wasm-ctor-eval.cpp

/*
 * Copyright 2017 WebAssembly Community Group participants
 *
 * Licensed under the Apache License, Version 2.0 (the "License");
 * you may not use this file except in compliance with the License.
 * You may obtain a copy of the License at
 *
 *     http://www.apache.org/licenses/LICENSE-2.0
 *
 * Unless required by applicable law or agreed to in writing, software
 * distributed under the License is distributed on an "AS IS" BASIS,
 * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
 * See the License for the specific language governing permissions and
 * limitations under the License.
 */

//
// Loads wasm plus a list of functions that are global ctors, i.e.,
// are to be executed. It then executes as many of them as it can,
// applying their changes to memory as needed, then writes it. In
// other words, this executes code at compile time to speed up
// startup later.
//

#include <memory>

#include "pass.h"
#include "support/command-line.h"
#include "support/file.h"
#include "support/colors.h"
#include "wasm-io.h"
#include "wasm-interpreter.h"
#include "wasm-builder.h"
#include "wasm-validator.h"
#include "ir/memory-utils.h"
#include "ir/global-utils.h"
#include "ir/import-utils.h"
#include "ir/literal-utils.h"
#include "ir/module-utils.h"

using namespace wasm;

struct FailToEvalException {
  std::string why;
  FailToEvalException(std::string why) : why(why) {}
};

// We do not have access to imported globals
class EvallingGlobalManager {
  // values of globals
  std::map<Name, Literal> globals;

  // globals that are dangerous to modify in the module
  std::set<Name> dangerousGlobals;

  // whether we are done adding new globals
  bool sealed = false;

public:
  void addDangerous(Name name) {
    dangerousGlobals.insert(name);
  }

  void seal() {
    sealed = true;
  }

  // for equality purposes, we just care about the globals
  // and whether they have changed
  bool operator==(const EvallingGlobalManager& other) {
    return globals == other.globals;
  }
  bool operator!=(const EvallingGlobalManager& other) {
    return !(*this == other);
  }

  Literal& operator[](Name name) {
    if (dangerousGlobals.count(name) > 0) {
      std::string extra;
      if (name == "___dso_handle") {
        extra = "\nrecommendation: build with -s NO_EXIT_RUNTIME=1 so that calls to atexit that use ___dso_handle are not emitted";
      }
      throw FailToEvalException(std::string("tried to access a dangerous (import-initialized) global: ") + name.str + extra);
    }
    return globals[name];
  }

  struct Iterator {
    Name first;
    Literal second;
    bool found;

    Iterator() : found(false) {}
    Iterator(Name name, Literal value) : first(name), second(value), found(true) {}

    bool operator==(const Iterator& other) {
      return first == other.first && second == other.second && found == other.found;
    }
    bool operator!=(const Iterator& other) {
      return !(*this == other);
    }
  };

  Iterator find(Name name) {
    if (globals.find(name) == globals.end()) {
      return end();
    }
    return Iterator(name, globals[name]);
  }

  Iterator end() {
    return Iterator();
  }
};

// Use a ridiculously large stack size.
static Index STACK_SIZE = 32 * 1024 * 1024;

// Start the stack at a ridiculously large location, and do so in
// a way that works regardless if the stack goes up or down.
static Index STACK_START = 1024 * 1024 * 1024 + STACK_SIZE;

// Bound the stack location in both directions, so we have bounds
// that do not depend on the direction it grows.
static Index STACK_LOWER_LIMIT = STACK_START - STACK_SIZE;
static Index STACK_UPPER_LIMIT = STACK_START + STACK_SIZE;

class EvallingModuleInstance : public ModuleInstanceBase<EvallingGlobalManager, EvallingModuleInstance> {
public:
  EvallingModuleInstance(Module& wasm, ExternalInterface* externalInterface) : ModuleInstanceBase(wasm, externalInterface) {
    // if any global in the module has a non-const constructor, it is using a global import,
    // which we don't have, and is illegal to use
    ModuleUtils::iterDefinedGlobals(wasm, [&](Global* global) {
      if (!global->init->is<Const>()) {
        // some constants are ok to use
        if (auto* get = global->init->dynCast<GetGlobal>()) {
          auto name = get->name;
          auto* import = wasm.getGlobal(name);
          if (import->module == Name(ENV) && (
            import->base == STACKTOP || // stack constants are special, we handle them
            import->base == STACK_MAX
          )) {
            return; // this is fine
          }
        }
        // this global is dangerously initialized by an import, so if it is used, we must fail
        globals.addDangerous(global->name);
      }
    });
  }

  std::vector<char> stack;

