[clang] [Clang][ByteCode] Store type information inside the custom stack (PR #224903)

[Nikolas Klauser via cfe-commits]

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@@ -25,15 +25,34 @@ InterpStack::~InterpStack() {
     std::free(Chunk->Next);
   if (Chunk)
     std::free(Chunk);
+
+#if __has_cpp_attribute(no_unique_address)
+#ifdef __GNUC__
+#pragma GCC diagnostic push
+// Clang and GCC complain that `offsetof` isn't allowed on non-standard-layout
+// types. However, it works just fine.
+#pragma GCC diagnostic ignored "-Winvalid-offsetof"
+#endif
+  TYPE_SWITCH(PrimType(), {
+    using Frame = StackFrame<T>;
+    static_assert(offsetof(Frame, type) == sizeof(Frame) - 1);
+    // Currently we don't need to use extra memory to store the type information
+    // for any PrimType on 64 bit platforms. Nothing breaks if this changes, but
+    // it would result in 8 extra bytes used just for the type information.
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philnik777 wrote:

Yes, exactly. This is simply here to make sure it's a conscious decision to not have tail padding somewhere and not just someone thinking "it really doesn't matter where the padding is - it's unused anyway".

https://github.com/llvm/llvm-project/pull/224903