[llvm] [AArch64][SelectionDAG] Add CodeGen support for scalar FEAT_CPA (PR #105669)

[Rodolfo Wottrich via llvm-commits]

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@@ -2578,6 +2581,98 @@ SDValue DAGCombiner::foldSubToAvg(SDNode *N, const SDLoc &DL) {
   return SDValue();
 }
 
+/// Try to fold a pointer arithmetic node.
+/// This needs to be done separately from normal addition, because pointer
+/// addition is not commutative.
+/// This function was adapted from DAGCombiner::visitPTRADD() from the Morello
+/// project, which is based on CHERI.
+SDValue DAGCombiner::visitPTRADD(SDNode *N) {
+  SDValue N0 = N->getOperand(0);
+  SDValue N1 = N->getOperand(1);
+  EVT PtrVT = N0.getValueType();
+  EVT IntVT = N1.getValueType();
+  SDLoc DL(N);
+
+  // fold (ptradd undef, y) -> undef
+  if (N0.isUndef())
+    return N0;
+
+  // fold (ptradd x, undef) -> undef
+  if (N1.isUndef())
+    return DAG.getUNDEF(PtrVT);
+
+  // fold (ptradd x, 0) -> x
+  if (isNullConstant(N1))
+    return N0;
+
+  if (N0.getOpcode() == ISD::PTRADD &&
+      !reassociationCanBreakAddressingModePattern(ISD::PTRADD, DL, N, N0, N1)) {
+    SDValue X = N0.getOperand(0);
+    SDValue Y = N0.getOperand(1);
+    SDValue Z = N1;
+    bool N0OneUse = N0.hasOneUse();
+    bool YIsConstant = DAG.isConstantIntBuildVectorOrConstantInt(Y);
+    bool ZIsConstant = DAG.isConstantIntBuildVectorOrConstantInt(Z);
+
+    // (ptradd (ptradd x, y), z) -> (ptradd (ptradd x, z), y) if:
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rgwott wrote:

I have been scratching my head because of this. I see your point and agree, although this is probably a rare corner case. If I understand correctly, the only choice we have is to remove this fold completely, as we cannot know the sign of the variable during codegen.

https://github.com/llvm/llvm-project/pull/105669