  // create C stack space for us to use. We do *NOT* care about their contents,
  // assuming the stack top was unwound. the memory may have been modified,
  // but it should not be read afterwards, doing so would be undefined behavior
  void setupEnvironment() {
    // prepare scratch memory
    stack.resize(2 * STACK_SIZE);
    // tell the module to accept writes up to the stack end
    auto total = STACK_START + STACK_SIZE;
    memorySize = total / Memory::kPageSize;
  }

  // flatten memory into a single segment
  void flattenMemory() {
    MemoryUtils::flatten(wasm.memory);
  }
};

struct CtorEvalExternalInterface : EvallingModuleInstance::ExternalInterface {
  Module* wasm;
  EvallingModuleInstance* instance;

  void init(Module& wasm_, EvallingModuleInstance& instance_) override {
    wasm = &wasm_;
    instance = &instance_;
  }

  void importGlobals(EvallingGlobalManager& globals, Module& wasm_) override {
    // fill usable values for stack imports, and globals initialized to them
    ImportInfo imports(wasm_);
    if (auto* stackTop = imports.getImportedGlobal(ENV, STACKTOP)) {
      globals[stackTop->name] = Literal(int32_t(STACK_START));
      if (auto* stackTop = GlobalUtils::getGlobalInitializedToImport(wasm_, ENV, STACKTOP)) {
        globals[stackTop->name] = Literal(int32_t(STACK_START));
      }
    }
    if (auto* stackMax = imports.getImportedGlobal(ENV, STACK_MAX)) {
      globals[stackMax->name] = Literal(int32_t(STACK_START));
      if (auto* stackMax = GlobalUtils::getGlobalInitializedToImport(wasm_, ENV, STACK_MAX)) {
        globals[stackMax->name] = Literal(int32_t(STACK_START));
      }
    }
    // fill in fake values for everything else, which is dangerous to use
    ModuleUtils::iterDefinedGlobals(wasm_, [&](Global* defined) {
      if (globals.find(defined->name) == globals.end()) {
        globals[defined->name] = Literal::makeZero(defined->type);
      }
    });
    ModuleUtils::iterImportedGlobals(wasm_, [&](Global* import) {
      if (globals.find(import->name) == globals.end()) {
        globals[import->name] = Literal::makeZero(import->type);
      }
    });
  }

  Literal callImport(Function* import, LiteralList& arguments) override {
    std::string extra;
    if (import->module == ENV && import->base == "___cxa_atexit") {
      extra = "\nrecommendation: build with -s NO_EXIT_RUNTIME=1 so that calls to atexit are not emitted";
    }
    throw FailToEvalException(std::string("call import: ") + import->module.str + "." + import->base.str + extra);
  }

  Literal callTable(Index index, LiteralList& arguments, Type result, EvallingModuleInstance& instance) override {
    // we assume the table is not modified (hmm)
    // look through the segments, try to find the function
    for (auto& segment : wasm->table.segments) {
      Index start;
      // look for the index in this segment. if it has a constant offset, we look in
      // the proper range. if it instead gets a global, we rely on the fact that when
      // not dynamically linking then the table is loaded at offset 0.
      if (auto* c = segment.offset->dynCast<Const>()) {
        start = c->value.getInteger();
      } else if (segment.offset->is<GetGlobal>()) {
        start = 0;
      } else {
        WASM_UNREACHABLE(); // wasm spec only allows const and global.get there
      }
      auto end = start + segment.data.size();
      if (start <= index && index < end) {
        auto name = segment.data[index - start];
        // if this is one of our functions, we can call it; if it was imported, fail
        auto* func = wasm->getFunction(name);
        if (!func->imported()) {
          return instance.callFunctionInternal(name, arguments);
        } else {
          throw FailToEvalException(std::string("callTable on imported function: ") + name.str);
        }
      }
    }
    throw FailToEvalException(std::string("callTable on index not found in static segments: ") + std::to_string(index));
  }

