;
+; We only do this if the resulting mask is legal to avoid introducing an
+; illegal shuffle that is expanded into a sub-optimal sequence of instructions
+; during lowering stage.
+
+
+define <4 x i32> @swizzle_1(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 3, i32 2, i32 0, i32 1>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 2, i32 0, i32 1>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_1
+; Mask: [1,0,3,2]
+; CHECK: pshufd $-79
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_2(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 3, i32 1, i32 0, i32 2>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 1, i32 0, i32 2>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_2
+; Mask: [2,1,3,0]
+; CHECK: pshufd $54
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_3(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 2, i32 3, i32 1, i32 0>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 2, i32 3, i32 1, i32 0>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_3
+; Mask: [1,0,3,2]
+; CHECK: pshufd $-79
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_4(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 2, i32 1, i32 3, i32 0>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 2, i32 1, i32 3, i32 0>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_4
+; Mask: [3,1,0,2]
+; CHECK: pshufd $-121
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_5(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 1, i32 2, i32 3, i32 0>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1, i32 2, i32 3, i32 0>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_5
+; Mask: [2,3,0,1]
+; CHECK: pshufd $78
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_6(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 1, i32 2, i32 0, i32 3>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1, i32 2, i32 0, i32 3>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_6
+; Mask: [2,0,1,3]
+; CHECK: pshufd $-46
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_7(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 1, i32 2>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 0, i32 3, i32 1, i32 2>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_7
+; Mask: [0,2,3,1]
+; CHECK: pshufd $120
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_8(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 3, i32 0, i32 2, i32 1>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 0, i32 2, i32 1>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_8
+; Mask: [1,3,2,0]
+; CHECK: pshufd $45
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_9(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 3, i32 0, i32 1, i32 2>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 3, i32 0, i32 1, i32 2>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_9
+; Mask: [2,3,0,1]
+; CHECK: pshufd $78
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_10(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 2, i32 0, i32 1, i32 3>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 2, i32 0, i32 1, i32 3>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_10
+; Mask: [1,2,0,3]
+; CHECK: pshufd $-55
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_11(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 2, i32 0, i32 3, i32 1>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 2, i32 0, i32 3, i32 1>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_11
+; Mask: [3,2,1,0]
+; CHECK: pshufd $27
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_12(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 0, i32 2, i32 3, i32 1>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 0, i32 2, i32 3, i32 1>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_12
+; Mask: [0,3,1,2]
+; CHECK: pshufd $-100
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_13(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 1, i32 3, i32 0, i32 2>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1, i32 3, i32 0, i32 2>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_13
+; Mask: [3,2,1,0]
+; CHECK: pshufd $27
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x i32> @swizzle_14(<4 x i32> %v) {
+  %1 = shufflevector <4 x i32> %v, <4 x i32> undef, <4 x i32> <i32 1, i32 3, i32 2, i32 0>
+  %2 = shufflevector <4 x i32> %1, <4 x i32> undef, <4 x i32> <i32 1, i32 3, i32 2, i32 0>
+  ret <4 x i32> %2
+}
+; CHECK-LABEL: swizzle_14
+; Mask: [3,0,2,1]
+; CHECK: pshufd $99
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_15(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 3, i32 2, i32 0, i32 1>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 3, i32 2, i32 0, i32 1>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_15
+; Mask: [1,0,3,2]
+; CHECK: pshufd $-79
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_16(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 3, i32 1, i32 0, i32 2>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 3, i32 1, i32 0, i32 2>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_16
+; Mask: [2,1,3,0]
+; CHECK: pshufd $54
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_17(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 2, i32 3, i32 1, i32 0>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 2, i32 3, i32 1, i32 0>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_17
+; Mask: [1,0,3,2]
+; CHECK: pshufd $-79
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_18(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 2, i32 1, i32 3, i32 0>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 2, i32 1, i32 3, i32 0>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_18
+; Mask: [3,1,0,2]
+; CHECK: pshufd $-121
