[X86][AVX2] Broadcast subvectors

AVX2 can only broadcast from the zero'th element of a vector, but if the broadcastable element is the zero'th element of a 128-bit subvector its advantageous to extract the subvector, broadcast from that and avoid the loading of shuffle mask data that would be needed for VPERMPS/VPERMD. The only exception being when the source type is 4f64 or 4i64 which can directly use the immediate shuffle VPERMPD/VPERMQ directly.

Differential Revision: http://reviews.llvm.org/D16050

llvm-svn: 258081

1 files changed, 21 insertions, 3 deletions

diff --git a/llvm/lib/Target/X86/X86ISelLowering.cpp b/llvm/lib/Target/X86/X86ISelLowering.cpp
index 866d11f62dc..322a0133b29 100644
--- a/llvm/lib/Target/X86/X86ISelLowering.cpp
+++ b/llvm/lib/Target/X86/X86ISelLowering.cpp
@@ -8291,10 +8291,28 @@ static SDValue lowerVectorShuffleAsBroadcast(SDLoc DL, MVT VT, SDValue V,
     V = DAG.getLoad(SVT, DL, Ld->getChain(), NewAddr,
                     DAG.getMachineFunction().getMachineMemOperand(
                         Ld->getMemOperand(), Offset, SVT.getStoreSize()));
-  } else if (BroadcastIdx != 0 || !Subtarget->hasAVX2()) {
-    // We can't broadcast from a vector register without AVX2, and we can only
-    // broadcast from the zero-element of a vector register.
+  } else if (!Subtarget->hasAVX2()) {
+    // We can't broadcast from a vector register without AVX2.
     return SDValue();
+  } else if (BroadcastIdx != 0) {
+    // We can only broadcast from the zero-element of a vector register,
+    // but it can be advantageous to broadcast from the zero-element of a
+    // subvector.
+    if (!VT.is256BitVector() && !VT.is512BitVector())
+      return SDValue();
+
+    // VPERMQ/VPERMPD can perform the cross-lane shuffle directly.
+    if (VT == MVT::v4f64 || VT == MVT::v4i64)
+      return SDValue();
+
+    // Only broadcast the zero-element of a 128-bit subvector.
+    unsigned EltSize = VT.getScalarSizeInBits();
+    if (((BroadcastIdx * EltSize) % 128) != 0)
+      return SDValue();
+
+    MVT ExtVT = MVT::getVectorVT(VT.getScalarType(), 128 / EltSize);
+    V = DAG.getNode(ISD::EXTRACT_SUBVECTOR, DL, ExtVT, V,
+                    DAG.getIntPtrConstant(BroadcastIdx, DL));
   }
 
   V = DAG.getNode(X86ISD::VBROADCAST, DL, BroadcastVT, V);