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(); 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. 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. 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. 111 bool customReadBarrier() const { return CustomReadBarriers; } 112 113 /// Returns true if this strategy is expecting the use of gc.statepoints, 114 /// and false otherwise. 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. 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. 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. 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. 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. 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. 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