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_19(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 1, i32 2, i32 3, i32 0>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 1, i32 2, i32 3, i32 0>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_19
+; Mask: [2,3,0,1]
+; CHECK: pshufd $78
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_20(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 1, i32 2, i32 0, i32 3>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 1, i32 2, i32 0, i32 3>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_20
+; Mask: [2,0,1,3]
+; CHECK: pshufd $-46
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_21(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 0, i32 3, i32 1, i32 2>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 0, i32 3, i32 1, i32 2>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_21
+; Mask: [0,2,3,1]
+; CHECK: pshufd $120
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_22(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 3, i32 0, i32 2, i32 1>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 3, i32 0, i32 2, i32 1>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_22
+; Mask: [1,3,2,0]
+; CHECK: pshufd $45
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_23(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 3, i32 0, i32 1, i32 2>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 3, i32 0, i32 1, i32 2>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_23
+; Mask: [2,3,0,1]
+; CHECK: pshufd $78
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_24(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 2, i32 0, i32 1, i32 3>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 2, i32 0, i32 1, i32 3>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_24
+; Mask: [1,2,0,3]
+; CHECK: pshufd $-55
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_25(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 2, i32 0, i32 3, i32 1>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 2, i32 0, i32 3, i32 1>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_25
+; Mask: [3,2,1,0]
+; CHECK: pshufd $27
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_26(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 0, i32 2, i32 3, i32 1>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 0, i32 2, i32 3, i32 1>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_26
+; Mask: [0,3,1,2]
+; CHECK: pshufd $-100
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_27(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 1, i32 3, i32 0, i32 2>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 1, i32 3, i32 0, i32 2>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_27
+; Mask: [3,2,1,0]
+; CHECK: pshufd $27
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_28(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 1, i32 3, i32 2, i32 0>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 1, i32 3, i32 2, i32 0>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_28
+; Mask: [3,0,2,1]
+; CHECK: pshufd $99
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+
+define <4 x float> @swizzle_29(<4 x float> %v) {
+  %1 = shufflevector <4 x float> %v, <4 x float> undef, <4 x i32> <i32 3, i32 1, i32 2, i32 0>
+  %2 = shufflevector <4 x float> %1, <4 x float> undef, <4 x i32> <i32 1, i32 0, i32 2, i32 3>
+  ret <4 x float> %2
+}
+; CHECK-LABEL: swizzle_29
+; Mask: [1,3,2,0]
+; CHECK: pshufd $45
+; CHECK-NOT: pshufd
+; CHECK-NEXT: ret
+
+; Make sure that we combine the shuffles from each function below into a single
+; legal shuffle (either pshuflw or pshufb depending on the masks).
+
+define <8 x i16> @swizzle_30(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 3, i32 1, i32 2, i32 0, i32 7, i32 5, i32 6, i32 4>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 1, i32 0, i32 2, i32 3, i32 7, i32 5, i32 6, i32 4>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_30
+; Mask: [1,3,2,0,5,7,6,4]
+; CHECK: pshuflw $45
+; CHECK-NOT: pshufb
+; CHECK-NEXT: ret
+
+
+define <8 x i16> @swizzle_31(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 3, i32 0, i32 2, i32 1, i32 7, i32 5, i32 6, i32 4>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 3, i32 0, i32 2, i32 1, i32 7, i32 5, i32 6, i32 4>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_31
+; Mask: [1,3,2,0,4,5,6,7]
+; CHECK: pshuflw $45
+; CHECK-NOT: pshufb
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_32(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 1, i32 2, i32 3, i32 0, i32 7, i32 5, i32 6, i32 4>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 1, i32 2, i32 3, i32 0, i32 7, i32 5, i32 6, i32 4>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_32
+; Mask: [2,3,0,1,4,5,6,7] --> equivalent to pshufd mask [1,0,2,3]
+; CHECK: pshufd $-31
+; CHECK-NOT: pshufb
+; CHECK: ret
+
+define <8 x i16> @swizzle_33(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 4, i32 6, i32 5, i32 7, i32 2, i32 3, i32 1, i32 0>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 4, i32 6, i32 5, i32 7, i32 2, i32 3, i32 1, i32 0>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_33
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_34(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 4, i32 7, i32 6, i32 5, i32 1, i32 2, i32 0, i32 3>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 4, i32 7, i32 6, i32 5, i32 1, i32 2, i32 0, i32 3>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_34
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_35(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 7, i32 4, i32 6, i32 5, i32 1, i32 3, i32 0, i32 2>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 7, i32 4, i32 6, i32 5, i32 1, i32 3, i32 0, i32 2>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_35
