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

[Timm Baeder via cfe-commits]

https://github.com/tbaederr created https://github.com/llvm/llvm-project/pull/68679

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

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

>From 10d205da4c220227589ce210bf921f86ead4d05f Mon Sep 17 00:00:00 2001
From: =?UTF-8?q?Timm=20B=C3=A4der?= <tbaeder at redhat.com>
Date: Tue, 10 Oct 2023 08:52:43 +0200
Subject: [PATCH] [clang][Interp] Support AddOffset with 128bit offsets

---
 clang/lib/AST/Interp/Interp.h   | 2 +-
 clang/test/AST/Interp/intap.cpp | 9 +++++++++
 2 files changed, 10 insertions(+), 1 deletion(-)

diff --git a/clang/lib/AST/Interp/Interp.h b/clang/lib/AST/Interp/Interp.h
index 47dc1d08c9c4d8b..620d797da27023f 100644
--- a/clang/lib/AST/Interp/Interp.h
+++ b/clang/lib/AST/Interp/Interp.h
@@ -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 - Ptr.getIndex(), Offset.bitWidth());
   if constexpr (Op == ArithOp::Add) {
     // If the new offset would be negative, bail out.
     if (Offset.isNegative() && (Offset.isMin() || -Offset > Index))
diff --git a/clang/test/AST/Interp/intap.cpp b/clang/test/AST/Interp/intap.cpp
index b3f02d2b769531d..e9e68438bb597aa 100644
--- a/clang/test/AST/Interp/intap.cpp
+++ b/clang/test/AST/Interp/intap.cpp
@@ -76,6 +76,15 @@ namespace i128 {
                                            // 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,"");
+}
+
 #else
 /// No int128 support, so no expected directives.