1 //===-- llvm/CodeGen/GCStrategy.h - Garbage collection ----------*- C++ -*-===//
3 // The LLVM Compiler Infrastructure
5 // This file is distributed under the University of Illinois Open Source
6 // License. See LICENSE.TXT for details.
8 //===----------------------------------------------------------------------===//
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
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.
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:
25 // Garbage collection is generally only possible at certain points in code.
26 // GCStrategy can request that the collector insert such points:
28 // - At and after any call to a subroutine
29 // - Before returning from the current function
30 // - Before backwards branches (loops)
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.
36 // This information can used to emit the metadata tables which are required by
37 // the target garbage collector runtime.
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.
46 // The read and write barrier support can be used with either implementation.
48 //===----------------------------------------------------------------------===//
50 #ifndef LLVM_CODEGEN_GCSTRATEGY_H
51 #define LLVM_CODEGEN_GCSTRATEGY_H
53 #include "llvm/ADT/Optional.h"
54 #include "llvm/CodeGen/GCMetadata.h"
55 #include "llvm/CodeGen/MachineFunction.h"
56 #include "llvm/Support/Registry.h"
60 /// GCStrategy describes a garbage collector algorithm's code generation
61 /// requirements, and provides overridable hooks for those needs which cannot
62 /// be abstractly described. GCStrategy objects currently must be looked up
63 /// through the GCModuleInfo analysis pass. They are owned by the analysis
64 /// pass and recreated every time that pass is invalidated.
68 friend class GCModuleInfo;
71 bool UseStatepoints; /// Uses gc.statepoints as opposed to gc.roots,
72 /// if set, none of the other options can be
73 /// anything but their default values.
75 unsigned NeededSafePoints; ///< Bitmask of required safe points.
76 bool CustomReadBarriers; ///< Default is to insert loads.
77 bool CustomWriteBarriers; ///< Default is to insert stores.
78 bool CustomRoots; ///< Default is to pass through to backend.
79 bool InitRoots; ///< If set, roots are nulled during lowering.
80 bool UsesMetadata; ///< If set, backend must emit metadata tables.
84 virtual ~GCStrategy() {}
86 /// Return the name of the GC strategy. This is the value of the collector
87 /// name string specified on functions which use this strategy.
88 const std::string &getName() const { return Name; }
90 /// By default, write barriers are replaced with simple store
91 /// instructions. If true, then performCustomLowering must instead lower
93 bool customWriteBarrier() const { return CustomWriteBarriers; }
95 /// By default, read barriers are replaced with simple load
96 /// instructions. If true, then performCustomLowering must instead lower
98 bool customReadBarrier() const { return CustomReadBarriers; }
100 /// Returns true if this strategy is expecting the use of gc.statepoints,
101 /// and false otherwise.
102 bool useStatepoints() const { return UseStatepoints; }
104 /** @name Statepoint Specific Properties */
107 /// If the value specified can be reliably distinguished, returns true for
108 /// pointers to GC managed locations and false for pointers to non-GC
109 /// managed locations. Note a GCStrategy can always return 'None' (i.e. an
110 /// empty optional indicating it can't reliably distinguish.
111 virtual Optional<bool> isGCManagedPointer(const Value *V) const {
116 /** @name GCRoot Specific Properties
117 * These properties and overrides only apply to collector strategies using
122 /// True if safe points of any kind are required. By default, none are
124 bool needsSafePoints() const {
125 return NeededSafePoints != 0;
128 /// True if the given kind of safe point is required. By default, none are
130 bool needsSafePoint(GC::PointKind Kind) const {
131 return (NeededSafePoints & 1 << Kind) != 0;
134 /// By default, roots are left for the code generator so it can generate a
135 /// stack map. If true, then performCustomLowering must delete them.
136 bool customRoots() const { return CustomRoots; }
138 /// If set, gcroot intrinsics should initialize their allocas to null
139 /// before the first use. This is necessary for most GCs and is enabled by
141 bool initializeRoots() const { return InitRoots; }
143 /// If set, appropriate metadata tables must be emitted by the back-end
144 /// (assembler, JIT, or otherwise). For statepoint, this method is
145 /// currently unsupported. The stackmap information can be found in the
146 /// StackMap section as described in the documentation.
147 bool usesMetadata() const { return UsesMetadata; }
151 /// initializeCustomLowering/performCustomLowering - If any of the actions
152 /// are set to custom, performCustomLowering must be overriden to transform
153 /// the corresponding actions to LLVM IR. initializeCustomLowering is
154 /// optional to override. These are the only GCStrategy methods through
155 /// which the LLVM IR can be modified. These methods apply mostly to
156 /// gc.root based implementations, but can be overriden to provide custom
157 /// barrier lowerings with gc.statepoint as well.
159 virtual bool initializeCustomLowering(Module &F) {
163 virtual bool performCustomLowering(Function &F) {
164 llvm_unreachable("GCStrategy subclass specified a configuration which"
165 "requires a custom lowering without providing one");
169 /// Subclasses of GCStrategy are made available for use during compilation by
170 /// adding them to the global GCRegistry. This can done either within the
171 /// LLVM source tree or via a loadable plugin. An example registeration
173 /// static GCRegistry::Add<CustomGC> X("custom-name",
174 /// "my custom supper fancy gc strategy");
176 /// Note that to use a custom GCMetadataPrinter w/gc.roots, you must also
177 /// register your GCMetadataPrinter subclass with the
178 /// GCMetadataPrinterRegistery as well.
179 typedef Registry<GCStrategy> GCRegistry;