如果屏蔽[NSString stringWithFormat:@"%@%@", num, str]; 没有显示添加autoreleasepool内存不会突增

[chipengliu]

情况1：Run loop without autoReleasePool中把下面代码屏蔽，运行demo，没有内存突增

//[NSString stringWithFormat:@"%@%@", num, str];

反编译伪代码代码

void __cdecl _26__ViewController_runTest___block_invoke_14(__block_literal_2 *.block_descriptor)
{
  void *v1; // rax
  void *v2; // rax
  Foundation::NSArray::NSMutableArray *v3; // ST18_8
  double v4; // xmm0_8
  void *v5; // rax
  __int64 v6; // ST08_8
  Foundation::NSString::NSString *str; // [rsp+38h] [rbp-28h]
  Foundation::NSValue::NSNumber *num; // [rsp+40h] [rbp-20h]
  int i; // [rsp+4Ch] [rbp-14h]
  __block_literal_2 *v10; // [rsp+50h] [rbp-10h]
  __block_literal_2 *_block_descriptor; // [rsp+58h] [rbp-8h]

  _block_descriptor = .block_descriptor;
  v10 = .block_descriptor;
  for ( i = 0; i < 500000; ++i )
  {
    v1 = objc_msgSend(&OBJC_CLASS___NSNumber, "numberWithInt:", (unsigned int)i);
    num = (Foundation::NSValue::NSNumber *)objc_retainAutoreleasedReturnValue(v1);
    v2 = objc_msgSend(&OBJC_CLASS___NSString, "stringWithFormat:", CFSTR("%d "), (unsigned int)i);
    str = (Foundation::NSString::NSString *)objc_retainAutoreleasedReturnValue(v2);
    
    if ( !(i % 50000) )
    {
      v3 = (&.block_descriptor->self->_memoryUsageList2)[3];
      v4 = getMemoryUsage();
      v5 = objc_msgSend(&OBJC_CLASS___NSNumber, "numberWithDouble:", v4);
      v6 = objc_retainAutoreleasedReturnValue(v5);
      objc_msgSend(v3, "addObject:", v6);
      objc_release(v6);
    }
    objc_storeStrong(&str, 0LL);
    objc_storeStrong(&num, 0LL);
  }
}

---

情况2：或者将屏蔽代码改为，结果和上面一样，也没有内存突增

[NSString stringWithFormat:@"%@", str];

反编译伪代码代码

void __cdecl _26__ViewController_runTest___block_invoke_14(__block_literal_2 *.block_descriptor)
{
  void *v1; // rax
  void *v2; // rax
  Foundation::NSString::NSString *v3; // rax
  void *v4; // rax
  __int64 v5; // rax
  Foundation::NSArray::NSMutableArray *v6; // ST18_8
  double v7; // xmm0_8
  void *v8; // rax
  __int64 v9; // ST08_8
  Foundation::NSString::NSString *str; // [rsp+38h] [rbp-28h]
  Foundation::NSValue::NSNumber *num; // [rsp+40h] [rbp-20h]
  int i; // [rsp+4Ch] [rbp-14h]
  __block_literal_2 *v13; // [rsp+50h] [rbp-10h]
  __block_literal_2 *_block_descriptor; // [rsp+58h] [rbp-8h]

  _block_descriptor = .block_descriptor;
  v13 = .block_descriptor;
  for ( i = 0; i < 500000; ++i )
  {
    v1 = objc_msgSend(&OBJC_CLASS___NSNumber, "numberWithInt:", (unsigned int)i);
    num = (Foundation::NSValue::NSNumber *)objc_retainAutoreleasedReturnValue(v1);
    v2 = objc_msgSend(&OBJC_CLASS___NSString, "stringWithFormat:", CFSTR("%d "), (unsigned int)i);
    v3 = (Foundation::NSString::NSString *)objc_retainAutoreleasedReturnValue(v2);
    str = v3;
    v4 = objc_msgSend(&OBJC_CLASS___NSString, "stringWithFormat:", CFSTR("%@%@"), num, v3);
    v5 = objc_retainAutoreleasedReturnValue(v4);
    objc_release(v5);
    
    if ( !(i % 50000) )
    {
      v6 = (&.block_descriptor->self->_memoryUsageList2)[3];
      v7 = getMemoryUsage();
      v8 = objc_msgSend(&OBJC_CLASS___NSNumber, "numberWithDouble:", v7);
      v9 = objc_retainAutoreleasedReturnValue(v8);
      objc_msgSend(v6, "addObject:", v9);
      objc_release(v9);
    }
    objc_storeStrong(&str, 0LL);
    objc_storeStrong(&num, 0LL);
  }
}

---

情况3：

//            [NSString stringWithFormat:@"%@%@", num, str];
            [NSString stringWithFormat:@"1%@人", str];
            [NSString stringWithFormat:@"2%@", num];

这时候出现的内存的突增

---

在不采取autoreleasepool的情况下，基于上面3种操作，请问为什么内存的表现不一样？ 请问这是ARC内部自动帮我们优化的吗？如果是对unused warnning优化，那么第二种情况和第一种一样呢？个人怀疑是NSString构造（或者说是对象的构造）实现有关系。望回答