llvm-project/lld/wasm/MarkLive.cpp

//===- MarkLive.cpp -------------------------------------------------------===//
//
// Part of the LLVM Project, under the Apache License v2.0 with LLVM Exceptions.
// See https://llvm.org/LICENSE.txt for license information.
// SPDX-License-Identifier: Apache-2.0 WITH LLVM-exception
//
//===----------------------------------------------------------------------===//
//
// This file implements --gc-sections, which is a feature to remove unused
// chunks from the output. Unused chunks are those that are not reachable from
// known root symbols or chunks. This feature is implemented as a mark-sweep
// garbage collector.
//
// Here's how it works. Each InputChunk has a "Live" bit. The bit is off by
// default. Starting with the GC-roots, visit all reachable chunks and set their
// Live bits. The Writer will then ignore chunks whose Live bits are off, so
// that such chunk are not appear in the output.
//
//===----------------------------------------------------------------------===//

#include "MarkLive.h"
#include "Config.h"
#include "InputChunks.h"
#include "InputEvent.h"
#include "InputGlobal.h"
#include "SymbolTable.h"
#include "Symbols.h"

#define DEBUG_TYPE "lld"

using namespace llvm;
using namespace llvm::wasm;

void lld::wasm::markLive() {
  if (!config->gcSections)
    return;

  LLVM_DEBUG(dbgs() << "markLive\n");
  SmallVector<InputChunk *, 256> q;

  std::function<void(Symbol*)> enqueue = [&](Symbol *sym) {
    if (!sym || sym->isLive())
      return;
    LLVM_DEBUG(dbgs() << "markLive: " << sym->getName() << "\n");
    sym->markLive();
    if (InputChunk *chunk = sym->getChunk())
      q.push_back(chunk);

    // The ctor functions are all referenced by the synthetic CallCtors
    // function.  However, this function does not contain relocations so we
    // have to manually mark the ctors as live if CallCtors itself is live.
    if (sym == WasmSym::callCtors) {
      if (config->passiveSegments)
        enqueue(WasmSym::initMemory);
      if (config->isPic)
        enqueue(WasmSym::applyRelocs);
      for (const ObjFile *obj : symtab->objectFiles) {
        const WasmLinkingData &l = obj->getWasmObj()->linkingData();
        for (const WasmInitFunc &f : l.InitFunctions) {
          auto* initSym = obj->getFunctionSymbol(f.Symbol);
          if (!initSym->isDiscarded())
            enqueue(initSym);
        }
      }
    }
  };

  // Add GC root symbols.
  if (!config->entry.empty())
    enqueue(symtab->find(config->entry));

  // We need to preserve any exported symbol
  for (Symbol *sym : symtab->getSymbols())
    if (sym->isExported())
      enqueue(sym);

  // For relocatable output, we need to preserve all the ctor functions
  if (config->relocatable) {
    for (const ObjFile *obj : symtab->objectFiles) {
      const WasmLinkingData &l = obj->getWasmObj()->linkingData();
      for (const WasmInitFunc &f : l.InitFunctions)
        enqueue(obj->getFunctionSymbol(f.Symbol));
    }
  }

  if (config->isPic)
    enqueue(WasmSym::callCtors);

  // Follow relocations to mark all reachable chunks.
  while (!q.empty()) {
    InputChunk *c = q.pop_back_val();

    for (const WasmRelocation reloc : c->getRelocations()) {
      if (reloc.Type == R_WASM_TYPE_INDEX_LEB)
        continue;
      Symbol *sym = c->file->getSymbol(reloc.Index);

      // If the function has been assigned the special index zero in the table,
      // the relocation doesn't pull in the function body, since the function
      // won't actually go in the table (the runtime will trap attempts to call
      // that index, since we don't use it).  A function with a table index of
      // zero is only reachable via "call", not via "call_indirect".  The stub
      // functions used for weak-undefined symbols have this behaviour (compare
      // equal to null pointer, only reachable via direct call).
      if (reloc.Type == R_WASM_TABLE_INDEX_SLEB ||
          reloc.Type == R_WASM_TABLE_INDEX_I32) {
        auto *funcSym = cast<FunctionSymbol>(sym);
        if (funcSym->hasTableIndex() && funcSym->getTableIndex() == 0)
          continue;
      }

      enqueue(sym);
    }
  }

  // Report garbage-collected sections.
  if (config->printGcSections) {
    for (const ObjFile *obj : symtab->objectFiles) {
      for (InputChunk *c : obj->functions)
        if (!c->live)
          message("removing unused section " + toString(c));
      for (InputChunk *c : obj->segments)
        if (!c->live)
          message("removing unused section " + toString(c));
      for (InputGlobal *g : obj->globals)
        if (!g->live)
          message("removing unused section " + toString(g));
      for (InputEvent *e : obj->events)
        if (!e->live)
          message("removing unused section " + toString(e));
    }
    for (InputChunk *c : symtab->syntheticFunctions)
      if (!c->live)
        message("removing unused section " + toString(c));
    for (InputGlobal *g : symtab->syntheticGlobals)
      if (!g->live)
        message("removing unused section " + toString(g));
  }
}