[clang] [X86_64] fix empty structure vaarg in c++ (PR #77907)

[Phoebe Wang via cfe-commits]

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@@ -3014,6 +3014,11 @@ Address X86_64ABIInfo::EmitVAArg(CodeGenFunction &CGF, Address VAListAddr,
   ABIArgInfo AI = classifyArgumentType(Ty, 0, neededInt, neededSSE,
                                        /*isNamedArg*/false);
 
+  // Empty records are ignored for parameter passing purposes.
+  if (AI.isIgnore()) {
+    return CGF.CreateMemTemp(Ty);
+  }
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phoebewang wrote:

Does it behave differently between C and C++? Maybe adding a C test for it?

Besides, do not use parentheses for single line code.

https://github.com/llvm/llvm-project/pull/77907