c++ - 优化 volatile 变量

Question

我的理解是，C++ 标准要求对 volatile 对象的访问严格按照抽象机的规则进行评估。我确信这意味着无法更改给定 volatile 变量的加载和存储次数，也无法对这些访问进行重新排序。

但是关于其他非 volatile 访问的重新排序呢？

是否可以从 if 语句的两个分支中提升或隐藏完全冗余的 volatile 访问？例如。假设没有违反数据依赖关系可以

if (e) {
    = non-volatile-load;
    non-volatile-store =;
    t = volatile-load;
    = non-volatile-load;
    non-volatile-store =;
} else {
    = non-volatile-load;
    non-volatile-store =;
    t = volatile-load;
    = non-volatile-load;
    non-volatile-store =;
}

优化为

t = volatile-load;
if (e) {
    = non-volatile-load;
    non-volatile-store =;
    = non-volatile-load;
    non-volatile-store =;
} else {
    = non-volatile-load;
    non-volatile-store =;
    = non-volatile-load;
    non-volatile-store =;
}

或到

if (e) {
    = non-volatile-load;
    non-volatile-store =;
    = non-volatile-load;
    non-volatile-store =;
} else {
    = non-volatile-load;
    non-volatile-store =;
    = non-volatile-load;
    non-volatile-store =;
}
t = volatile-load;

如果 volatile-load 是 volatile-store 会怎样？

Answer 1

在as-if rule下,...

... conforming implementations ... are required to emulate (only) the observable behavior of the abstract machine as explained below

和observable behavior在下面指定为(强调我的)...

– Accesses through volatile glvalues are evaluated strictly according to the rules of the abstract machine.

所以是的，编译器可能会重新排序对非 volatile 变量的访问(当然在线程环境中的内存栅栏范围内；volatile 不提供这样的语义，atomic 确实)。