[DAGCombiner] sub X, 0/1 --> add X, 0/-1

This extends the existing transform for:
add X, 0/1 --> sub X, 0/-1
...to allow the sibling subtraction fold.

This pattern could regress with the proposed change in D57401.

llvm-svn: 352680

1 files changed, 22 insertions, 10 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 9837f5b4497..d2e235189ec 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2256,6 +2256,23 @@ static SDValue getAsCarry(const TargetLowering &TLI, SDValue V) {
   return SDValue();
 }
 
+/// Given the operands of an add/sub operation, see if the 2nd operand is a
+/// masked 0/1 whose source operand is actually known to be 0/-1. If so, invert
+/// the opcode and bypass the mask operation.
+static SDValue foldAddSubMasked1(bool IsAdd, SDValue N0, SDValue N1,
+                                 SelectionDAG &DAG, const SDLoc &DL) {
+  if (N1.getOpcode() != ISD::AND || !isOneOrOneSplat(N1->getOperand(1)))
+    return SDValue();
+
+  EVT VT = N0.getValueType();
+  if (DAG.ComputeNumSignBits(N1.getOperand(0)) != VT.getScalarSizeInBits())
+    return SDValue();
+
+  // add N0, (and (AssertSext X, i1), 1) --> sub N0, X
+  // sub N0, (and (AssertSext X, i1), 1) --> add N0, X
+  return DAG.getNode(IsAdd ? ISD::SUB : ISD::ADD, DL, VT, N0, N1.getOperand(0));
+}
+
 SDValue DAGCombiner::visitADDLike(SDValue N0, SDValue N1, SDNode *LocReference) {
   EVT VT = N0.getValueType();
   SDLoc DL(LocReference);
@@ -2268,16 +2285,8 @@ SDValue DAGCombiner::visitADDLike(SDValue N0, SDValue N1, SDNode *LocReference)
                                    N1.getOperand(0).getOperand(1),
                                    N1.getOperand(1)));
 
-  if (N1.getOpcode() == ISD::AND) {
-    SDValue AndOp0 = N1.getOperand(0);
-    unsigned NumSignBits = DAG.ComputeNumSignBits(AndOp0);
-    unsigned DestBits = VT.getScalarSizeInBits();
-
-    // (add z, (and (sbbl x, x), 1)) -> (sub z, (sbbl x, x))
-    // and similar xforms where the inner op is either ~0 or 0.
-    if (NumSignBits == DestBits && isOneOrOneSplat(N1->getOperand(1)))
-      return DAG.getNode(ISD::SUB, DL, VT, N0, AndOp0);
-  }
+  if (SDValue V = foldAddSubMasked1(true, N0, N1, DAG, DL))
+    return V;
 
   // add (sext i1), X -> sub X, (zext i1)
   if (N0.getOpcode() == ISD::SIGN_EXTEND &&
@@ -2727,6 +2736,9 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
   if (SDValue V = foldAddSubOfSignBit(N, DAG))
     return V;
 
+  if (SDValue V = foldAddSubMasked1(false, N0, N1, DAG, SDLoc(N)))
+    return V;
+
   // fold Y = sra (X, size(X)-1); sub (xor (X, Y), Y) -> (abs X)
   if (TLI.isOperationLegalOrCustom(ISD::ABS, VT)) {
     if (N0.getOpcode() == ISD::XOR && N1.getOpcode() == ISD::SRA) {