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_36(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 4, i32 6, i32 7, i32 5, i32 0, i32 1, i32 3, i32 2>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 4, i32 6, i32 7, i32 5, i32 0, i32 1, i32 3, i32 2>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_36
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_37(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 1, i32 0, i32 3, i32 2, i32 7, i32 5, i32 6, i32 4>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 1, i32 0, i32 3, i32 2, i32 7, i32 4, i32 6, i32 5>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_37
+; Mask: [0,1,2,3,4,7,6,5]
+; CHECK: pshufhw $108
+; CHECK-NOT: pshufb
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_38(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 5, i32 6, i32 4, i32 7, i32 0, i32 2, i32 1, i32 3>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 5, i32 6, i32 4, i32 7, i32 0, i32 2, i32 1, i32 3>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_38
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_39(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 5, i32 4, i32 6, i32 7, i32 3, i32 2, i32 1, i32 0>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 5, i32 4, i32 6, i32 7, i32 3, i32 2, i32 1, i32 0>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_39
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_40(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 6, i32 4, i32 7, i32 5, i32 1, i32 0, i32 3, i32 2>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 6, i32 4, i32 7, i32 5, i32 1, i32 0, i32 3, i32 2>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_40
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_41(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 6, i32 7, i32 5, i32 4, i32 0, i32 1, i32 3, i32 2>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 6, i32 7, i32 5, i32 4, i32 0, i32 1, i32 3, i32 2>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_41
+; CHECK: pshufb
+; CHECK-NOT: pshufb
+; CHECK-NOT: shufpd
+; CHECK: ret
+
+
+define <8 x i16> @swizzle_42(<8 x i16> %v) {
+  %1 = shufflevector <8 x i16> %v, <8 x i16> undef, <8 x i32> <i32 0, i32 1, i32 3, i32 2, i32 7, i32 6, i32 4, i32 5>
+  %2 = shufflevector <8 x i16> %1, <8 x i16> undef, <8 x i32> <i32 0, i32 1, i32 3, i32 2, i32 7, i32 6, i32 4, i32 5>
+  ret <8 x i16> %2
+}
+; CHECK-LABEL: swizzle_42
+; Mask: [0,1,2,3,5,4,7,6]
+; CHECK: pshufhw $-79
+; CHECK-NOT: pshufb
+; CHECK: ret
+
+
diff --git a/llvm/test/CodeGen/X86/swizzle-avx2.ll b/llvm/test/CodeGen/X86/swizzle-avx2.ll
new file mode 100644
index 00000000000..29dfa6c2dcc
--- /dev/null
+++ b/llvm/test/CodeGen/X86/swizzle-avx2.ll
@@ -0,0 +1,91 @@
+; RUN: llc < %s -march=x86-64 -mtriple=x86_64-unknown-linux-gnu -mcpu=core-avx2 | FileCheck %s
+
+; Test that we correctly fold a shuffle that performs a swizzle of another
+; shuffle node according to the rule
+;  shuffle (shuffle (x, undef, M0), undef, M1) -> shuffle(x, undef, M2)
+;
+; We only do this if the resulting mask is legal to avoid introducing an
+; illegal shuffle that is expanded into a sub-optimal sequence of instructions
+; during lowering stage.
+
+; Check that we produce a single vector permute / shuffle in all cases.
+
+define <8 x i32> @swizzle_1(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 3, i32 1, i32 2, i32 0, i32 7, i32 5, i32 6, i32 4>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 1, i32 0, i32 2, i32 3, i32 7, i32 5, i32 6, i32 4>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_1
+; CHECK: vpermd
+; CHECK-NOT: vpermd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_2(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 6, i32 7, i32 4, i32 5, i32 0, i32 1, i32 2, i32 3>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 6, i32 7, i32 4, i32 5, i32 0, i32 1, i32 2, i32 3>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_2
+; CHECK: vpshufd $78
+; CHECK-NOT: vpermd
+; CHECK-NOT: vpshufd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_3(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 4, i32 5, i32 6, i32 7, i32 2, i32 3, i32 0, i32 1>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 4, i32 5, i32 6, i32 7, i32 2, i32 3, i32 0, i32 1>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_3
+; CHECK: vpshufd $78
+; CHECK-NOT: vpermd
+; CHECK-NOT: vpshufd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_4(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 4, i32 7, i32 5, i32 6, i32 3, i32 2, i32 0, i32 1>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 4, i32 7, i32 5, i32 6, i32 3, i32 2, i32 0, i32 1>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_4
+; CHECK: vpermd
+; CHECK-NOT: vpermd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_5(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 7, i32 4, i32 6, i32 5, i32 0, i32 2, i32 1, i32 3>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 7, i32 4, i32 6, i32 5, i32 0, i32 2, i32 1, i32 3>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_5
+; CHECK: vpermd
+; CHECK-NOT: vpermd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_6(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 2, i32 1, i32 3, i32 0, i32 4, i32 7, i32 6, i32 5>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 2, i32 1, i32 3, i32 0, i32 4, i32 7, i32 6, i32 5>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_6
+; CHECK: vpermd
+; CHECK-NOT: vpermd
+; CHECK: ret
+
+
+define <8 x i32> @swizzle_7(<8 x i32> %v) {
+  %1 = shufflevector <8 x i32> %v, <8 x i32> undef, <8 x i32> <i32 0, i32 3, i32 1, i32 2, i32 5, i32 4, i32 6, i32 7>
+  %2 = shufflevector <8 x i32> %1, <8 x i32> undef, <8 x i32> <i32 0, i32 3, i32 1, i32 2, i32 5, i32 4, i32 6, i32 7>
+  ret <8 x i32> %2
+}
+; CHECK-LABEL: swizzle_7
+; CHECK: vpermd
+; CHECK-NOT: vpermd
+; CHECK: ret
+
+