[clang][Interp][NFC] Refector OffsetHelper

There was a FIXME comment for this. Stop getting the values in
OffsetHelper and let the caller do that instead, so we can control
whether the value(s) are removed from the stack at all.

Also use ArithOp instead of the unclear boolean for Add.

Author: tbaederr

clang/lib/AST/Interp/Interp.h

@@ -1082,11 +1082,9 @@ bool InitElemPop(InterpState &S, CodePtr OpPC, uint32_t Idx) {
 // AddOffset, SubOffset
 //===----------------------------------------------------------------------===//
 
-template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
-  // Fetch the pointer and the offset.
-  const T &Offset = S.Stk.pop<T>();
-  const Pointer &Ptr = S.Stk.pop<Pointer>();
-
+template <class T, ArithOp Op>
+bool OffsetHelper(InterpState &S, CodePtr OpPC, const T &Offset,
+                  const Pointer &Ptr) {
   if (!CheckRange(S, OpPC, Ptr, CSK_ArrayToPointer))
     return false;
 
@@ -1113,7 +1111,8 @@ template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
     const unsigned Bits = Offset.bitWidth();
     APSInt APOffset(Offset.toAPSInt().extend(Bits + 2), false);
     APSInt APIndex(Index.toAPSInt().extend(Bits + 2), false);
-    APSInt NewIndex = Add ? (APIndex + APOffset) : (APIndex - APOffset);
+    APSInt NewIndex =
+        (Op == ArithOp::Add) ? (APIndex + APOffset) : (APIndex - APOffset);
     S.CCEDiag(S.Current->getSource(OpPC), diag::note_constexpr_array_index)
         << NewIndex
         << /*array*/ static_cast<int>(!Ptr.inArray())
@@ -1122,7 +1121,7 @@ template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
   };
 
   unsigned MaxOffset = MaxIndex - Ptr.getIndex();
-  if constexpr (Add) {
+  if constexpr (Op == ArithOp::Add) {
     // If the new offset would be negative, bail out.
     if (Offset.isNegative() && (Offset.isMin() || -Offset > Index))
       return InvalidOffset();
@@ -1144,7 +1143,7 @@ template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
   int64_t WideIndex = static_cast<int64_t>(Index);
   int64_t WideOffset = static_cast<int64_t>(Offset);
   int64_t Result;
-  if constexpr (Add)
+  if constexpr (Op == ArithOp::Add)
     Result = WideIndex + WideOffset;
   else
     Result = WideIndex - WideOffset;
@@ -1155,12 +1154,16 @@ template <class T, bool Add> bool OffsetHelper(InterpState &S, CodePtr OpPC) {
 
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool AddOffset(InterpState &S, CodePtr OpPC) {
-  return OffsetHelper<T, true>(S, OpPC);
+  const T &Offset = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  return OffsetHelper<T, ArithOp::Add>(S, OpPC, Offset, Ptr);
 }
 
 template <PrimType Name, class T = typename PrimConv<Name>::T>
 bool SubOffset(InterpState &S, CodePtr OpPC) {
-  return OffsetHelper<T, false>(S, OpPC);
+  const T &Offset = S.Stk.pop<T>();
+  const Pointer &Ptr = S.Stk.pop<Pointer>();
+  return OffsetHelper<T, ArithOp::Sub>(S, OpPC, Offset, Ptr);
 }
 
 template <ArithOp Op>
@@ -1172,10 +1175,9 @@ static inline bool IncDecPtrHelper(InterpState &S, CodePtr OpPC) {
   S.Stk.push<Pointer>(Ptr.deref<Pointer>());
 
   // Now the current Ptr again and a constant 1.
-  // FIXME: We shouldn't have to push these two on the stack.
-  S.Stk.push<Pointer>(Ptr.deref<Pointer>());
-  S.Stk.push<OneT>(OneT::from(1));
-  if (!OffsetHelper<OneT, Op == ArithOp::Add>(S, OpPC))
+  Pointer P = Ptr.deref<Pointer>();
+  OneT One = OneT::from(1);
+  if (!OffsetHelper<OneT, Op>(S, OpPC, One, P))
     return false;
 
   // Store the new value.