[SelectionDAGBuilder] Use address width when lowering ptrtoaddr

Instead of just deferring to ptrtoint, we should truncate to the index
width and then perform the ZextOrTrunc.
This is effectively NFC since ptrtoint ends up doing the same thing, but
handling it explicitly is cleaner and will make it easier to eventually
upstream the changes needed for CHERI support.

Reviewed By: nikic, arsenm

Pull Request: llvm/llvm-project#139423

Author: arichardson

llvm/lib/CodeGen/SelectionDAG/SelectionDAGBuilder.cpp

@@ -3971,8 +3971,14 @@ void SelectionDAGBuilder::visitSIToFP(const User &I) {
 }
 
 void SelectionDAGBuilder::visitPtrToAddr(const User &I) {
-  // FIXME: this is not correct for pointers with addr width != pointer width
-  visitPtrToInt(I);
+  SDValue N = getValue(I.getOperand(0));
+  // By definition the type of the ptrtoaddr must be equal to the address type.
+  const auto &TLI = DAG.getTargetLoweringInfo();
+  EVT AddrVT = TLI.getValueType(DAG.getDataLayout(), I.getType());
+  // The address width must be smaller or equal to the pointer representation
+  // width, so we lower ptrtoaddr as a truncate (possibly folded to a no-op).
+  N = DAG.getNode(ISD::TRUNCATE, getCurSDLoc(), AddrVT, N);
+  setValue(&I, N);
 }
 
 void SelectionDAGBuilder::visitPtrToInt(const User &I) {