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
57namespace llvm {
58
59class 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.
66class GCStrategy {
67private:
68  friend class GCModuleInfo;
69
70  std::string Name;
71
72protected:
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
80public:
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.
132using GCRegistry = Registry<GCStrategy>;
133
134} // end namespace llvm
135
136#endif // LLVM_CODEGEN_GCSTRATEGY_H
137