[DAGCombiner] Set disjoint flag in add->or and xor->or combines

[lukel97]

We check DAG.haveNoCommonBitsSet so the operands will be known to be disjoint.

I couldn't think of a codegen test case since most targets aren't checking hasDisjoint yet, apart from RISCV in the or_is_add pattern, but it also falls back to computeKnownBits.

[llvmbot]

@llvm/pr-subscribers-llvm-selectiondag

Author: Luke Lau (lukel97)

Changes

We check DAG.haveNoCommonBitsSet so the operands will be known to be disjoint.

I couldn't think of a codegen test case since most targets aren't checking hasDisjoint yet, apart from RISCV in the or_is_add pattern, but it also falls back to computeKnownBits.

---

Full diff: https://github.com/llvm/llvm-project/pull/86925.diff

1 Files Affected:

• (modified) llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp (+10-4)

diff --git a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
index 36abe27d262176..6dd3fbb3c97e0c 100644
--- a/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/DAGCombiner.cpp
@@ -2887,8 +2887,11 @@ SDValue DAGCombiner::visitADD(SDNode *N) {
 
   // fold (a+b) -> (a|b) iff a and b share no bits.
   if ((!LegalOperations || TLI.isOperationLegal(ISD::OR, VT)) &&
-      DAG.haveNoCommonBitsSet(N0, N1))
-    return DAG.getNode(ISD::OR, DL, VT, N0, N1);
+      DAG.haveNoCommonBitsSet(N0, N1)) {
+    SDNodeFlags Flags;
+    Flags.setDisjoint(true);
+    return DAG.getNode(ISD::OR, DL, VT, N0, N1, Flags);
+  }
 
   // Fold (add (vscale * C0), (vscale * C1)) to (vscale * (C0 + C1)).
   if (N0.getOpcode() == ISD::VSCALE && N1.getOpcode() == ISD::VSCALE) {
@@ -9289,8 +9292,11 @@ SDValue DAGCombiner::visitXOR(SDNode *N) {
 
   // fold (a^b) -> (a|b) iff a and b share no bits.
   if ((!LegalOperations || TLI.isOperationLegal(ISD::OR, VT)) &&
-      DAG.haveNoCommonBitsSet(N0, N1))
-    return DAG.getNode(ISD::OR, DL, VT, N0, N1);
+      DAG.haveNoCommonBitsSet(N0, N1)) {
+    SDNodeFlags Flags;
+    Flags.setDisjoint(true);
+    return DAG.getNode(ISD::OR, DL, VT, N0, N1, Flags);
+  }
 
   // look for 'add-like' folds:
   // XOR(N0,MIN_SIGNED_VALUE) == ADD(N0,MIN_SIGNED_VALUE)

[arsenm]

Side note, we should fix the bad API of SDNodeFlags

[lukel97]

Yeah, this was gnarly but I just tried to follow what we do for nonneg