[clang] [clang][CodeGen] Return RValue from `EmitVAArg` (PR #94635)

[John McCall via cfe-commits]

================
@@ -1328,15 +1328,15 @@ void AggExprEmitter::VisitChooseExpr(const ChooseExpr *CE) {
 
 void AggExprEmitter::VisitVAArgExpr(VAArgExpr *VE) {
   Address ArgValue = Address::invalid();
-  Address ArgPtr = CGF.EmitVAArg(VE, ArgValue);
+  RValue ArgPtr = CGF.EmitVAArg(VE, ArgValue);
 
   // If EmitVAArg fails, emit an error.
-  if (!ArgPtr.isValid()) {
+  if (!ArgValue.isValid()) {
     CGF.ErrorUnsupported(VE, "aggregate va_arg expression");
     return;
   }
 
-  EmitFinalDestCopy(VE->getType(), CGF.MakeAddrLValue(ArgPtr, VE->getType()));
+  EmitFinalDestCopy(VE->getType(), ArgPtr);
----------------
rjmccall wrote:

CodeGen handles aggregates by keeping them in memory.  Everything to do with aggregate expression emission generally works by passing around a destination address that the value should be placed in — you evaluate the expression into that address, then just put that address into the `RValue` you return.  If you don't pass an address down, generally operations have to evaluate into a temporary, which can create redundant `memcpy`s.

In `AggExprEmitter`, the destination address is the `Dest` member.  We may just be doing an assignment into `Dest` and not an initialization, though, which has important semantic differences in some cases and is (confusingly, sorry) tracked with `.isPotentiallyAliased()`. The easiest way to handle that is to make sure the final copy is done with `EmitFinalDestCopy`.  You'll need to add a method like this to `CodeGenFunction`:

```c++
void EmitAggFinalDestCopy(QualType type, AggValueSlot dest, const LValue &src, ExprValueKind srcKind);
```

which makes an `AggExprEmitter` and calls `EmitFinalDestCopy` on it.

https://github.com/llvm/llvm-project/pull/94635