Fixed DAGCombiner bug (found and localized by James Malloy):

The DAGCombiner tries to optimise a BUILD_VECTOR by checking if it
consists purely of get_vector_elts from one or two source vectors. If
so, it either makes a concat_vectors node or a shufflevector node.

However, it doesn't check the element type width of the underlying
vector, so if you have this sequence:

Node0: v4i16 = ...
Node1: i32 = extract_vector_elt Node0
Node2: i32 = extract_vector_elt Node0
Node3: v16i8 = BUILD_VECTOR Node1, Node2, ...

It will attempt to:

Node0:    v4i16 = ...
NewNode1: v16i8 = concat_vectors Node0, ...

Where this is actually invalid because the element width is completely
different. This causes an assertion failure on DAG legalization stage.

Fix:
If output item type of BUILD_VECTOR differs from input item type.
Make concat_vectors based on input element type and then bitcast it to the output vector type. So the case described above will transformed to:
Node0:    v4i16 = ...
NewNode1: v8i16 = concat_vectors Node0, ...
NewNode2: v16i8 = bitcast NewNode1



git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@162195 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 23 insertions, 3 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 4e29879bef..1c485a01fe 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -7876,9 +7876,29 @@ SDValue DAGCombiner::visitBUILD_VECTOR(SDNode *N) {
       if (VecIn1.getValueType().getSizeInBits()*2 != VT.getSizeInBits())
         return SDValue();
 
-      // Widen the input vector by adding undef values.
-      VecIn1 = DAG.getNode(ISD::CONCAT_VECTORS, N->getDebugLoc(), VT,
-                           VecIn1, DAG.getUNDEF(VecIn1.getValueType()));
+      // If the element type of the input vector is not the same as
+      // the output element type, make concat_vectors based on input element
+      // type and then bitcast it to the output vector type.
+      //
+      // In another words avoid nodes like this:
+      //  <NODE> v16i8 = concat_vectors v4i16 v4i16
+      // Replace it with this one:
+      //  <NODE0> v8i16 = concat_vectors v4i16 v4i16
+      //  <NODE1> v16i8 = bitcast NODE0
+      EVT ItemType = VecIn1.getValueType().getVectorElementType();
+      if (ItemType != VT.getVectorElementType()) {
+        EVT ConcatVT = EVT::getVectorVT(*DAG.getContext(),
+                                ItemType,
+                                VecIn1.getValueType().getVectorNumElements()*2);
+        // Widen the input vector by adding undef values.
+        VecIn1 = DAG.getNode(ISD::CONCAT_VECTORS, dl, ConcatVT,
+                             VecIn1, DAG.getUNDEF(VecIn1.getValueType()));
+        VecIn1 = DAG.getNode(ISD::BITCAST, dl, VT, VecIn1);
+      } else
+        // Widen the input vector by adding undef values.
+        VecIn1 = DAG.getNode(ISD::CONCAT_VECTORS, dl, VT,
+                             VecIn1, DAG.getUNDEF(VecIn1.getValueType()));
+
     }
 
     // If VecIn2 is unused then change it to undef.