GCStrategy.h revision 321369 
1//===- llvm/CodeGen/GCStrategy.h - Garbage collection -----------*- C++ -*-===//
2//
3//                     The LLVM Compiler Infrastructure
4//
5// This file is distributed under the University of Illinois Open Source
6// License. See LICENSE.TXT for details.
7//
8//===----------------------------------------------------------------------===//
9//
10// GCStrategy coordinates code generation algorithms and implements some itself
11// in order to generate code compatible with a target code generator as
12// specified in a function's 'gc' attribute. Algorithms are enabled by setting
13// flags in a subclass's constructor, and some virtual methods can be
14// overridden.
15//
16// GCStrategy is relevant for implementations using either gc.root or
17// gc.statepoint based lowering strategies, but is currently focused mostly on
18// options for gc.root.  This will change over time.
19//
20// When requested by a subclass of GCStrategy, the gc.root implementation will
21// populate GCModuleInfo and GCFunctionInfo with that about each Function in
22// the Module that opts in to garbage collection.  Specifically:
23//
24// - Safe points
25//   Garbage collection is generally only possible at certain points in code.
26//   GCStrategy can request that the collector insert such points:
27//
28//     - At and after any call to a subroutine
29//     - Before returning from the current function
30//     - Before backwards branches (loops)
31//
32// - Roots
33//   When a reference to a GC-allocated object exists on the stack, it must be
34//   stored in an alloca registered with llvm.gcoot.
35//
36// This information can used to emit the metadata tables which are required by
37// the target garbage collector runtime.
38//
39// When used with gc.statepoint, information about safepoint and roots can be
40// found in the binary StackMap section after code generation.  Safepoint
41// placement is currently the responsibility of the frontend, though late
42// insertion support is planned.  gc.statepoint does not currently support
43// custom stack map formats; such can be generated by parsing the standard
44// stack map section if desired.
45//
46// The read and write barrier support can be used with either implementation.
47//
48//===----------------------------------------------------------------------===//
49
50#ifndef LLVM_CODEGEN_GCSTRATEGY_H
51#define LLVM_CODEGEN_GCSTRATEGY_H
52
53#include "llvm/ADT/None.h"
54#include "llvm/ADT/Optional.h"
55#include "llvm/Support/Registry.h"
56#include <string>
57
58namespace llvm {
59
60class Type;
61
62namespace GC {
63
64/// PointKind - Used to indicate whether the address of the call instruction
65/// or the address after the call instruction is listed in the stackmap.  For
66/// most runtimes, PostCall safepoints are appropriate.
67///
68enum PointKind {
69  PreCall, ///< Instr is a call instruction.
70  PostCall ///< Instr is the return address of a call.
71};
72
73} // end namespace GC
74
75/// GCStrategy describes a garbage collector algorithm's code generation
76/// requirements, and provides overridable hooks for those needs which cannot
77/// be abstractly described.  GCStrategy objects must be looked up through
78/// the Function.  The objects themselves are owned by the Context and must
79/// be immutable.
80class GCStrategy {
81private:
82  friend class GCModuleInfo;
83
84  std::string Name;
85
86protected:
87  bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots,
88                               /// if set, none of the other options can be
89                               /// anything but their default values.
90
91  unsigned NeededSafePoints = 0;    ///< Bitmask of required safe points.
92  bool CustomReadBarriers = false;  ///< Default is to insert loads.
93  bool CustomWriteBarriers = false; ///< Default is to insert stores.
94  bool CustomRoots = false;      ///< Default is to pass through to backend.
95  bool InitRoots= true;          ///< If set, roots are nulled during lowering.
96  bool UsesMetadata = false;     ///< If set, backend must emit metadata tables.
97
98public:
99  GCStrategy();
100  virtual ~GCStrategy() = default;
101
102  /// Return the name of the GC strategy.  This is the value of the collector
103  /// name string specified on functions which use this strategy.
104  const std::string &getName() const { return Name; }
105
106  /// By default, write barriers are replaced with simple store
107  /// instructions. If true, you must provide a custom pass to lower
108  /// calls to @llvm.gcwrite.
109  bool customWriteBarrier() const { return CustomWriteBarriers; }
110
111  /// By default, read barriers are replaced with simple load
112  /// instructions. If true, you must provide a custom pass to lower
113  /// calls to @llvm.gcread.
114  bool customReadBarrier() const { return CustomReadBarriers; }
115
116  /// Returns true if this strategy is expecting the use of gc.statepoints,
117  /// and false otherwise.
118  bool useStatepoints() const { return UseStatepoints; }
119
120  /** @name Statepoint Specific Properties */
121  ///@{
122
123  /// If the type specified can be reliably distinguished, returns true for
124  /// pointers to GC managed locations and false for pointers to non-GC
125  /// managed locations.  Note a GCStrategy can always return 'None' (i.e. an
126  /// empty optional indicating it can't reliably distinguish.
127  virtual Optional<bool> isGCManagedPointer(const Type *Ty) const {
128    return None;
129  }
130  ///@}
131
132  /** @name GCRoot Specific Properties
133   * These properties and overrides only apply to collector strategies using
134   * GCRoot.
135   */
136  ///@{
137
138  /// True if safe points of any kind are required. By default, none are
139  /// recorded.
140  bool needsSafePoints() const { return NeededSafePoints != 0; }
141
142  /// True if the given kind of safe point is required. By default, none are
143  /// recorded.
144  bool needsSafePoint(GC::PointKind Kind) const {
145    return (NeededSafePoints & 1 << Kind) != 0;
146  }
147
148  /// By default, roots are left for the code generator so it can generate a
149  /// stack map. If true, you must provide a custom pass to lower
150  /// calls to @llvm.gcroot.
151  bool customRoots() const { return CustomRoots; }
152
153  /// If set, gcroot intrinsics should initialize their allocas to null
154  /// before the first use. This is necessary for most GCs and is enabled by
155  /// default.
156  bool initializeRoots() const { return InitRoots; }
157
158  /// If set, appropriate metadata tables must be emitted by the back-end
159  /// (assembler, JIT, or otherwise). For statepoint, this method is
160  /// currently unsupported.  The stackmap information can be found in the
161  /// StackMap section as described in the documentation.
162  bool usesMetadata() const { return UsesMetadata; }
163
164  ///@}
165};
166
167/// Subclasses of GCStrategy are made available for use during compilation by
168/// adding them to the global GCRegistry.  This can done either within the
169/// LLVM source tree or via a loadable plugin.  An example registeration
170/// would be:
171/// static GCRegistry::Add<CustomGC> X("custom-name",
172///        "my custom supper fancy gc strategy");
173///
174/// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
175/// register your GCMetadataPrinter subclass with the
176/// GCMetadataPrinterRegistery as well.
177using GCRegistry = Registry<GCStrategy>;
178
179} // end namespace llvm
180
181#endif // LLVM_CODEGEN_GCSTRATEGY_H
182