[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
diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index a06f35e..72f4581 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -11490,7 +11490,7 @@
}
// 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 @@
// 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;
+ }
}
}