[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

Fixed recommit of r226811.

llvm-svn: 226816

1 files changed, 19 insertions, 1 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index a06f35ee937..72f458118da 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11490,7 +11490,7 @@ SDValue DAGCombiner::visitVECTOR_SHUFFLE(SDNode *N) {
   }
 
   // If it is a splat, check if the argument vector is another splat or a
-  // build_vector with all scalar elements the same.
+  // build_vector.
   if (SVN->isSplat() && SVN->getSplatIndex() < (int)NumElts) {
     SDNode *V = N0.getNode();
 
@@ -11527,6 +11527,24 @@ SDValue DAGCombiner::visitVECTOR_SHUFFLE(SDNode *N) {
       // Splat of <x, x, x, x>, return <x, x, x, x>
       if (AllSame)
         return N0;
+
+      // If the splatted element is a constant, just build the vector out of
+      // constants directly.
+      const SDValue &Splatted = V->getOperand(SVN->getSplatIndex());
+      if (isa<ConstantSDNode>(Splatted) || isa<ConstantFPSDNode>(Splatted)) {
+        SmallVector<SDValue, 8> Ops;
+        for (unsigned i = 0; i != NumElts; ++i) {
+          Ops.push_back(Splatted);
+        }
+        SDValue NewBV = DAG.getNode(ISD::BUILD_VECTOR, SDLoc(N),
+          V->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 (V->getValueType(0) != VT)
+           NewBV = DAG.getNode(ISD::BITCAST, SDLoc(N), VT, NewBV);
+        return NewBV;
+      }
     }
   }