[clang][Interp] Support AddOffset with 128bit offsets (llvm#68679)

We do a similar thing a few lines above for `Index`:

```c++
  // Get a version of the index comparable to the type.
  T Index = T::from(Ptr.getIndex(), Offset.bitWidth());
```

Author: tbaederr

clang/lib/AST/Interp/Boolean.h

@@ -42,6 +42,7 @@ class Boolean final {
   bool operator>(unsigned RHS) const { return static_cast<unsigned>(V) > RHS; }
 
   Boolean operator-() const { return Boolean(V); }
+  Boolean operator-(const Boolean &Other) const { return Boolean(V - Other.V); }
   Boolean operator~() const { return Boolean(true); }
 
   explicit operator int8_t() const { return V; }

clang/lib/AST/Interp/Integral.h

@@ -88,6 +88,9 @@ template <unsigned Bits, bool Signed> class Integral final {
   }
 
   Integral operator-() const { return Integral(-V); }
+  Integral operator-(const Integral &Other) const {
+    return Integral(V - Other.V);
+  }
   Integral operator~() const { return Integral(~V); }
 
   template <unsigned DstBits, bool DstSign>

clang/lib/AST/Interp/IntegralAP.h

@@ -59,6 +59,9 @@ template <bool Signed> class IntegralAP final {
   IntegralAP() : V(APSInt::getMaxValue(1024, Signed)) {}
 
   IntegralAP operator-() const { return IntegralAP(-V); }
+  IntegralAP operator-(const IntegralAP &Other) const {
+    return IntegralAP(V - Other.V);
+  }
   bool operator>(IntegralAP RHS) const { return V > RHS.V; }
   bool operator>=(IntegralAP RHS) const { return V >= RHS.V; }
   bool operator<(IntegralAP RHS) const { return V < RHS.V; }

clang/lib/AST/Interp/Interp.h

@@ -1421,7 +1421,7 @@ bool OffsetHelper(InterpState &S, CodePtr OpPC, const T &Offset,
   // Get a version of the index comparable to the type.
   T Index = T::from(Ptr.getIndex(), Offset.bitWidth());
   // Compute the largest index into the array.
-  unsigned MaxIndex = Ptr.getNumElems();
+  T MaxIndex = T::from(Ptr.getNumElems(), Offset.bitWidth());
 
   // Helper to report an invalid offset, computed as APSInt.
   auto InvalidOffset = [&]() {
@@ -1437,7 +1437,7 @@ bool OffsetHelper(InterpState &S, CodePtr OpPC, const T &Offset,
     return false;
   };
 
-  unsigned MaxOffset = MaxIndex - Ptr.getIndex();
+  T MaxOffset = T::from(MaxIndex - Index, Offset.bitWidth());
   if constexpr (Op == ArithOp::Add) {
     // If the new offset would be negative, bail out.
     if (Offset.isNegative() && (Offset.isMin() || -Offset > Index))

clang/test/AST/Interp/intap.cpp

@@ -90,4 +90,14 @@ namespace i128 {
                                            // expected-error {{must be initialized by a constant expression}} \
                                            // expected-note {{is outside the range of representable values of type}}
 }
+
+namespace AddSubOffset {
+  constexpr __int128 A = 1;
+  constexpr int arr[] = {1,2,3};
+  constexpr const int *P = arr + A;
+  static_assert(*P == 2, "");
+  constexpr const int *P2 = P - A;
+  static_assert(*P2 == 1,"");
+}
+
 #endif