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_IR_GCSTRATEGY_H
51 #define LLVM_IR_GCSTRATEGY_H
52 
53 #include "llvm/ADT/Optional.h"
54 #include "llvm/IR/Function.h"
55 #include "llvm/IR/Module.h"
56 #include "llvm/IR/Value.h"
57 #include "llvm/Support/ErrorHandling.h"
58 #include "llvm/Support/Registry.h"
59 #include <string>
60 
61 namespace llvm {
62 namespace GC {
63 /// PointKind - Used to indicate whether the address of the call instruction
64 /// or the address after the call instruction is listed in the stackmap.  For
65 /// most runtimes, PostCall safepoints are appropriate.
66 ///
67 enum PointKind {
68   PreCall, ///< Instr is a call instruction.
69   PostCall ///< Instr is the return address of a call.
70 };
71 }
72 
73 /// GCStrategy describes a garbage collector algorithm's code generation
74 /// requirements, and provides overridable hooks for those needs which cannot
75 /// be abstractly described.  GCStrategy objects must be looked up through
76 /// the Function.  The objects themselves are owned by the Context and must
77 /// be immutable.
78 class GCStrategy {
79 private:
80   std::string Name;
81   friend class GCModuleInfo;
82 
83 protected:
84   bool UseStatepoints; /// Uses gc.statepoints as opposed to gc.roots,
85                        /// if set, none of the other options can be
86                        /// anything but their default values.
87 
88   unsigned NeededSafePoints; ///< Bitmask of required safe points.
89   bool CustomReadBarriers;   ///< Default is to insert loads.
90   bool CustomWriteBarriers;  ///< Default is to insert stores.
91   bool CustomRoots;          ///< Default is to pass through to backend.
92   bool InitRoots;            ///< If set, roots are nulled during lowering.
93   bool UsesMetadata;         ///< If set, backend must emit metadata tables.
94 
95 public:
96   GCStrategy();
~GCStrategy()97   virtual ~GCStrategy() {}
98 
99   /// Return the name of the GC strategy.  This is the value of the collector
100   /// name string specified on functions which use this strategy.
getName()101   const std::string &getName() const { return Name; }
102 
103   /// By default, write barriers are replaced with simple store
104   /// instructions. If true, you must provide a custom pass to lower
105   /// calls to @llvm.gcwrite.
customWriteBarrier()106   bool customWriteBarrier() const { return CustomWriteBarriers; }
107 
108   /// By default, read barriers are replaced with simple load
109   /// instructions. If true, you must provide a custom pass to lower
110   /// calls to @llvm.gcread.
customReadBarrier()111   bool customReadBarrier() const { return CustomReadBarriers; }
112 
113   /// Returns true if this strategy is expecting the use of gc.statepoints,
114   /// and false otherwise.
useStatepoints()115   bool useStatepoints() const { return UseStatepoints; }
116 
117   /** @name Statepoint Specific Properties */
118   ///@{
119 
120   /// If the value specified can be reliably distinguished, returns true for
121   /// pointers to GC managed locations and false for pointers to non-GC
122   /// managed locations.  Note a GCStrategy can always return 'None' (i.e. an
123   /// empty optional indicating it can't reliably distinguish.
isGCManagedPointer(const Value * V)124   virtual Optional<bool> isGCManagedPointer(const Value *V) const {
125     return None;
126   }
127   ///@}
128 
129   /** @name GCRoot Specific Properties
130    * These properties and overrides only apply to collector strategies using
131    * GCRoot.
132    */
133   ///@{
134 
135   /// True if safe points of any kind are required. By default, none are
136   /// recorded.
needsSafePoints()137   bool needsSafePoints() const { return NeededSafePoints != 0; }
138 
139   /// True if the given kind of safe point is required. By default, none are
140   /// recorded.
needsSafePoint(GC::PointKind Kind)141   bool needsSafePoint(GC::PointKind Kind) const {
142     return (NeededSafePoints & 1 << Kind) != 0;
143   }
144 
145   /// By default, roots are left for the code generator so it can generate a
146   /// stack map. If true, you must provide a custom pass to lower
147   /// calls to @llvm.gcroot.
customRoots()148   bool customRoots() const { return CustomRoots; }
149 
150   /// If set, gcroot intrinsics should initialize their allocas to null
151   /// before the first use. This is necessary for most GCs and is enabled by
152   /// default.
initializeRoots()153   bool initializeRoots() const { return InitRoots; }
154 
155   /// If set, appropriate metadata tables must be emitted by the back-end
156   /// (assembler, JIT, or otherwise). For statepoint, this method is
157   /// currently unsupported.  The stackmap information can be found in the
158   /// StackMap section as described in the documentation.
usesMetadata()159   bool usesMetadata() const { return UsesMetadata; }
160 
161   ///@}
162 };
163 
164 /// Subclasses of GCStrategy are made available for use during compilation by
165 /// adding them to the global GCRegistry.  This can done either within the
166 /// LLVM source tree or via a loadable plugin.  An example registeration
167 /// would be:
168 /// static GCRegistry::Add<CustomGC> X("custom-name",
169 ///        "my custom supper fancy gc strategy");
170 ///
171 /// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
172 /// register your GCMetadataPrinter subclass with the
173 /// GCMetadataPrinterRegistery as well.
174 typedef Registry<GCStrategy> GCRegistry;
175 }
176 
177 #endif
178