DAGCombiner: Constant folding around pre-increment loads/stores

Previously, even when a pre-increment load or store was generated,
we often needed to keep a copy of the original base register for use
with other offsets. If all of these offsets are constants (including
the offset which was combined into the addressing mode), then this is
clearly unnecessary. This change adjusts these other offsets to use the
new incremented address.

git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@174746 91177308-0d34-0410-b5e6-96231b3b80d8

1 files changed, 89 insertions, 0 deletions

diff --git a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index d694bc782c..472919c3e3 100644
--- a/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -6917,6 +6917,16 @@ bool DAGCombiner::CombineToPreIndexedLoadStore(SDNode *N) {
   ISD::MemIndexedMode AM = ISD::UNINDEXED;
   if (!TLI.getPreIndexedAddressParts(N, BasePtr, Offset, AM, DAG))
     return false;
+
+  // Backends without true r+i pre-indexed forms may need to pass a
+  // constant base with a variable offset so that constant coercion
+  // will work with the patterns in canonical form.
+  bool Swapped = false;
+  if (isa<ConstantSDNode>(BasePtr)) {
+    std::swap(BasePtr, Offset);
+    Swapped = true;
+  }
+
   // Don't create a indexed load / store with zero offset.
   if (isa<ConstantSDNode>(Offset) &&
       cast<ConstantSDNode>(Offset)->isNullValue())
@@ -6942,6 +6952,48 @@ bool DAGCombiner::CombineToPreIndexedLoadStore(SDNode *N) {
       return false;
   }
 
+  // If the offset is a constant, there may be other adds of constants that
+  // can be folded with this one. We should do this to avoid having to keep
+  // a copy of the original base pointer.
+  SmallVector<SDNode *, 16> OtherUses;
+  if (isa<ConstantSDNode>(Offset))
+    for (SDNode::use_iterator I = BasePtr.getNode()->use_begin(),
+         E = BasePtr.getNode()->use_end(); I != E; ++I) {
+      SDNode *Use = *I;
+      if (Use == Ptr.getNode())
+        continue;
+
+      if (Use->isPredecessorOf(N))
+        continue;
+
+      if (Use->getOpcode() != ISD::ADD && Use->getOpcode() != ISD::SUB) {
+        OtherUses.clear();
+        break;
+      }
+
+      SDValue Op0 = Use->getOperand(0), Op1 = Use->getOperand(1);
+      if (Op1.getNode() == BasePtr.getNode())
+        std::swap(Op0, Op1);
+      assert(Op0.getNode() == BasePtr.getNode() &&
+             "Use of ADD/SUB but not an operand");
+
+      if (!isa<ConstantSDNode>(Op1)) {
+        OtherUses.clear();
+        break;
+      }
+
+      // FIXME: In some cases, we can be smarter about this.
+      if (Op1.getValueType() != Offset.getValueType()) {
+        OtherUses.clear();
+        break;
+      }
+
+      OtherUses.push_back(Use);
+    }
+
+  if (Swapped)
+    std::swap(BasePtr, Offset);
+
   // Now check for #3 and #4.
   bool RealUse = false;
 
@@ -6991,6 +7043,43 @@ bool DAGCombiner::CombineToPreIndexedLoadStore(SDNode *N) {
   // Finally, since the node is now dead, remove it from the graph.
   DAG.DeleteNode(N);
 
+  if (Swapped)
+    std::swap(BasePtr, Offset);
+
+  // Replace other uses of BasePtr that can be updated to use Ptr
+  for (unsigned i = 0, e = OtherUses.size(); i != e; ++i) {
+    unsigned OffsetIdx = 1;
+    if (OtherUses[i]->getOperand(OffsetIdx).getNode() == BasePtr.getNode())
+      OffsetIdx = 0;
+    assert(OtherUses[i]->getOperand(!OffsetIdx).getNode() ==
+           BasePtr.getNode() && "Expected BasePtr operand");
+
+    APInt OV =
+      cast<ConstantSDNode>(Offset)->getAPIntValue();
+    if (AM == ISD::PRE_DEC)
+      OV = -OV;
+
+    ConstantSDNode *CN =
+      cast<ConstantSDNode>(OtherUses[i]->getOperand(OffsetIdx));
+    APInt CNV = CN->getAPIntValue();
+    if (OtherUses[i]->getOpcode() == ISD::SUB && OffsetIdx == 1)
+      CNV += OV;
+    else
+      CNV -= OV;
+
+    SDValue NewOp1 = Result.getValue(isLoad ? 1 : 0);
+    SDValue NewOp2 = DAG.getConstant(CNV, CN->getValueType(0));
+    if (OffsetIdx == 0)
+      std::swap(NewOp1, NewOp2);
+
+    SDValue NewUse = DAG.getNode(OtherUses[i]->getOpcode(),
+                                 OtherUses[i]->getDebugLoc(),
+                                 OtherUses[i]->getValueType(0), NewOp1, NewOp2);
+    DAG.ReplaceAllUsesOfValueWith(SDValue(OtherUses[i], 0), NewUse);
+    removeFromWorkList(OtherUses[i]);
+    DAG.DeleteNode(OtherUses[i]);
+  }
+
   // Replace the uses of Ptr with uses of the updated base value.
   DAG.ReplaceAllUsesOfValueWith(Ptr, Result.getValue(isLoad ? 1 : 0));
   removeFromWorkList(Ptr.getNode());