[DAGCombiner] add (sext i1 X), 1 --> zext (not i1 X)

Besides better codegen, the motivation is to be able to canonicalize this pattern 
in IR (currently we don't) knowing that the backend is prepared for that.

This may also allow removing code for special constant cases in 
DAGCombiner::foldSelectOfConstants() that was added in D30180.

Differential Revision: https://reviews.llvm.org/D31944

llvm-svn: 301457

1 files changed, 23 insertions, 6 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 1251ae6262b..0c289714c8f 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -1866,14 +1866,31 @@ SDValue DAGCombiner::visitADD(SDNode *N) {
   if (isNullConstant(N1))
     return N0;
 
-  // fold ((c1-A)+c2) -> (c1+c2)-A
   if (isConstantOrConstantVector(N1, /* NoOpaque */ true)) {
-    if (N0.getOpcode() == ISD::SUB)
-      if (isConstantOrConstantVector(N0.getOperand(0), /* NoOpaque */ true)) {
-        return DAG.getNode(ISD::SUB, DL, VT,
-                           DAG.getNode(ISD::ADD, DL, VT, N1, N0.getOperand(0)),
-                           N0.getOperand(1));
+    // fold ((c1-A)+c2) -> (c1+c2)-A
+    if (N0.getOpcode() == ISD::SUB &&
+        isConstantOrConstantVector(N0.getOperand(0), /* NoOpaque */ true)) {
+      // FIXME: Adding 2 constants should be handled by FoldConstantArithmetic.
+      return DAG.getNode(ISD::SUB, DL, VT,
+                         DAG.getNode(ISD::ADD, DL, VT, N1, N0.getOperand(0)),
+                         N0.getOperand(1));
+    }
+
+    // add (sext i1 X), 1 -> zext (not i1 X)
+    // We don't transform this pattern:
+    //   add (zext i1 X), -1 -> sext (not i1 X)
+    // because most (?) targets generate better code for the zext form.
+    if (N0.getOpcode() == ISD::SIGN_EXTEND && N0.hasOneUse() &&
+        isOneConstantOrOneSplatConstant(N1)) {
+      SDValue X = N0.getOperand(0);
+      if ((!LegalOperations ||
+           (TLI.isOperationLegal(ISD::XOR, X.getValueType()) &&
+            TLI.isOperationLegal(ISD::ZERO_EXTEND, VT))) &&
+          X.getScalarValueSizeInBits() == 1) {
+        SDValue Not = DAG.getNOT(DL, X, X.getValueType());
+        return DAG.getNode(ISD::ZERO_EXTEND, DL, VT, Not);
       }
+    }
   }
 
   if (SDValue NewSel = foldBinOpIntoSelect(N))