[clang] [HLSL] Implement output parameter (PR #101083)

[Damyan Pepper via cfe-commits]

================
@@ -4689,6 +4719,32 @@ void CodeGenFunction::EmitCallArg(CallArgList &args, const Expr *E,
   assert(type->isReferenceType() == E->isGLValue() &&
          "reference binding to unmaterialized r-value!");
 
+  // Add writeback for HLSLOutParamExpr.
+  if (const HLSLOutArgExpr *OE = dyn_cast<HLSLOutArgExpr>(E)) {
+    LValue LV = EmitLValue(E);
+    llvm::Type *ElTy = ConvertTypeForMem(LV.getType());
+    llvm::Value *Addr, *BaseAddr;
+    if (LV.isExtVectorElt()) {
+      llvm::Constant *VecElts = LV.getExtVectorElts();
+      BaseAddr = LV.getExtVectorAddress().getBasePointer();
+      Addr = Builder.CreateGEP(
+          ElTy, BaseAddr,
+          {Builder.getInt32(0), VecElts->getAggregateElement((unsigned)0)});
+    } else // LV.getAddress() will assert if this is not a simple LValue.
+      Addr = BaseAddr = LV.getAddress().getBasePointer();
+
+    llvm::TypeSize Sz =
+        CGM.getDataLayout().getTypeAllocSize(ConvertTypeForMem(LV.getType()));
+
+    llvm::Value *LifetimeSize = EmitLifetimeStart(Sz, BaseAddr);
+
+    Address TmpAddr(Addr, ElTy, LV.getAlignment());
+    // TODO-HLSLOutArgExp: Fix me!!!
----------------
damyanp wrote:

Is it more obvious to an expert what needs fixing here?

https://github.com/llvm/llvm-project/pull/101083