[clang] fix: compatible C++ empty record with align UB with gcc (PR #72197)

[Aaron Ballman via cfe-commits]

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@@ -296,10 +296,16 @@ bool CodeGen::isEmptyRecord(ASTContext &Context, QualType T, bool AllowArrays,
     return false;
 
   // If this is a C++ record, check the bases first.
-  if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD))
+  if (const CXXRecordDecl *CXXRD = dyn_cast<CXXRecordDecl>(RD)) {
     for (const auto &I : CXXRD->bases())
       if (!isEmptyRecord(Context, I.getType(), true, AsIfNoUniqueAddr))
         return false;
+    // C++ object size >= 1 byte, empty struct is 1 byte.
+    // FIXME: an alignment on a empty record is a UB, may just warning it，
+    // this code just want to compatible gcc.
+    if (Context.getTypeSize(T) > 8)
+      return false;
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AaronBallman wrote:

This is an ABI break; should we be adding ABI tags so you can get the old behavior if needed? (CC @rjmccall @efriedma-quic)

https://github.com/llvm/llvm-project/pull/72197