  int8_t load8s(Address addr) override { return doLoad<int8_t>(addr); }
  uint8_t load8u(Address addr) override { return doLoad<uint8_t>(addr); }
  int16_t load16s(Address addr) override { return doLoad<int16_t>(addr); }
  uint16_t load16u(Address addr) override { return doLoad<uint16_t>(addr); }
  int32_t load32s(Address addr) override { return doLoad<int32_t>(addr); }
  uint32_t load32u(Address addr) override { return doLoad<uint32_t>(addr); }
  int64_t load64s(Address addr) override { return doLoad<int64_t>(addr); }
  uint64_t load64u(Address addr) override { return doLoad<uint64_t>(addr); }

  void store8(Address addr, int8_t value) override { doStore<int8_t>(addr, value); }
  void store16(Address addr, int16_t value) override { doStore<int16_t>(addr, value); }
  void store32(Address addr, int32_t value) override { doStore<int32_t>(addr, value); }
  void store64(Address addr, int64_t value) override { doStore<int64_t>(addr, value); }

  void growMemory(Address /*oldSize*/, Address newSize) override {
    throw FailToEvalException("grow memory");
  }

  void trap(const char* why) override {
    throw FailToEvalException(std::string("trap: ") + why);
  }

private:
  // TODO: handle unaligned too, see shell-interface

  template<typename T>
  T* getMemory(Address address) {
    // if memory is on the stack, use the stack
    if (address >= STACK_LOWER_LIMIT) {
      if (address >= STACK_UPPER_LIMIT) {
        throw FailToEvalException("stack usage too high");
      }
      Address relative = address - STACK_LOWER_LIMIT;
      // in range, all is good, use the stack
      return (T*)(&instance->stack[relative]);
    }

    // otherwise, this must be in the singleton segment. resize as needed
    if (wasm->memory.segments.size() == 0) {
      std::vector<char> temp;
      Builder builder(*wasm);
      wasm->memory.segments.push_back(
        Memory::Segment(
          builder.makeConst(Literal(int32_t(0))),
          temp
        )
      );
    }
    assert(wasm->memory.segments[0].offset->cast<Const>()->value.getInteger() == 0);
    auto max = address + sizeof(T);
    auto& data = wasm->memory.segments[0].data;
    if (max > data.size()) {
      data.resize(max);
    }
    return (T*)(&data[address]);
  }

  template<typename T>
  void doStore(Address address, T value) {
    // do a memcpy to avoid undefined behavior if unaligned
    memcpy(getMemory<T>(address), &value, sizeof(T));
  }

  template<typename T>
  T doLoad(Address address) {
    // do a memcpy to avoid undefined behavior if unaligned
    T ret;
    memcpy(&ret, getMemory<T>(address), sizeof(T));
    return ret;
  }
};

