include/llvm/CodeGen/GCStrategy.h

   1 //===- llvm/CodeGen/GCStrategy.h - Garbage collection -----------*- C++ -*-===//
   2 //
   3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
   4 // See https://llvm.org/LICENSE.txt for license information.
   5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
   6 //
   7 //===----------------------------------------------------------------------===//
   8 //
   9 // GCStrategy coordinates code generation algorithms and implements some itself
  10 // in order to generate code compatible with a target code generator as
  11 // specified in a function's 'gc' attribute. Algorithms are enabled by setting
  12 // flags in a subclass's constructor, and some virtual methods can be
  13 // overridden.
  14 //
  15 // GCStrategy is relevant for implementations using either gc.root or
  16 // gc.statepoint based lowering strategies, but is currently focused mostly on
  17 // options for gc.root.  This will change over time.
  18 //
  19 // When requested by a subclass of GCStrategy, the gc.root implementation will
  20 // populate GCModuleInfo and GCFunctionInfo with that about each Function in
  21 // the Module that opts in to garbage collection.  Specifically:
  22 //
  23 // - Safe points
  24 //   Garbage collection is generally only possible at certain points in code.
  25 //   GCStrategy can request that the collector insert such points:
  26 //
  27 //     - At and after any call to a subroutine
  28 //     - Before returning from the current function
  29 //     - Before backwards branches (loops)
  30 //
  31 // - Roots
  32 //   When a reference to a GC-allocated object exists on the stack, it must be
  33 //   stored in an alloca registered with llvm.gcoot.
  34 //
  35 // This information can used to emit the metadata tables which are required by
  36 // the target garbage collector runtime.
  37 //
  38 // When used with gc.statepoint, information about safepoint and roots can be
  39 // found in the binary StackMap section after code generation.  Safepoint
  40 // placement is currently the responsibility of the frontend, though late
  41 // insertion support is planned.  gc.statepoint does not currently support
  42 // custom stack map formats; such can be generated by parsing the standard
  43 // stack map section if desired.
  44 //
  45 // The read and write barrier support can be used with either implementation.
  46 //
  47 //===----------------------------------------------------------------------===//
  48
  49 #ifndef LLVM_CODEGEN_GCSTRATEGY_H
  50 #define LLVM_CODEGEN_GCSTRATEGY_H
  51
  52 #include "llvm/ADT/None.h"
  53 #include "llvm/ADT/Optional.h"
  54 #include "llvm/Support/Registry.h"
  55 #include <string>
  56
  57 namespace llvm {
  58
  59 class Type;
  60
  61 /// GCStrategy describes a garbage collector algorithm's code generation
  62 /// requirements, and provides overridable hooks for those needs which cannot
  63 /// be abstractly described.  GCStrategy objects must be looked up through
  64 /// the Function.  The objects themselves are owned by the Context and must
  65 /// be immutable.
  66 class GCStrategy {
  67 private:
  68   friend class GCModuleInfo;
  69
  70   std::string Name;
  71
  72 protected:
  73   bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots,
  74                                /// if set, none of the other options can be
  75                                /// anything but their default values.
  76
  77   bool NeededSafePoints = false;    ///< if set, calls are inferred to be safepoints
  78   bool UsesMetadata = false;     ///< If set, backend must emit metadata tables.
  79
  80 public:
  81   GCStrategy();
  82   virtual ~GCStrategy() = default;
  83
  84   /// Return the name of the GC strategy.  This is the value of the collector
  85   /// name string specified on functions which use this strategy.
  86   const std::string &getName() const { return Name; }
  87
  88   /// Returns true if this strategy is expecting the use of gc.statepoints,
  89   /// and false otherwise.
  90   bool useStatepoints() const { return UseStatepoints; }
  91
  92   /** @name Statepoint Specific Properties */
  93   ///@{
  94
  95   /// If the type specified can be reliably distinguished, returns true for
  96   /// pointers to GC managed locations and false for pointers to non-GC
  97   /// managed locations.  Note a GCStrategy can always return 'None' (i.e. an
  98   /// empty optional indicating it can't reliably distinguish.
  99   virtual Optional<bool> isGCManagedPointer(const Type *Ty) const {
 100     return None;
 101   }
 102   ///@}
 103
 104   /** @name GCRoot Specific Properties
 105    * These properties and overrides only apply to collector strategies using
 106    * GCRoot.
 107    */
 108   ///@{
 109
 110   /// True if safe points need to be inferred on call sites
 111   bool needsSafePoints() const { return NeededSafePoints; }
 112
 113   /// If set, appropriate metadata tables must be emitted by the back-end
 114   /// (assembler, JIT, or otherwise). For statepoint, this method is
 115   /// currently unsupported.  The stackmap information can be found in the
 116   /// StackMap section as described in the documentation.
 117   bool usesMetadata() const { return UsesMetadata; }
 118
 119   ///@}
 120 };
 121
 122 /// Subclasses of GCStrategy are made available for use during compilation by
 123 /// adding them to the global GCRegistry.  This can done either within the
 124 /// LLVM source tree or via a loadable plugin.  An example registeration
 125 /// would be:
 126 /// static GCRegistry::Add<CustomGC> X("custom-name",
 127 ///        "my custom supper fancy gc strategy");
 128 ///
 129 /// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
 130 /// register your GCMetadataPrinter subclass with the
 131 /// GCMetadataPrinterRegistery as well.
 132 using GCRegistry = Registry<GCStrategy>;
 133
 134 } // end namespace llvm
 135
 136 #endif // LLVM_CODEGEN_GCSTRATEGY_H