+/// \brief Pointer to the context, with optional RAUW support.
+///
+/// Either a raw (non-null) pointer to the \a LLVMContext, or an owned pointer
+/// to \a ReplaceableMetadataImpl (which has a reference to \a LLVMContext).
+class ContextAndReplaceableUses {
+ PointerUnion<LLVMContext *, ReplaceableMetadataImpl *> Ptr;
+
+ ContextAndReplaceableUses() = delete;
+ ContextAndReplaceableUses(ContextAndReplaceableUses &&) = delete;
+ ContextAndReplaceableUses(const ContextAndReplaceableUses &) = delete;
+ ContextAndReplaceableUses &operator=(ContextAndReplaceableUses &&) = delete;
+ ContextAndReplaceableUses &
+ operator=(const ContextAndReplaceableUses &) = delete;
+
+public:
+ ContextAndReplaceableUses(LLVMContext &Context) : Ptr(&Context) {}
+ ContextAndReplaceableUses(
+ std::unique_ptr<ReplaceableMetadataImpl> ReplaceableUses)
+ : Ptr(ReplaceableUses.release()) {
+ assert(getReplaceableUses() && "Expected non-null replaceable uses");
+ }
+ ~ContextAndReplaceableUses() { delete getReplaceableUses(); }
+
+ operator LLVMContext &() { return getContext(); }
+
+ /// \brief Whether this contains RAUW support.
+ bool hasReplaceableUses() const {
+ return Ptr.is<ReplaceableMetadataImpl *>();
+ }
+ LLVMContext &getContext() const {
+ if (hasReplaceableUses())
+ return getReplaceableUses()->getContext();
+ return *Ptr.get<LLVMContext *>();
+ }
+ ReplaceableMetadataImpl *getReplaceableUses() const {
+ if (hasReplaceableUses())
+ return Ptr.get<ReplaceableMetadataImpl *>();
+ return nullptr;
+ }
+
+ /// \brief Assign RAUW support to this.
+ ///
+ /// Make this replaceable, taking ownership of \c ReplaceableUses (which must
+ /// not be null).
+ void
+ makeReplaceable(std::unique_ptr<ReplaceableMetadataImpl> ReplaceableUses) {
+ assert(ReplaceableUses && "Expected non-null replaceable uses");
+ assert(&ReplaceableUses->getContext() == &getContext() &&
+ "Expected same context");
+ delete getReplaceableUses();
+ Ptr = ReplaceableUses.release();
+ }
+
+ /// \brief Drop RAUW support.
+ ///
+ /// Cede ownership of RAUW support, returning it.
+ std::unique_ptr<ReplaceableMetadataImpl> takeReplaceableUses() {
+ assert(hasReplaceableUses() && "Expected to own replaceable uses");
+ std::unique_ptr<ReplaceableMetadataImpl> ReplaceableUses(
+ getReplaceableUses());
+ Ptr = &ReplaceableUses->getContext();
+ return ReplaceableUses;
+ }
+};
+
+struct TempMDNodeDeleter {
+ inline void operator()(MDNode *Node) const;
+};
+
+#define HANDLE_MDNODE_LEAF(CLASS) \
+ typedef std::unique_ptr<CLASS, TempMDNodeDeleter> Temp##CLASS;
+#define HANDLE_MDNODE_BRANCH(CLASS) HANDLE_MDNODE_LEAF(CLASS)
+#include "llvm/IR/Metadata.def"
+
+/// \brief Metadata node.
+///
+/// Metadata nodes can be uniqued, like constants, or distinct. Temporary
+/// metadata nodes (with full support for RAUW) can be used to delay uniquing
+/// until forward references are known. The basic metadata node is an \a
+/// MDTuple.
+///
+/// There is limited support for RAUW at construction time. At construction
+/// time, if any operand is a temporary node (or an unresolved uniqued node,
+/// which indicates a transitive temporary operand), the node itself will be
+/// unresolved. As soon as all operands become resolved, it will drop RAUW
+/// support permanently.
+///
+/// If an unresolved node is part of a cycle, \a resolveCycles() needs
+/// to be called on some member of the cycle once all temporary nodes have been
+/// replaced.