何时以及为什么我会使用-fno-elide-constructors？

Question

我正在学习C++而且我遇到过-fno-elide-constructors,下面我已经包含了手册页中的描述.

   -fno-elide-constructors
       The C++ standard allows an implementation to omit creating a
       temporary which is only used to initialize another object of the
       same type.  Specifying this option disables that optimization, and
       forces G++ to call the copy constructor in all cases.

因此,使用此选项,我可以禁用此特定类型的编译器优化.我有一个程序创建2个对象并将它们一起添加,并在使用BASIC4TRACE库调用每个函数时进行打印.我编译了相同的程序来测试函数调用的差异,同时使用此选项两次,一次使用,一次不使用,给出此输出.

Without optimizations

BASIC4TRACE: (0x7fff7504a7c0)->Object(const char *)
BASIC4TRACE: (0x7fff7504a7d0)->Object(const char *)
BASIC4TRACE: (0x7fff7504a770)->Object(const char *)
BASIC4TRACE: op+(const Object&, const Object&)
BASIC4TRACE: (0x7fff7504a720)->Object()
BASIC4TRACE: (0x7fff7504a780)->Object(const Object&)
BASIC4TRACE: (0x7fff7504a720)->~Object()
BASIC4TRACE: op+(const Object&, const Object&)
BASIC4TRACE: (0x7fff7504a720)->Object()
BASIC4TRACE: (0x7fff7504a790)->Object(const Object&)
BASIC4TRACE: (0x7fff7504a720)->~Object()
BASIC4TRACE: (0x7fff7504a7f0)->Object(const Object&)
BASIC4TRACE: (0x7fff7504a790)->~Object()
BASIC4TRACE: (0x7fff7504a780)->~Object()
BASIC4TRACE: (0x7fff7504a770)->~Object()
BASIC4TRACE: (0x7fff7504a7e0)->Object(const Object&)
BASIC4TRACE: (0x7fff7504a7f0)->~Object()
BASIC4TRACE: (0x7fff7504a7e0)->~Object()
BASIC4TRACE: (0x7fff7504a7d0)->~Object()
BASIC4TRACE: (0x7fff7504a7c0)->~Object()

With optimizations

BASIC4TRACE: (0x7fffbfc8bbf0)->Object(const char *)
BASIC4TRACE: (0x7fffbfc8bc00)->Object(const char *)
BASIC4TRACE: (0x7fffbfc8bbb0)->Object(const char *)
BASIC4TRACE: op+(const Object&, const Object&)
BASIC4TRACE: (0x7fffbfc8bbc0)->Object()
BASIC4TRACE: op+(const Object&, const Object&)
BASIC4TRACE: (0x7fffbfc8bc10)->Object()
BASIC4TRACE: (0x7fffbfc8bbc0)->~Object()
BASIC4TRACE: (0x7fffbfc8bbb0)->~Object()
BASIC4TRACE: (0x7fffbfc8bc10)->~Object()
BASIC4TRACE: (0x7fffbfc8bc00)->~Object()
BASIC4TRACE: (0x7fffbfc8bbf0)->~Object()

正如您所看到的,调用次数存在显着差异.所以我的问题是我什么时候才能使用这个选项？是否存在此类优化导致问题的特定情况？我真的不能想到一种情况,我不希望我的代码尽可能地进行优化,所以我很难弄清楚这有什么好处.

Answer 1

正如Benjamin Lindley对这个问题所评论的那样,如果你的程序依赖于拷贝构造函数的副作用,那么你的代码编写得非常糟糕.应始终编写您的副本构造函数,以便这样的优化是安全的.不仅因为编译器可能执行这样的优化,还因为其他人类读者将很难理解正在发生的事情.

也就是说,该选项仍然可以肯定是有用的.确切地说,当复制构造函数被省略时,未指定,并且看似无关的更改可能会改变复制省略是否发生.因此,-fno-elide-constructors使GCC更具确定性,有时候,这有助于调试,因为这意味着您可以更少地担心仅通过添加调试打印语句开始工作的代码(这会产生副作用,从而禁用复制省略).

您可能想要使用的另一个原因-fno-elide-constructors是,如果您希望代码在执行较少复制省略的其他编译器上运行良好.如果无用的复制构造函数导致明显的减速,您可以重新编写代码,以便无论副本是否被省略,它都会很快.