1 //===- Core/UndefinedAtom.h - An Undefined Atom ---------------------------===// 2 // 3 // The LLVM Linker 4 // 5 // This file is distributed under the University of Illinois Open Source 6 // License. See LICENSE.TXT for details. 7 // 8 //===----------------------------------------------------------------------===// 9 10 #ifndef LLD_CORE_UNDEFINED_ATOM_H 11 #define LLD_CORE_UNDEFINED_ATOM_H 12 13 #include "lld/Core/Atom.h" 14 15 namespace lld { 16 17 /// An UndefinedAtom has no content. 18 /// It exists as a placeholder for a future atom. 19 class UndefinedAtom : public Atom { 20 public: 21 /// Whether this undefined symbol needs to be resolved, 22 /// or whether it can just evaluate to nullptr. 23 /// This concept is often called "weak", but that term 24 /// is overloaded to mean other things too. 25 enum CanBeNull { 26 /// Normal symbols must be resolved at build time 27 canBeNullNever, 28 29 /// This symbol can be missing at runtime and will evalute to nullptr. 30 /// That is, the static linker still must find a definition (usually 31 /// is some shared library), but at runtime, the dynamic loader 32 /// will allow the symbol to be missing and resolved to nullptr. 33 /// 34 /// On Darwin this is generated using a function prototype with 35 /// __attribute__((weak_import)). 36 /// On linux this is generated using a function prototype with 37 /// __attribute__((weak)). 38 /// On Windows this feature is not supported. 39 canBeNullAtRuntime, 40 41 /// This symbol can be missing at build time. 42 /// That is, the static linker will not error if a definition for 43 /// this symbol is not found at build time. Instead, the linker 44 /// will build an executable that lets the dynamic loader find the 45 /// symbol at runtime. 46 /// This feature is not supported on Darwin nor Windows. 47 /// On linux this is generated using a function prototype with 48 /// __attribute__((weak)). 49 canBeNullAtBuildtime 50 }; 51 52 virtual CanBeNull canBeNull() const = 0; 53 54 static bool classof(const Atom *a) { 55 return a->definition() == definitionUndefined; 56 } 57 58 static bool classof(const UndefinedAtom *) { return true; } 59 60 protected: 61 UndefinedAtom() : Atom(definitionUndefined) {} 62 63 ~UndefinedAtom() override = default; 64 }; 65 66 } // namespace lld 67 68 #endif // LLD_CORE_UNDEFINED_ATOM_H 69
