Coverage Report

Created: 2018-08-19 14:04

/Users/buildslave/jenkins/workspace/clang-stage2-coverage-R/llvm/tools/lld/wasm/MarkLive.cpp
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//===- MarkLive.cpp -------------------------------------------------------===//
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//
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//                             The LLVM Linker
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//
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// This file is distributed under the University of Illinois Open Source
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// License. See LICENSE.TXT for details.
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//
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//===----------------------------------------------------------------------===//
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//
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// This file implements --gc-sections, which is a feature to remove unused
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// chunks from the output. Unused chunks are those that are not reachable from
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// known root symbols or chunks. This feature is implemented as a mark-sweep
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// garbage collector.
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//
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// Here's how it works. Each InputChunk has a "Live" bit. The bit is off by
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// default. Starting with the GC-roots, visit all reachable chunks and set their
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// Live bits. The Writer will then ignore chunks whose Live bits are off, so
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// that such chunk are not appear in the output.
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//
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//===----------------------------------------------------------------------===//
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#include "MarkLive.h"
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#include "Config.h"
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#include "InputChunks.h"
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#include "InputGlobal.h"
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#include "SymbolTable.h"
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#include "Symbols.h"
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#define DEBUG_TYPE "lld"
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using namespace llvm;
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using namespace llvm::wasm;
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using namespace lld;
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using namespace lld::wasm;
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0
void lld::wasm::markLive() {
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  if (!Config->GcSections)
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    return;
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  LLVM_DEBUG(dbgs() << "markLive\n");
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  SmallVector<InputChunk *, 256> Q;
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  auto Enqueue = [&](Symbol *Sym) {
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    if (!Sym || Sym->isLive())
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      return;
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    LLVM_DEBUG(dbgs() << "markLive: " << Sym->getName() << "\n");
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    Sym->markLive();
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    if (InputChunk *Chunk = Sym->getChunk())
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      Q.push_back(Chunk);
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  };
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  // Add GC root symbols.
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  if (!Config->Entry.empty())
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    Enqueue(Symtab->find(Config->Entry));
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  Enqueue(WasmSym::CallCtors);
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0
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  // We need to preserve any exported symbol
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  for (Symbol *Sym : Symtab->getSymbols())
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    if (Sym->isExported())
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      Enqueue(Sym);
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  // The ctor functions are all used in the synthetic __wasm_call_ctors
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  // function, but since this function is created in-place it doesn't contain
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  // relocations which mean we have to manually mark the ctors.
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  for (const ObjFile *Obj : Symtab->ObjectFiles) {
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    const WasmLinkingData &L = Obj->getWasmObj()->linkingData();
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    for (const WasmInitFunc &F : L.InitFunctions)
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      Enqueue(Obj->getFunctionSymbol(F.Symbol));
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  }
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  // Follow relocations to mark all reachable chunks.
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  while (!Q.empty()) {
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    InputChunk *C = Q.pop_back_val();
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    for (const WasmRelocation Reloc : C->getRelocations()) {
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      if (Reloc.Type == R_WEBASSEMBLY_TYPE_INDEX_LEB)
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        continue;
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      Symbol *Sym = C->File->getSymbol(Reloc.Index);
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      // If the function has been assigned the special index zero in the table,
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      // the relocation doesn't pull in the function body, since the function
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      // won't actually go in the table (the runtime will trap attempts to call
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      // that index, since we don't use it).  A function with a table index of
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      // zero is only reachable via "call", not via "call_indirect".  The stub
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      // functions used for weak-undefined symbols have this behaviour (compare
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      // equal to null pointer, only reachable via direct call).
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      if (Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_SLEB ||
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          Reloc.Type == R_WEBASSEMBLY_TABLE_INDEX_I32) {
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        FunctionSymbol *FuncSym = cast<FunctionSymbol>(Sym);
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        if (FuncSym->hasTableIndex() && FuncSym->getTableIndex() == 0)
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          continue;
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      }
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      Enqueue(Sym);
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    }
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  }
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  // Report garbage-collected sections.
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  if (Config->PrintGcSections) {
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    for (const ObjFile *Obj : Symtab->ObjectFiles) {
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      for (InputChunk *C : Obj->Functions)
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        if (!C->Live)
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          message("removing unused section " + toString(C));
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      for (InputChunk *C : Obj->Segments)
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        if (!C->Live)
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          message("removing unused section " + toString(C));
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      for (InputGlobal *G : Obj->Globals)
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        if (!G->Live)
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          message("removing unused section " + toString(G));
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    }
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    for (InputChunk *C : Symtab->SyntheticFunctions)
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      if (!C->Live)
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        message("removing unused section " + toString(C));
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    for (InputGlobal *G : Symtab->SyntheticGlobals)
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      if (!G->Live)
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        message("removing unused section " + toString(G));
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  }
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}