1 //===- llvm/CodeGen/GCStrategy.h - Garbage collection -----------*- C++ -*-===// 2 // 3 // Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions. 4 // See https://llvm.org/LICENSE.txt for license information. 5 // SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception 6 // 7 //===----------------------------------------------------------------------===// 8 // 9 // GCStrategy coordinates code generation algorithms and implements some itself 10 // in order to generate code compatible with a target code generator as 11 // specified in a function's 'gc' attribute. Algorithms are enabled by setting 12 // flags in a subclass's constructor, and some virtual methods can be 13 // overridden. 14 // 15 // GCStrategy is relevant for implementations using either gc.root or 16 // gc.statepoint based lowering strategies, but is currently focused mostly on 17 // options for gc.root. This will change over time. 18 // 19 // When requested by a subclass of GCStrategy, the gc.root implementation will 20 // populate GCModuleInfo and GCFunctionInfo with that about each Function in 21 // the Module that opts in to garbage collection. Specifically: 22 // 23 // - Safe points 24 // Garbage collection is generally only possible at certain points in code. 25 // GCStrategy can request that the collector insert such points: 26 // 27 // - At and after any call to a subroutine 28 // - Before returning from the current function 29 // - Before backwards branches (loops) 30 // 31 // - Roots 32 // When a reference to a GC-allocated object exists on the stack, it must be 33 // stored in an alloca registered with llvm.gcoot. 34 // 35 // This information can used to emit the metadata tables which are required by 36 // the target garbage collector runtime. 37 // 38 // When used with gc.statepoint, information about safepoint and roots can be 39 // found in the binary StackMap section after code generation. Safepoint 40 // placement is currently the responsibility of the frontend, though late 41 // insertion support is planned. 42 // 43 // The read and write barrier support can be used with either implementation. 44 // 45 //===----------------------------------------------------------------------===// 46 47 #ifndef LLVM_IR_GCSTRATEGY_H 48 #define LLVM_IR_GCSTRATEGY_H 49 50 #include "llvm/Support/Registry.h" 51 #include <optional> 52 #include <string> 53 54 namespace llvm { 55 56 class Type; 57 58 /// GCStrategy describes a garbage collector algorithm's code generation 59 /// requirements, and provides overridable hooks for those needs which cannot 60 /// be abstractly described. GCStrategy objects must be looked up through 61 /// the Function. The objects themselves are owned by the Context and must 62 /// be immutable. 63 class GCStrategy { 64 private: 65 friend class GCModuleInfo; 66 67 std::string Name; 68 69 protected: 70 bool UseStatepoints = false; /// Uses gc.statepoints as opposed to gc.roots, 71 /// if set, NeededSafePoints and UsesMetadata 72 /// should be left at their default values. 73 74 bool UseRS4GC = false; /// If UseStatepoints is set, this determines whether 75 /// the RewriteStatepointsForGC pass should rewrite 76 /// this function's calls. 77 /// This should only be set if UseStatepoints is set. 78 79 bool NeededSafePoints = false; ///< if set, calls are inferred to be safepoints 80 bool UsesMetadata = false; ///< If set, backend must emit metadata tables. 81 82 public: 83 GCStrategy(); 84 virtual ~GCStrategy() = default; 85 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. getName()88 const std::string &getName() const { return Name; } 89 90 /// Returns true if this strategy is expecting the use of gc.statepoints, 91 /// and false otherwise. useStatepoints()92 bool useStatepoints() const { return UseStatepoints; } 93 94 /** @name Statepoint Specific Properties */ 95 ///@{ 96 97 /// If the type specified can be reliably distinguished, returns true for 98 /// pointers to GC managed locations and false for pointers to non-GC 99 /// managed locations. Note a GCStrategy can always return 'std::nullopt' 100 /// (i.e. an empty optional indicating it can't reliably distinguish. isGCManagedPointer(const Type * Ty)101 virtual std::optional<bool> isGCManagedPointer(const Type *Ty) const { 102 return std::nullopt; 103 } 104 105 /// Returns true if the RewriteStatepointsForGC pass should run on functions 106 /// using this GC. useRS4GC()107 bool useRS4GC() const { 108 assert((!UseRS4GC || useStatepoints()) && 109 "GC strategy has useRS4GC but not useStatepoints set"); 110 return UseRS4GC; 111 } 112 113 ///@} 114 115 /// If set, appropriate metadata tables must be emitted by the back-end 116 /// (assembler, JIT, or otherwise). The default stackmap information can be 117 /// found in the StackMap section as described in the documentation. usesMetadata()118 bool usesMetadata() const { return UsesMetadata; } 119 120 /** @name GCRoot Specific Properties 121 * These properties and overrides only apply to collector strategies using 122 * GCRoot. 123 */ 124 ///@{ 125 126 /// True if safe points need to be inferred on call sites needsSafePoints()127 bool needsSafePoints() const { return NeededSafePoints; } 128 129 ///@} 130 }; 131 132 /// Subclasses of GCStrategy are made available for use during compilation by 133 /// adding them to the global GCRegistry. This can done either within the 134 /// LLVM source tree or via a loadable plugin. An example registeration 135 /// would be: 136 /// static GCRegistry::Add<CustomGC> X("custom-name", 137 /// "my custom supper fancy gc strategy"); 138 /// 139 /// Note that to use a custom GCMetadataPrinter, you must also 140 /// register your GCMetadataPrinter subclass with the 141 /// GCMetadataPrinterRegistery as well. 142 using GCRegistry = Registry<GCStrategy>; 143 144 /// Lookup the GCStrategy object associated with the given gc name. 145 std::unique_ptr<GCStrategy> getGCStrategy(const StringRef Name); 146 147 } // end namespace llvm 148 149 #endif // LLVM_IR_GCSTRATEGY_H 150