As-if rule

wikipedia As-if rule

The standard for the C++ programming language allows compilers for this language to apply any optimizing[1] transformation to a program during compilation, provided that such optimizations make no change in the "observable behavior" of the program, as specified in the standard; this mostly means that any actions the program performs on its environment occur in the specified order. This rule is commonly referred to as the as-if rule.[2]

NOTE: 这对于理解很多compiler optimization是非常重要的

cppreference The as-if rule

Explanation

NOTE: 此处的解释是更加详细的。

Exception

本段总结打破"As-if rule"的一些optimization，重要参考了如下文章:

1、wikipedia As-if rule

2、cppreference Copy elision # Notes

3、stackoverflow What are copy elision and return value optimization? # A

wikipedia As-if rule

The rule has three main exceptions.

The first is that programs exhibiting(展示) undefined behavior are exempt(豁免的); since the observable behavior is not well-defined anyway, any transformation is valid.

NOTE: 首先，程序表现出不明确的行为是豁免的; 由于可观察行为没有明确定义，*任何*转换都是有效的。

The other two exceptions concern the copying of objects, and are called copy elision and the return value optimization.

NOTE: 通过下面的内容可知，上述总结是不全面的，或者说不是最新的

cppreference Copy elision # Notes

Copy elision is the only allowed form of optimization (until C++14)one of the two allowed forms of optimization, alongside allocation elision and extension, (since C++14) that can change the observable side-effects. Because some compilers do not perform copy elision in every situation where it is allowed (e.g., in debug mode), programs that rely on the side-effects of copy/move constructors and destructors are not portable.

NOTE: 上面这段话，初读是难以理解的，下面是注解版本:

Optimization that can change the observable side-effects:

一、until C++14

1、Copy elision

二、since C++14

1、Copy elision

2、Allocation elision and extension

"Optimization that can change the observable side-effects"意味中打破As-if-rule，因此，这段话总结了compiler打破As-if rule的optimization。