[NFC] Add a SDValue overload for SelectionDAG::getMemBasePlusOffset()

Summary:
This change is preparatory work to use this helper functions in more places.
Currently the function only allows integer constants offsets, but there
are cases where we can use an existing SDValue parameter.

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, craig.topper

Reviewed By: spatel, craig.topper

Subscribers: craig.topper, merge_guards_bot, hiraditya, llvm-commits

Tags: #llvm

Differential Revision: https://reviews.llvm.org/D71205

2 files changed, 9 insertions, 1 deletions

diff --git a/llvm/include/llvm/CodeGen/SelectionDAG.h b/llvm/include/llvm/CodeGen/SelectionDAG.h
index ffc4f8e2f01..7f331d15c8a 100644
--- a/llvm/include/llvm/CodeGen/SelectionDAG.h
+++ b/llvm/include/llvm/CodeGen/SelectionDAG.h
@@ -1140,6 +1140,7 @@ public:
   /// Returns sum of the base pointer and offset.
   /// Unlike getObjectPtrOffset this does not set NoUnsignedWrap by default.
   SDValue getMemBasePlusOffset(SDValue Base, int64_t Offset, const SDLoc &DL);
+  SDValue getMemBasePlusOffset(SDValue Base, SDValue Offset, const SDLoc &DL);
 
   SDValue getMaskedLoad(EVT VT, const SDLoc &dl, SDValue Chain, SDValue Base,
                         SDValue Offset, SDValue Mask, SDValue Src0, EVT MemVT,
diff --git a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
index 378e3367ec7..3379d5b0a09 100644
--- a/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
+++ b/llvm/lib/CodeGen/SelectionDAG/SelectionDAG.cpp
@@ -5735,7 +5735,14 @@ static SDValue getMemsetStringVal(EVT VT, const SDLoc &dl, SelectionDAG &DAG,
 SDValue SelectionDAG::getMemBasePlusOffset(SDValue Base, int64_t Offset,
                                            const SDLoc &DL) {
   EVT VT = Base.getValueType();
-  return getNode(ISD::ADD, DL, VT, Base, getConstant(Offset, DL, VT));
+  return getMemBasePlusOffset(Base, getConstant(Offset, DL, VT), DL);
+}
+
+SDValue SelectionDAG::getMemBasePlusOffset(SDValue Ptr, SDValue Offset,
+                                           const SDLoc &DL) {
+  assert(Offset.getValueType().isInteger());
+  EVT BasePtrVT = Ptr.getValueType();
+  return getNode(ISD::ADD, DL, BasePtrVT, Ptr, Offset);
 }
 
 /// Returns true if memcpy source is constant data.