[NFC] Use SelectionDAG::getMemBasePlusOffset() instead of getNode(ISD::ADD)

Summary:
To find potential opportunities to use getMemBasePlusOffset() I looked at
all ISD::ADD uses found with the regex getNode\(ISD::ADD,.+,.+Ptr
in lib/CodeGen/SelectionDAG. If this patch is accepted I will convert
the files in the individual backends too.

The motivation for this change is our out-of-tree CHERI backend
(https://github.com/CTSRD-CHERI/llvm-project). We use a separate register
type to store pointers (128-bit capabilities, which are effectively
unforgeable and monotonic fat pointers). These capabilities permit a
reduced set of operations and therefore use a separate ValueType (iFATPTR).
to represent pointers implemented as capabilities.
Therefore, we need to avoid using ISD::ADD for our patterns that operate
on pointers and need to use a function that chooses ISD::ADD or a new
ISD::PTRADD opcode depending on the value type.

We originally added a new DAG.getPointerAdd() function, but after this
patch series we can modify the implementation of getMemBasePlusOffset()
instead. Avoiding direct uses of ISD::ADD for pointer types will
significantly reduce the amount of assertion/instruction selection
failures for us in future upstream merges.

Reviewers: spatel

Reviewed By: spatel

Subscribers: merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71207

1 files changed, 2 insertions, 4 deletions

diff --git a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
index 53f5c1c547e..5a239290198 100644
--- a/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/TargetLowering.cpp
@@ -3282,11 +3282,9 @@ SDValue TargetLowering::SimplifySetCC(EVT VT, SDValue N0, SDValue N1,
         EVT newVT = EVT::getIntegerVT(*DAG.getContext(), bestWidth);
         if (newVT.isRound() &&
             shouldReduceLoadWidth(Lod, ISD::NON_EXTLOAD, newVT)) {
-          EVT PtrType = Lod->getOperand(1).getValueType();
           SDValue Ptr = Lod->getBasePtr();
           if (bestOffset != 0)
-            Ptr = DAG.getNode(ISD::ADD, dl, PtrType, Lod->getBasePtr(),
-                              DAG.getConstant(bestOffset, dl, PtrType));
+            Ptr = DAG.getMemBasePlusOffset(Ptr, bestOffset, dl);
           unsigned NewAlign = MinAlign(Lod->getAlignment(), bestOffset);
           SDValue NewLoad = DAG.getLoad(
               newVT, dl, Lod->getChain(), Ptr,
@@ -6906,7 +6904,7 @@ SDValue TargetLowering::getVectorElementPointer(SelectionDAG &DAG,
 
   Index = DAG.getNode(ISD::MUL, dl, IdxVT, Index,
                       DAG.getConstant(EltSize, dl, IdxVT));
-  return DAG.getNode(ISD::ADD, dl, IdxVT, VecPtr, Index);
+  return DAG.getMemBasePlusOffset(VecPtr, Index, dl);
 }
 
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