1 //===- MarkLive.cpp -------------------------------------------------------===// 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 // This file implements --gc-sections, which is a feature to remove unused 11 // chunks from the output. Unused chunks are those that are not reachable from 12 // known root symbols or chunks. This feature is implemented as a mark-sweep 13 // garbage collector. 14 // 15 // Here's how it works. Each InputChunk has a "Live" bit. The bit is off by 16 // default. Starting with the GC-roots, visit all reachable chunks and set their 17 // Live bits. The Writer will then ignore chunks whose Live bits are off, so 18 // that such chunk are not appear in the output. 19 // 20 //===----------------------------------------------------------------------===// 21 22 #include "MarkLive.h" 23 #include "Config.h" 24 #include "InputChunks.h" 25 #include "SymbolTable.h" 26 #include "Symbols.h" 27 28 #define DEBUG_TYPE "lld" 29 30 using namespace llvm; 31 using namespace llvm::wasm; 32 using namespace lld; 33 using namespace lld::wasm; 34 35 void lld::wasm::markLive() { 36 if (!Config->GcSections) 37 return; 38 39 DEBUG(dbgs() << "markLive\n"); 40 SmallVector<InputChunk *, 256> Q; 41 42 auto Enqueue = [&](Symbol *Sym) { 43 if (!Sym) 44 return; 45 InputChunk *Chunk = Sym->getChunk(); 46 if (!Chunk || Chunk->Live) 47 return; 48 Chunk->Live = true; 49 Q.push_back(Chunk); 50 }; 51 52 // Add GC root symbols. 53 if (!Config->Entry.empty()) 54 Enqueue(Symtab->find(Config->Entry)); 55 Enqueue(WasmSym::CallCtors); 56 57 // By default we export all non-hidden, so they are gc roots too 58 for (Symbol *Sym : Symtab->getSymbols()) 59 if (!Sym->isHidden()) 60 Enqueue(Sym); 61 62 // The ctor functions are all used in the synthetic __wasm_call_ctors 63 // function, but since this function is created in-place it doesn't contain 64 // relocations which mean we have to manually mark the ctors. 65 for (const ObjFile *Obj : Symtab->ObjectFiles) { 66 const WasmLinkingData &L = Obj->getWasmObj()->linkingData(); 67 for (const WasmInitFunc &F : L.InitFunctions) 68 Enqueue(Obj->getFunctionSymbol(F.Symbol)); 69 } 70 71 // Follow relocations to mark all reachable chunks. 72 while (!Q.empty()) { 73 InputChunk *C = Q.pop_back_val(); 74 75 for (const WasmRelocation Reloc : C->getRelocations()) { 76 if (Reloc.Type == R_WEBASSEMBLY_TYPE_INDEX_LEB) 77 continue; 78 Symbol *Sym = C->File->getSymbol(Reloc.Index); 79 80 // If the function has been assigned the special index zero in the table, 81 // the relocation doesn't pull in the function body, since the function 82 // won't actually go in the table (the runtime will trap attempts to call 83 // that index, since we don't use it). A function with a table index of 84 // zero is only reachable via "call", not via "call_indirect". The stub 85 // functions used for weak-undefined symbols have this behaviour (compare 86 // equal to null pointer, only reachable via direct call). 87 if (Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_SLEB || 88 Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_I32) { 89 FunctionSymbol *FuncSym = cast<FunctionSymbol>(Sym); 90 if (FuncSym->hasTableIndex() && FuncSym->getTableIndex() == 0) 91 continue; 92 } 93 94 Enqueue(Sym); 95 } 96 } 97 98 // Report garbage-collected sections. 99 if (Config->PrintGcSections) { 100 for (const ObjFile *Obj : Symtab->ObjectFiles) { 101 for (InputChunk *C : Obj->Functions) 102 if (!C->Live) 103 message("removing unused section " + toString(C)); 104 for (InputChunk *C : Obj->Segments) 105 if (!C->Live) 106 message("removing unused section " + toString(C)); 107 } 108 } 109 } 110