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

[John McCall via cfe-commits]

================
@@ -7071,6 +7071,102 @@ class ArraySectionExpr : public Expr {
   void setRBracketLoc(SourceLocation L) { RBracketLoc = L; }
 };
 
+/// This class represents temporary values used to represent inout and out
+/// arguments in HLSL. From the callee perspective these parameters are more or
+/// less __restrict__ T&. They are guaranteed to not alias any memory. inout
+/// parameters are initialized by the caller, and out parameters are references
+/// to uninitialized memory.
+///
+/// In the caller, the argument expression creates a temporary in local memory
+/// and the address of the temporary is passed into the callee. There may be
+/// implicit conversion sequences to initialize the temporary, and on expiration
+/// of the temporary an inverse conversion sequence is applied as a write-back
+/// conversion to the source l-value.
+///
+/// This AST node has three sub-expressions:
+///  - An OpaqueValueExpr with a source that is the argument lvalue expression.
+///  - An OpaqueValueExpr with a source that is a an implicit conversion
+///    sequence from the source lvalue to the argument type.
+///  - A BinaryOperatorExpr that assigns the first sub-expression with the
+///    value from an implict conversion sequence from the second expression to
+///    the argument expression's type.
+class HLSLOutArgExpr : public Expr {
+  friend class ASTStmtReader;
+
+  enum {
+    BaseLValue,
+    CastedTemporary,
+    WritebackCast,
+    NumSubExprs,
+  };
+
+  Stmt *SubExprs[NumSubExprs];
+  bool IsInOut;
+
+  HLSLOutArgExpr(QualType Ty, OpaqueValueExpr *B, OpaqueValueExpr *OpV,
+                 Expr *WB, bool IsInOut)
+      : Expr(HLSLOutArgExprClass, Ty, VK_LValue, OK_Ordinary),
+        IsInOut(IsInOut) {
+    SubExprs[BaseLValue] = B;
+    SubExprs[CastedTemporary] = OpV;
+    SubExprs[WritebackCast] = WB;
+    assert(!Ty->isDependentType() && "HLSLOutArgExpr given a dependent type!");
+  }
+
+  explicit HLSLOutArgExpr(EmptyShell Shell)
+      : Expr(HLSLOutArgExprClass, Shell) {}
+
+public:
+  static HLSLOutArgExpr *Create(const ASTContext &C, QualType Ty,
+                                OpaqueValueExpr *Base, OpaqueValueExpr *OpV,
+                                Expr *WB, bool IsInOut);
+  static HLSLOutArgExpr *CreateEmpty(const ASTContext &Ctx);
+
+  const OpaqueValueExpr *getOpaqueArgLValue() const {
+    return cast<OpaqueValueExpr>(SubExprs[BaseLValue]);
+  }
+  OpaqueValueExpr *getOpaqueArgLValue() {
+    return cast<OpaqueValueExpr>(SubExprs[BaseLValue]);
+  }
+
+  const Expr *getArgLValue() const {
----------------
rjmccall wrote:

```suggestion
  /// Return the l-value expression that was written as the argument
  /// in source.  Everything else here is implicitly generated.
  const Expr *getArgLValue() const {
```

People writing source tools will appreciate this being explicitly called out.

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