[SelectionDAG] Teach getNode to simplify a couple easy cases of EXTRACT_SUBVECTOR

Summary:
This teaches getNode to simplify extracting from Undef. This is similar to what is done for EXTRACT_VECTOR_ELT. It also adds support for extracting from CONCAT_VECTOR when we can reuse one of the inputs to the concat. These seem like simple non-target specific optimizations.

For X86 we currently handle undef in extractSubvector, but not all EXTRACT_SUBVECTOR creations go through there.

Ultimately, my motivation here is to simplify extractSubvector and remove custom lowering for EXTRACT_SUBVECTOR since we don't do anything but handle undef and BUILD_VECTOR optimizations, but those should be DAG combines.

Reviewers: RKSimon, delena

Reviewed By: RKSimon

Subscribers: llvm-commits

Differential Revision: https://reviews.llvm.org/D29000

llvm-svn: 292876

1 files changed, 13 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index d7b4b4291a1..5a82ff1e3ab 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -4084,6 +4084,19 @@ SDValue SelectionDAG::getNode(unsigned Opcode, const SDLoc &DL, EVT VT,
       if (VT.getSimpleVT() == N1.getSimpleValueType())
         return N1;
 
+      // EXTRACT_SUBVECTOR of an UNDEF is an UNDEF.
+      if (N1.isUndef())
+        return getUNDEF(VT);
+
+      // EXTRACT_SUBVECTOR of CONCAT_VECTOR can be simplified if the pieces of
+      // the concat have the same type as the extract.
+      if (N2C && N1.getOpcode() == ISD::CONCAT_VECTORS &&
+          N1.getNumOperands() > 0 &&
+          VT == N1.getOperand(0).getValueType()) {
+        unsigned Factor = VT.getVectorNumElements();
+        return N1.getOperand(N2C->getZExtValue() / Factor);
+      }
+
       // EXTRACT_SUBVECTOR of INSERT_SUBVECTOR is often created
       // during shuffle legalization.
       if (N1.getOpcode() == ISD::INSERT_SUBVECTOR && N2 == N1.getOperand(2) &&