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 /// GCStrategy describes a garbage collector algorithm's code generation
63 /// requirements, and provides overridable hooks for those needs which cannot
64 /// be abstractly described.  GCStrategy objects must be looked up through
65 /// the Function.  The objects themselves are owned by the Context and must
66 /// be immutable.
67 class GCStrategy {
68 private:
69   friend class GCModuleInfo;
70 
71   std::string Name;
72 
73 protected:
74   bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots,
75                                /// if set, none of the other options can be
76                                /// anything but their default values.
77 
78   bool NeededSafePoints = false;    ///< if set, calls are inferred to be safepoints
79   bool UsesMetadata = false;     ///< If set, backend must emit metadata tables.
80 
81 public:
82   GCStrategy();
83   virtual ~GCStrategy() = default;
84 
85   /// Return the name of the GC strategy.  This is the value of the collector
86   /// name string specified on functions which use this strategy.
getName()87   const std::string &getName() const { return Name; }
88 
89   /// Returns true if this strategy is expecting the use of gc.statepoints,
90   /// and false otherwise.
useStatepoints()91   bool useStatepoints() const { return UseStatepoints; }
92 
93   /** @name Statepoint Specific Properties */
94   ///@{
95 
96   /// If the type specified can be reliably distinguished, returns true for
97   /// pointers to GC managed locations and false for pointers to non-GC
98   /// managed locations.  Note a GCStrategy can always return 'None' (i.e. an
99   /// empty optional indicating it can't reliably distinguish.
isGCManagedPointer(const Type * Ty)100   virtual Optional<bool> isGCManagedPointer(const Type *Ty) const {
101     return None;
102   }
103   ///@}
104 
105   /** @name GCRoot Specific Properties
106    * These properties and overrides only apply to collector strategies using
107    * GCRoot.
108    */
109   ///@{
110 
111   /// True if safe points need to be inferred on call sites
needsSafePoints()112   bool needsSafePoints() const { return NeededSafePoints; }
113 
114   /// If set, appropriate metadata tables must be emitted by the back-end
115   /// (assembler, JIT, or otherwise). For statepoint, this method is
116   /// currently unsupported.  The stackmap information can be found in the
117   /// StackMap section as described in the documentation.
usesMetadata()118   bool usesMetadata() const { return UsesMetadata; }
119 
120   ///@}
121 };
122 
123 /// Subclasses of GCStrategy are made available for use during compilation by
124 /// adding them to the global GCRegistry.  This can done either within the
125 /// LLVM source tree or via a loadable plugin.  An example registeration
126 /// would be:
127 /// static GCRegistry::Add<CustomGC> X("custom-name",
128 ///        "my custom supper fancy gc strategy");
129 ///
130 /// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
131 /// register your GCMetadataPrinter subclass with the
132 /// GCMetadataPrinterRegistery as well.
133 using GCRegistry = Registry<GCStrategy>;
134 
135 } // end namespace llvm
136 
137 #endif // LLVM_CODEGEN_GCSTRATEGY_H
138