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

[Jessica Clarke via llvm-commits]

================
@@ -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:
+    //   * (ptradd x, y) has one use; and
+    //   * y is a constant; and
+    //   * z is not a constant.
+    // Serves to expose constant y for subsequent folding.
+    if (N0OneUse && YIsConstant && !ZIsConstant) {
+      SDValue Add = DAG.getNode(ISD::PTRADD, DL, IntVT, {X, Z});
+
+      // Calling visit() can replace the Add node with ISD::DELETED_NODE if
+      // there aren't any users, so keep a handle around whilst we visit it.
+      HandleSDNode ADDHandle(Add);
+
+      SDValue VisitedAdd = visit(Add.getNode());
+      if (VisitedAdd) {
+        // If visit() returns the same node, it means the SDNode was RAUW'd, and
+        // therefore we have to load the new value to perform the checks whether
+        // the reassociation fold is profitable.
+        if (VisitedAdd.getNode() == Add.getNode())
+          Add = ADDHandle.getValue();
+        else
+          Add = VisitedAdd;
+      }
+
+      return DAG.getMemBasePlusOffset(Add, Y, DL, SDNodeFlags());
+    }
+
+    bool ZOneUse = Z.hasOneUse();
+
+    // (ptradd (ptradd x, y), z) -> (ptradd x, (add y, z)) if:
+    //   * x is a null pointer; or
+    //   * y is a constant and z has one use; or
----------------
jrtc27 wrote:

Could you explain the reasoning behind dropping a couple of the conditions present in CHERI LLVM? I spent quite a while at the time tweaking them to catch everything I could reasonably throw at it, but there could be redundancy in the final version.

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