[Codegen][X86][AArch64][ARM][PowerPC] Inc-of-add vs sub-of-not (PR42457)

Summary:
This is the backend part of [[ https://bugs.llvm.org/show_bug.cgi?id=42457 | PR42457 ]].
In middle-end, we'd want to prefer the form with two adds - D63992,
but as this diff shows, not every target will prefer that pattern.

Out of 4 targets for which i added tests all seem to be ok with inc-of-add for scalars,
but only X86 prefer that same pattern for vectors.

Here i'm adding a new TLI hook, always defaulting to the inc-of-add,
but adding AArch64,ARM,PowerPC overrides to prefer inc-of-add only for scalars.

Reviewers: spatel, RKSimon, efriedma, t.p.northover, hfinkel

Reviewed By: efriedma

Subscribers: nemanjai, javed.absar, kristof.beyls, kbarton, jsji, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D64090

llvm-svn: 365010

1 files changed, 31 insertions, 0 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 2bfc3979360..3bd4fe277be 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2424,6 +2424,17 @@ SDValue DAGCombiner::visitADDLike(SDNode *N) {
       if (Xor)
         return DAG.getNode(ISD::SUB, DL, VT, A, Xor.getOperand(0));
     }
+
+    // Look for:
+    //   add (add x, y), 1
+    // And if the target does not like this form then turn into:
+    //   sub y, (xor x, -1)
+    if (!TLI.preferIncOfAddToSubOfNot(VT) && N0.hasOneUse() &&
+        N0.getOpcode() == ISD::ADD) {
+      SDValue Not = DAG.getNode(ISD::XOR, DL, VT, N0.getOperand(0),
+                                DAG.getAllOnesConstant(DL, VT));
+      return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(1), Not);
+    }
   }
 
   // (x - y) + -1  ->  add (xor y, -1), x
@@ -2584,6 +2595,17 @@ SDValue DAGCombiner::visitADDLikeCommutative(SDValue N0, SDValue N1,
   if (SDValue V = foldAddSubMasked1(true, N0, N1, DAG, DL))
     return V;
 
+  // Look for:
+  //   add (add x, 1), y
+  // And if the target does not like this form then turn into:
+  //   sub y, (xor x, -1)
+  if (!TLI.preferIncOfAddToSubOfNot(VT) && N0.hasOneUse() &&
+      N0.getOpcode() == ISD::ADD && isOneOrOneSplat(N0.getOperand(1))) {
+    SDValue Not = DAG.getNode(ISD::XOR, DL, VT, N0.getOperand(0),
+                              DAG.getAllOnesConstant(DL, VT));
+    return DAG.getNode(ISD::SUB, DL, VT, N1, Not);
+  }
+
   // Hoist one-use subtraction by non-opaque constant:
   //   (x - C) + y  ->  (x + y) - C
   // This is necessary because SUB(X,C) -> ADD(X,-C) doesn't work for vectors.
@@ -3108,6 +3130,15 @@ SDValue DAGCombiner::visitSUB(SDNode *N) {
     return DAG.getNode(ISD::ADD, DL, VT, Xor, N0.getOperand(0));
   }
 
+  // Look for:
+  //   sub y, (xor x, -1)
+  // And if the target does not like this form then turn into:
+  //   add (add x, y), 1
+  if (TLI.preferIncOfAddToSubOfNot(VT) && N1.hasOneUse() && isBitwiseNot(N1)) {
+    SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N0, N1.getOperand(0));
+    return DAG.getNode(ISD::ADD, DL, VT, Add, DAG.getConstant(1, DL, VT));
+  }
+
   // Hoist one-use addition by non-opaque constant:
   //   (x + C) - y  ->  (x - y) + C
   if (N0.hasOneUse() && N0.getOpcode() == ISD::ADD &&