[DAGCombiner] Fold together mul and shl when both are by a constant

This is intended to improve code generation for GEPs, as the index value is
shifted by the element size and in GEPs of multi-dimensional arrays the index
of higher dimensions is multiplied by the lower dimension size.

Differential Revision: http://reviews.llvm.org/D12197

llvm-svn: 245689

1 files changed, 8 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d58c33a499d..53d07b385e3 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -4455,6 +4455,14 @@ SDValue DAGCombiner::visitSHL(SDNode *N) {
     return DAG.getNode(ISD::ADD, SDLoc(N), VT, Shl0, Shl1);
   }
 
+  // fold (shl (mul x, c1), c2) -> (mul x, c1 << c2)
+  if (N1C && N0.getOpcode() == ISD::MUL && N0.getNode()->hasOneUse()) {
+    if (ConstantSDNode *N0C1 = isConstOrConstSplat(N0.getOperand(1))) {
+      SDValue Folded = DAG.FoldConstantArithmetic(ISD::SHL, SDLoc(N1), VT, N0C1, N1C);
+      return DAG.getNode(ISD::MUL, SDLoc(N), VT, N0.getOperand(0), Folded);
+    }
+  }
+
   if (N1C && !N1C->isOpaque())
     if (SDValue NewSHL = visitShiftByConstant(N, N1C))
       return NewSHL;