[X86] Delay creating index register negations during address matching until after we know for sure the match will succeed

If we're trying to match an LEA, its possible the LEA match will be deemed unprofitable. In which case the negation we created in matchAddress would be left dangling in the SelectionDAG. This could artificially increase use counts for other nodes in the DAG. Though I don't have an example of that. But it just seems like bad form to have dangling nodes in isel.

Differential Revision: https://reviews.llvm.org/D61047

llvm-svn: 360823

1 files changed, 15 insertions, 7 deletions

diff --git a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
index 3c6dfb45a56..1bc7af73a48 100644
--- a/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
+++ b/llvm/lib/Target/X86/X86ISelDAGToDAG.cpp
@@ -73,6 +73,7 @@ namespace {
     int JT;
     unsigned Align;    // CP alignment.
     unsigned char SymbolFlags;  // X86II::MO_*
+    bool NegateIndex = false;
 
     X86ISelAddressMode()
         : BaseType(RegBase), Base_FrameIndex(0), Scale(1), IndexReg(), Disp(0),
@@ -115,6 +116,8 @@ namespace {
         dbgs() << " Base.FrameIndex " << Base_FrameIndex << '\n';
       dbgs() << " Scale " << Scale << '\n'
              << "IndexReg ";
+      if (NegateIndex)
+        dbgs() << "negate ";
       if (IndexReg.getNode())
         IndexReg.getNode()->dump(DAG);
       else
@@ -271,6 +274,14 @@ namespace {
 
       Scale = getI8Imm(AM.Scale, DL);
 
+      // Negate the index if needed.
+      if (AM.NegateIndex) {
+        unsigned NegOpc = VT == MVT::i64 ? X86::NEG64r : X86::NEG32r;
+        SDValue Neg = SDValue(CurDAG->getMachineNode(NegOpc, DL, VT, MVT::i32,
+                                                     AM.IndexReg), 0);
+        AM.IndexReg = Neg;
+      }
+
       if (AM.IndexReg.getNode())
         Index = AM.IndexReg;
       else
@@ -1863,14 +1874,11 @@ bool X86DAGToDAGISel::matchAddressRecursively(SDValue N, X86ISelAddressMode &AM,
     }
 
     // Ok, the transformation is legal and appears profitable. Go for it.
-    SDValue Zero = CurDAG->getConstant(0, dl, N.getValueType());
-    SDValue Neg = CurDAG->getNode(ISD::SUB, dl, N.getValueType(), Zero, RHS);
-    AM.IndexReg = Neg;
+    // Negation will be emitted later to avoid creating dangling nodes if this
+    // was an unprofitable LEA.
+    AM.IndexReg = RHS;
+    AM.NegateIndex = true;
     AM.Scale = 1;
-
-    // Insert the new nodes into the topological ordering.
-    insertDAGNode(*CurDAG, N, Zero);
-    insertDAGNode(*CurDAG, N, Neg);
     return false;
   }