[PATCH] [X86, AVX] recognize shufflevector with zero input as a vperm2 (PR22984)

[Andrea Di Biagio]

Hi Sanjay,

The patch LGTM. Thanks!


================
Comment at: lib/Target/X86/X86ISelLowering.cpp:9081
@@ -9080,3 +9067,20 @@
-    return DAG.getNode(ISD::CONCAT_VECTORS, DL, VT, LoV, HiV);
+  bool IsV1Zero = ISD::isBuildVectorAllZeros(V1.getNode());
+  bool IsV2Zero = ISD::isBuildVectorAllZeros(V2.getNode());
+
+  // If either input operand is a zero vector, use VPERM2X128 because its mask
+  // allows us to replace the zero input with an implicit zero.
+  if (!IsV1Zero && !IsV2Zero) {
+    // Check for patterns which can be matched with a single insert of a 128-bit
+    // subvector.
+    bool OnlyUsesV1 = isShuffleEquivalent(V1, V2, Mask, {0, 1, 0, 1});
+    if (OnlyUsesV1 || isShuffleEquivalent(V1, V2, Mask, {0, 1, 4, 5})) {
+      MVT SubVT = MVT::getVectorVT(VT.getVectorElementType(),
+                                   VT.getVectorNumElements() / 2);
+      SDValue LoV = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SubVT, V1,
+                                DAG.getIntPtrConstant(0));
+      SDValue HiV = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, SubVT,
+                                OnlyUsesV1 ? V1 : V2, DAG.getIntPtrConstant(0));
+      return DAG.getNode(ISD::CONCAT_VECTORS, DL, VT, LoV, HiV);
+    }
   }
 
----------------
spatel wrote:
> Note: This mask {0, 1, 6, 7} is a v4x64 blend, but we've already tried "lowerVectorShuffleAsBlend()" above. Therefore, this check is redundant, and I've removed it. There was no change in the regression tests after removing this check.
Thanks for clarifying it. That was my only concern about this patch.

http://reviews.llvm.org/D8563

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