当在完全禁用优化(g++ -O0)的情况下编译同一段代码，然后在完全启用优化(g++ -O3)的情况下再次编译时，如何源代码本身的逻辑被改变了？

例如，编译器可以

• unwind loops和
• 做constant folding .

这两个优化使得代码执行速度更快，同时不影响原始源代码的完整性。在没有这些优化的情况下运行的任何代码都将在启用它们的情况下运行。

但是，编译器优化也会影响代码逻辑。以下是我所知道的两个例子:

• Removing copy constructors and assignment operators来自临时对象可能会消除可能的副作用。
• 重新排列包含浮点值的算法可能会影响浮点错误(希望需要 -ffast-math 参数)。

我很惊讶也很幸运能够了解这些，因为它们在错误的情况下可能成为巨大的潜在陷阱。

所以我想知道，是否还有其他情况下 c++ 编译器优化会影响代码逻辑？我专门在 g++ 编译器下查找有关 c++11(没有任何未定义行为)的信息，但欢迎提供其他编译器的提示。

最佳答案

“好像”规则:

An implementation is free to disregard any requirement of this International Standard as long as the result is as if the requirement had been obeyed, as far as can be determined from the observable behavior of the program. For instance, an actual implementation need not evaluate part of an expression if it can deduce that its value is not used and that no side effects affecting the observable behavior of the program are produced.

但是，该标准提到了一种允许的优化，它打破了“假设”规则:

When certain criteria are met, an implementation is allowed to omit the copy/move construction of a class object, even if the copy/move constructor and/or destructor for the object have side effects. In such cases, the implementation treats the source and target of the omitted copy/move operation as simply two different ways of referring to the same object, and the destruction of that object occurs at the later of the times when the two objects would have been destroyed without the optimization.123 This elision of copy/move operations, called copy elision, is permitted in the following circumstances (which may be combined to eliminate multiple copies):

— in a return statement in a function with a class return type, when the expression is the name of a non-volatile automatic object (other than a function or catch-clause parameter) with the same cvunqualified type as the function return type, the copy/move operation can be omitted by constructing the automatic object directly into the function’s return value

— in a throw-expression, when the operand is the name of a non-volatile automatic object (other than a function or catch-clause parameter) whose scope does not extend beyond the end of the innermost enclosing try-block (if there is one), the copy/move operation from the operand to the exception object (15.1) can be omitted by constructing the automatic object directly into the exception object

— when a temporary class object that has not been bound to a reference (12.2) would be copied/moved to a class object with the same cv-unqualified type, the copy/move operation can be omitted by constructing the temporary object directly into the target of the omitted copy/move

— when the exception-declaration of an exception handler (Clause 15) declares an object of the same type (except for cv-qualification) as the exception object (15.1), the copy/move operation can be omitted by treating the exception-declaration as an alias for the exception object if the meaning of the program will be unchanged except for the execution of constructors and destructors for the object declared by the exception-declaration.

关于c++ - 编译器优化如何影响代码逻辑？，我们在Stack Overflow上找到一个类似的问题： https://stackoverflow.com/questions/12946114/