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 //===- llvm/CodeGen/GCStrategy.h - Garbage collection -----------*- C++ -*-===//
 //
 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
 // See https://llvm.org/LICENSE.txt for license information.
 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
 //
 //===----------------------------------------------------------------------===//
 //
 // GCStrategy coordinates code generation algorithms and implements some itself
 // in order to generate code compatible with a target code generator as
 // specified in a function's 'gc' attribute. Algorithms are enabled by setting
 // flags in a subclass's constructor, and some virtual methods can be
 // overridden.
 //
 // GCStrategy is relevant for implementations using either gc.root or
 // gc.statepoint based lowering strategies, but is currently focused mostly on
 // options for gc.root.  This will change over time.
 //
 // When requested by a subclass of GCStrategy, the gc.root implementation will
 // populate GCModuleInfo and GCFunctionInfo with that about each Function in
 // the Module that opts in to garbage collection.  Specifically:
 //
 // - Safe points
 //   Garbage collection is generally only possible at certain points in code.
 //   GCStrategy can request that the collector insert such points:
 //
 //     - At and after any call to a subroutine
 //     - Before returning from the current function
 //     - Before backwards branches (loops)
 //
 // - Roots
 //   When a reference to a GC-allocated object exists on the stack, it must be
 //   stored in an alloca registered with llvm.gcoot.
 //
 // This information can used to emit the metadata tables which are required by
 // the target garbage collector runtime.
 //
 // When used with gc.statepoint, information about safepoint and roots can be
 // found in the binary StackMap section after code generation.  Safepoint
 // placement is currently the responsibility of the frontend, though late
 // insertion support is planned.
 //
 // The read and write barrier support can be used with either implementation.
 //
 //===----------------------------------------------------------------------===//
 
 #ifndef LLVM_IR_GCSTRATEGY_H
 #define LLVM_IR_GCSTRATEGY_H
 
 #include "llvm/Support/Registry.h"
 #include <optional>
 #include <string>
 
 namespace llvm {
 
 class Type;
 
 /// GCStrategy describes a garbage collector algorithm's code generation
 /// requirements, and provides overridable hooks for those needs which cannot
 /// be abstractly described.  GCStrategy objects must be looked up through
 /// the Function.  The objects themselves are owned by the Context and must
 /// be immutable.
 class GCStrategy {
 private:
   friend class GCModuleInfo;
 
   std::string Name;
 
 protected:
   bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots,
                                /// if set, NeededSafePoints and UsesMetadata
                                /// should be left at their default values.
 
   bool UseRS4GC = false; /// If UseStatepoints is set, this determines whether
                          /// the RewriteStatepointsForGC pass should rewrite
                          /// this function's calls.
                          /// This should only be set if UseStatepoints is set.
 
   bool NeededSafePoints = false;    ///< if set, calls are inferred to be safepoints
   bool UsesMetadata = false;     ///< If set, backend must emit metadata tables.
 
 public:
   GCStrategy();
   virtual ~GCStrategy() = default;
 
   /// Return the name of the GC strategy.  This is the value of the collector
   /// name string specified on functions which use this strategy.
   const std::string &getName() const { return Name; }
 
   /// Returns true if this strategy is expecting the use of gc.statepoints,
   /// and false otherwise.
   bool useStatepoints() const { return UseStatepoints; }
 
   /** @name Statepoint Specific Properties */
   ///@{
 
   /// If the type specified can be reliably distinguished, returns true for
   /// pointers to GC managed locations and false for pointers to non-GC
   /// managed locations.  Note a GCStrategy can always return 'std::nullopt'
   /// (i.e. an empty optional indicating it can't reliably distinguish.
   virtual std::optional<bool> isGCManagedPointer(const Type *Ty) const {
     return std::nullopt;
   }
 
   /// Returns true if the RewriteStatepointsForGC pass should run on functions
   /// using this GC.
   bool useRS4GC() const {
     assert((!UseRS4GC || useStatepoints()) &&
            "GC strategy has useRS4GC but not useStatepoints set");
     return UseRS4GC;
   }
 
   ///@}
 
   /// If set, appropriate metadata tables must be emitted by the back-end
   /// (assembler, JIT, or otherwise). The default stackmap information can be
   /// found in the StackMap section as described in the documentation.
   bool usesMetadata() const { return UsesMetadata; }
 
   /** @name GCRoot Specific Properties
    * These properties and overrides only apply to collector strategies using
    * GCRoot.
    */
   ///@{
 
   /// True if safe points need to be inferred on call sites
   bool needsSafePoints() const { return NeededSafePoints; }
 
   ///@}
 };
 
 /// Subclasses of GCStrategy are made available for use during compilation by
 /// adding them to the global GCRegistry.  This can done either within the
 /// LLVM source tree or via a loadable plugin.  An example registeration
 /// would be:
 /// static GCRegistry::Add<CustomGC> X("custom-name",
 ///        "my custom supper fancy gc strategy");
 ///
 /// Note that to use a custom GCMetadataPrinter, you must also
 /// register your GCMetadataPrinter subclass with the
 /// GCMetadataPrinterRegistery as well.
 using GCRegistry = Registry<GCStrategy>;
 
 extern template class LLVM_TEMPLATE_ABI Registry<GCStrategy>;
 
 /// Lookup the GCStrategy object associated with the given gc name.
 std::unique_ptr<GCStrategy> getGCStrategy(const StringRef Name);
 
 } // end namespace llvm
 
 #endif // LLVM_IR_GCSTRATEGY_H

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