void evalCtors(Module& wasm, std::vector<std::string> ctors) {
  CtorEvalExternalInterface interface;
  try {
    // create an instance for evalling
    EvallingModuleInstance instance(wasm, &interface);
    // flatten memory, so we do not depend on the layout of data segments
    instance.flattenMemory();
    // set up the stack area and other environment details
    instance.setupEnvironment();
    // we should not add new globals from here on; as a result, using
    // an imported global will fail, as it is missing and so looks new
    instance.globals.seal();
    // go one by one, in order, until we fail
    // TODO: if we knew priorities, we could reorder?
    for (auto& ctor : ctors) {
      std::cerr << "trying to eval " << ctor << '\n';
      // snapshot memory, as either the entire function is done, or none
      auto memoryBefore = wasm.memory;
      // snapshot globals (note that STACKTOP might be modified, but should
      // be returned, so that works out)
      auto globalsBefore = instance.globals;
      Export *ex = wasm.getExportOrNull(ctor);
      if (!ex) {
        Fatal() << "export not found: " << ctor;
      }
      try {
        instance.callFunction(ex->value, LiteralList());
      } catch (FailToEvalException& fail) {
        // that's it, we failed, so stop here, cleaning up partial
        // memory changes first
        std::cerr << "  ...stopping since could not eval: " << fail.why << "\n";
        wasm.memory = memoryBefore;
        return;
      }
      if (instance.globals != globalsBefore) {
        std::cerr << "  ...stopping since globals modified\n";
        wasm.memory = memoryBefore;
        return;
      }
      std::cerr << "  ...success on " << ctor << ".\n";
      // success, the entire function was evalled!
      // we can nop the function (which may be used elsewhere)
      // and remove the export
      auto* exp = wasm.getExport(ctor);
      auto* func = wasm.getFunction(exp->value);
      func->body = wasm.allocator.alloc<Nop>();
      wasm.removeExport(exp->name);
    }
  } catch (FailToEvalException& fail) {
    // that's it, we failed to even create the instance
    std::cerr << "  ...stopping since could not create module instance: " << fail.why << "\n";
    return;
  }
}

//
// main
//

int main(int argc, const char* argv[]) {
  Name entry;
  std::vector<std::string> passes;
  bool emitBinary = true;
  bool debugInfo = false;
  std::string ctorsString;

  Options options("wasm-ctor-eval", "Execute C++ global constructors ahead of time");
  options
      .add("--output", "-o", "Output file (stdout if not specified)",
           Options::Arguments::One,
           [](Options* o, const std::string& argument) {
             o->extra["output"] = argument;
             Colors::disable();
           })
      .add("--emit-text", "-S", "Emit text instead of binary for the output file",
           Options::Arguments::Zero,
           [&](Options *o, const std::string& argument) { emitBinary = false; })
      .add("--debuginfo", "-g", "Emit names section and debug info",
           Options::Arguments::Zero,
           [&](Options *o, const std::string& arguments) { debugInfo = true; })
      .add("--ctors", "-c", "Comma-separated list of global constructor functions to evaluate",
           Options::Arguments::One,
           [&](Options* o, const std::string& argument) {
             ctorsString = argument;
           })
      .add_positional("INFILE", Options::Arguments::One,
                      [](Options* o, const std::string& argument) {
                        o->extra["infile"] = argument;
                      });
  options.parse(argc, argv);

  auto input(read_file<std::string>(options.extra["infile"], Flags::Text, options.debug ? Flags::Debug : Flags::Release));

  Module wasm;

  {
    if (options.debug) std::cerr << "reading...\n";
    ModuleReader reader;
    reader.setDebug(options.debug);

    try {
      reader.read(options.extra["infile"], wasm);
    } catch (ParseException& p) {
      p.dump(std::cerr);
      Fatal() << "error in parsing input";
    }
  }

  if (!WasmValidator().validate(wasm)) {
    WasmPrinter::printModule(&wasm);
    Fatal() << "error in validating input";
  }

  // get list of ctors, and eval them
  std::vector<std::string> ctors;
  std::istringstream stream(ctorsString);
  std::string temp;
  while (std::getline(stream, temp, ',')) {
    ctors.push_back(temp);
  }
  evalCtors(wasm, ctors);

  // Do some useful optimizations after the evalling
  {
    PassRunner passRunner(&wasm);
    passRunner.add("memory-packing"); // we flattened it, so re-optimize
    passRunner.add("remove-unused-names");
    passRunner.add("dce");
    passRunner.add("merge-blocks");
    passRunner.add("vacuum");
    passRunner.add("remove-unused-module-elements");
    passRunner.run();
  }

  if (options.extra.count("output") > 0) {
    if (options.debug) std::cerr << "writing..." << std::endl;
    ModuleWriter writer;
    writer.setDebug(options.debug);
    writer.setBinary(emitBinary);
    writer.setDebugInfo(debugInfo);
    writer.write(wasm, options.extra["output"]);
  }
}