[PATCH] D148710: [DAGCombiner] Limit 'hoist add/sub binop w/ constant op' to the same divergency property

[Jay Foad via Phabricator via llvm-commits]

foad added inline comments.


================
Comment at: llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp:3874
   // Hoist one-use addition by non-opaque constant:
-  //   (x + C) - y  ->  (x - y) + C
-  if (N0.getOpcode() == ISD::ADD && N0.hasOneUse() &&
-      isConstantOrConstantVector(N0.getOperand(1), /*NoOpaques=*/true)) {
-    SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), N1);
-    return DAG.getNode(ISD::ADD, DL, VT, Sub, N0.getOperand(1));
-  }
-  // y - (x + C)  ->  (y - x) - C
-  if (N1.getOpcode() == ISD::ADD && N1.hasOneUse() &&
-      isConstantOrConstantVector(N1.getOperand(1), /*NoOpaques=*/true)) {
-    SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0, N1.getOperand(0));
-    return DAG.getNode(ISD::SUB, DL, VT, Sub, N1.getOperand(1));
-  }
-  // (x - C) - y  ->  (x - y) - C
-  // This is necessary because SUB(X,C) -> ADD(X,-C) doesn't work for vectors.
-  if (N0.getOpcode() == ISD::SUB && N0.hasOneUse() &&
-      isConstantOrConstantVector(N0.getOperand(1), /*NoOpaques=*/true)) {
-    SDValue Sub = DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), N1);
-    return DAG.getNode(ISD::SUB, DL, VT, Sub, N0.getOperand(1));
-  }
-  // (C - x) - y  ->  C - (x + y)
-  if (N0.getOpcode() == ISD::SUB && N0.hasOneUse() &&
-      isConstantOrConstantVector(N0.getOperand(0), /*NoOpaques=*/true)) {
-    SDValue Add = DAG.getNode(ISD::ADD, DL, VT, N0.getOperand(1), N1);
-    return DAG.getNode(ISD::SUB, DL, VT, N0.getOperand(0), Add);
+  if (N0->isDivergent() == N1->isDivergent()) {
+    //   (x + C) - y  ->  (x - y) + C
----------------
We also want to do this if x is divergent and y is uniform.


CHANGES SINCE LAST ACTION
  https://reviews.llvm.org/D148710/new/

https://reviews.llvm.org/D148710