[DAGCombine] Produce better code for constant splats

This solves PR22276.
Splats of constants would sometimes produce redundant shuffles, sometimes ridiculously so (see the PR for details). Fold these shuffles into BUILD_VECTORs early on instead.

Differential Revision: http://reviews.llvm.org/D7093

llvm-svn: 226811

1 files changed, 22 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index f75d5f4b2bd..c5ef77a3195 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -1513,9 +1513,10 @@ SDValue SelectionDAG::getVectorShuffle(EVT VT, SDLoc dl, SDValue N1,
     return getUNDEF(VT);
 
   // If Identity shuffle return that node.
-  bool Identity = true;
+  bool Identity = true, AllSame = true;
   for (unsigned i = 0; i != NElts; ++i) {
     if (MaskVec[i] >= 0 && MaskVec[i] != (int)i) Identity = false;
+    if (MaskVec[i] != MaskVec[0]) AllSame = false;
   }
   if (Identity && NElts)
     return N1;
@@ -1549,6 +1550,26 @@ SDValue SelectionDAG::getVectorShuffle(EVT VT, SDLoc dl, SDValue N1,
           if (C->isNullValue())
             return N1;
       }
+
+      // If the shuffle itself creates a constant splat, build the vector
+      // directly.
+      if (AllSame) {
+         const SDValue &Splatted = BV->getOperand(MaskVec[0]);
+         if (isa<ConstantSDNode>(Splatted) || isa<ConstantFPSDNode>(Splatted)) {
+           SmallVector<SDValue, 8> Ops;
+           for (unsigned i = 0; i != NElts; ++i) {
+             Ops.push_back(Splatted);
+           }
+           SDValue &NewBV = getNode(ISD::BUILD_VECTOR, dl,
+             BV->getValueType(0), Ops);
+
+           // We may have jumped through bitcasts, so the type of the
+           // BUILD_VECTOR may not match the type of the shuffle.
+           if (BV->getValueType(0) != VT)
+             NewBV = getNode(ISD::BITCAST, dl, VT, NewBV);
+           return NewBV;
+         }
+      }
     }
   }