LLVM  6.0.0svn
GCStrategy.h
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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 
58 namespace llvm {
59 
60 class Type;
61 
62 namespace 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 ///
68 enum 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.
80 class GCStrategy {
81 private:
82  friend class GCModuleInfo;
83 
84  std::string Name;
85 
86 protected:
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 
98 public:
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.
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.
178 
179 } // end namespace llvm
180 
181 #endif // LLVM_CODEGEN_GCSTRATEGY_H
Compute iterated dominance frontiers using a linear time algorithm.
Definition: AllocatorList.h:24
A global registry used in conjunction with static constructors to make pluggable components (like tar...
Definition: Registry.h:45
PointKind
PointKind - Used to indicate whether the address of the call instruction or the address after the cal...
Definition: GCStrategy.h:68
bool customReadBarrier() const
By default, read barriers are replaced with simple load instructions.
Definition: GCStrategy.h:114
An analysis pass which caches information about the entire Module.
Definition: GCMetadata.h:154
const std::string & getName() const
Return the name of the GC strategy.
Definition: GCStrategy.h:104
virtual Optional< bool > isGCManagedPointer(const Type *Ty) const
If the type specified can be reliably distinguished, returns true for pointers to GC managed location...
Definition: GCStrategy.h:127
bool customWriteBarrier() const
By default, write barriers are replaced with simple store instructions.
Definition: GCStrategy.h:109
bool useStatepoints() const
Returns true if this strategy is expecting the use of gc.statepoints, and false otherwise.
Definition: GCStrategy.h:118
The instances of the Type class are immutable: once they are created, they are never changed...
Definition: Type.h:46
Instr is the return address of a call.
Definition: GCStrategy.h:70
bool needsSafePoint(GC::PointKind Kind) const
True if the given kind of safe point is required.
Definition: GCStrategy.h:144
bool usesMetadata() const
If set, appropriate metadata tables must be emitted by the back-end (assembler, JIT, or otherwise).
Definition: GCStrategy.h:162
GCStrategy describes a garbage collector algorithm&#39;s code generation requirements, and provides overridable hooks for those needs which cannot be abstractly described.
Definition: GCStrategy.h:80
bool initializeRoots() const
If set, gcroot intrinsics should initialize their allocas to null before the first use...
Definition: GCStrategy.h:156
Instr is a call instruction.
Definition: GCStrategy.h:69
const unsigned Kind
bool needsSafePoints() const
True if safe points of any kind are required.
Definition: GCStrategy.h:140
bool customRoots() const
By default, roots are left for the code generator so it can generate a stack map. ...
Definition: GCStrategy.h:151