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

[Eli Friedman via cfe-commits]

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@@ -3014,6 +3014,16 @@ 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()) {
+    CharUnits Align = CGF.getContext().getTypeAlignInChars(Ty);
+    VAListAddr = CGF.Builder.CreateStructGEP(VAListAddr, 2);
+    llvm::Value *Load = CGF.Builder.CreateLoad(VAListAddr);
+    llvm::Value *Offset = llvm::ConstantInt::get(CGF.Int32Ty, 0);
+    Load = CGF.Builder.CreateGEP(CGF.Int8Ty, Load, Offset);
+    return Address(Load, CGF.ConvertTypeForMem(Ty), Align);
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efriedma-quic wrote:

I don't think the address here is guaranteed to be appropriately aligned.  It's probably better to create a temporary like I suggested before.

Also, a GEP with offset 0 does nothing.

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