[clang] [llvm] [BPF] Handle C++ records in BTF and CO-RE (PR #233715)

[Tamir Duberstein via cfe-commits]

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@@ -315,6 +315,14 @@ Address CodeGenFunction::GetAddressOfBaseClass(
     VBase = nullptr; // we no longer have a virtual step
   }
 
+  // A BPF CO-RE relocation is relative to the record that has the field, so
+  // the offset of the base subobject, which may differ in the target, would not
+  // be relocated.
+  if (IsInPreservedAIRegion && getTarget().getTriple().isBPF()) {
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tamird wrote:

The rejection needs to track base adjustments on the preserved address path. It misses the inverse conversion: for `struct A { int a; }; struct B { int b; }; struct D : A, B { int x; };`, `__builtin_preserve_access_index(&static_cast<D *>(p)->x)` with `B *p` hardcodes the local `B`-to-`D` adjustment of `-4` in `GetAddressOfDerivedClass()`. If the target changes to `D : B, A`, `B` moves to offset 0 while `x` stays at offset 8; the `D`-rooted relocation still yields an address four bytes short. The same guard also rejects `__builtin_preserve_access_index((consume(d), &d->x))` for `D *d` and `void consume(A *)`, even though the upcast belongs only to the side-effect call. Please reject or relocate base adjustments that contribute to the preserved address, while allowing unrelated conversions.

https://github.com/llvm/llvm-project/pull/233715