c++ - 是否可以在 Objective-C++ 中删除 dispatch_once？

Question

自 C++11 起，已知局部 static 变量以线程安全的方式初始化(​​除非给出了 -fno-threadsafe-statics)，如指定in this question .这是否意味着以下众所周知的模式:

+ (NSObject *)onlyOnce {
  static NSObject *object;
  static dispatch_once_t onceToken;
  dispatch_once(&onceToken, ^{
    object = [[NSObject alloc] init];
  });
  return object;
}

可以替换为更短的:

+ (NSObject *)onlyOnce {
  static NSObject *object = [[NSObject alloc] init];
  return object;
}

当使用 C++11 及更高版本的 C++ 语言方言将代码编译为 Objective-C++ 时？

Answer 1

TL;DR - 似乎可以以线程安全的方式使用 C++11 静态变量初始化，它具有与 dispatch_once 相同的性能特征。

根据 Stephan Lechner 的回答，我编写了最简单的代码来测试 C++ 静态初始化流程:

class Object {  
};

static Object *GetObjectCppStatic() {
  static Object *object = new Object();
  return object;
}

int main() {
  GetObjectCppStatic();
}

通过 clang++ test.cpp -O0 -fno-exceptions -S 将其编译为汇编(-O0 以避免内联，为 生成相同的通用代码-Os, -fno-exceptions 以简化生成的代码)，表明 GetObjectCppStatic 编译为:

__ZL18GetObjectCppStaticv:        ## @_ZL18GetObjectCppStaticv
  .cfi_startproc
## BB#0:
  pushq   %rbp
Lcfi6:
  .cfi_def_cfa_offset 16
Lcfi7:
  .cfi_offset %rbp, -16
  movq  %rsp, %rbp
Lcfi8:
  .cfi_def_cfa_register %rbp
  cmpb  $0, __ZGVZL18GetObjectCppStaticvE6object(%rip)
  jne LBB2_3
## BB#1:
  leaq  __ZGVZL18GetObjectCppStaticvE6object(%rip), %rdi
  callq   ___cxa_guard_acquire
  cmpl  $0, %eax
  je  LBB2_3
## BB#2:
  movl  $1, %eax
  movl  %eax, %edi
  callq   __Znwm
  leaq  __ZGVZL18GetObjectCppStaticvE6object(%rip), %rdi
  movq  %rax, __ZZL18GetObjectCppStaticvE6object(%rip)
  callq   ___cxa_guard_release
LBB2_3:
  movq  __ZZL18GetObjectCppStaticvE6object(%rip), %rax
  popq  %rbp
  retq
  .cfi_endproc

我们肯定可以看到由 libc++ ABI 实现的 ___cxa_guard_acquire 和 ___cxa_guard_release here .请注意，我们甚至不必向 clang 指定我们使用 C++11，因为显然这甚至在此之前就已默认支持。

所以我们知道这两种形式都确保了局部静态的线程安全初始化。但是性能呢？以下测试代码检查无争用(单线程)和高争用(多线程)两种方法:

#include <cstdio>
#include <dispatch/dispatch.h>
#include <mach/mach_time.h>

class Object {  
};

static double Measure(int times, void(^executionBlock)(), void(^finallyBlock)()) {
  struct mach_timebase_info timebaseInfo;
  mach_timebase_info(&timebaseInfo);

  uint64_t start = mach_absolute_time();
  for (int i = 0; i < times; ++i) {
    executionBlock();
  }
  finallyBlock();
  uint64_t end = mach_absolute_time();

  uint64_t timeTook = end - start;
  return ((double)timeTook * timebaseInfo.numer / timebaseInfo.denom) /
      NSEC_PER_SEC;
}

static Object *GetObjectDispatchOnce() {
  static Object *object;
  static dispatch_once_t onceToken;

  dispatch_once(&onceToken, ^{
    object = new Object();
  });

  return object;
}

static Object *GetObjectCppStatic() {
  static Object *object = new Object();
  return object;
}

int main() {
  printf("Single thread statistics:\n");
  printf("DispatchOnce took %g\n", Measure(10000000, ^{
    GetObjectDispatchOnce();
  }, ^{}));
  printf("CppStatic took %g\n", Measure(10000000, ^{
    GetObjectCppStatic();
  }, ^{}));

  printf("\n");

  dispatch_queue_t queue = dispatch_queue_create("queue", 
      DISPATCH_QUEUE_CONCURRENT);
  dispatch_group_t group = dispatch_group_create();

  printf("Multi thread statistics:\n");
  printf("DispatchOnce took %g\n", Measure(1000000, ^{
    dispatch_group_async(group, queue, ^{
      GetObjectDispatchOnce();
    });
  }, ^{
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
  }));
  printf("CppStatic took %g\n", Measure(1000000, ^{
    dispatch_group_async(group, queue, ^{
      GetObjectCppStatic();
    });
  }, ^{
    dispatch_group_wait(group, DISPATCH_TIME_FOREVER);
  }));
}

在 x64 上产生以下结果:

Single thread statistics:
DispatchOnce took 0.025486
CppStatic took 0.0232348

Multi thread statistics:
DispatchOnce took 0.285058
CppStatic took 0.32596

所以直到测量误差，这两种方法的性能特征似乎相似，主要是由于 double-check locking这是由他们两个执行的。对于 dispatch_once，这发生在 _dispatch_once 函数中:

void
_dispatch_once(dispatch_once_t *predicate,
    DISPATCH_NOESCAPE dispatch_block_t block)
{
  if (DISPATCH_EXPECT(*predicate, ~0l) != ~0l) {
    // ...
  } else {
    // ...
  }
}

在 C++ 静态初始化流程中，它发生在调用 ___cxa_guard_acquire 之前。