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. getName()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. customWriteBarrier()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. customReadBarrier()114 bool customReadBarrier() const { return CustomReadBarriers; } 115 116 /// Returns true if this strategy is expecting the use of gc.statepoints, 117 /// and false otherwise. useStatepoints()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. isGCManagedPointer(const Type * Ty)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. needsSafePoints()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. needsSafePoint(GC::PointKind Kind)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. customRoots()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. initializeRoots()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. usesMetadata()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. 177 using GCRegistry = Registry<GCStrategy>; 178 179 } // end namespace llvm 180 181 #endif // LLVM_CODEGEN_GCSTRATEGY_H 182