atomic_compare_exchange与大于而不是等于？ [英] atomic_compare_exchange with greater-than instead of equals?

问题描述

C ++ 11对原子变量

语义是：

与预期所指向的值 c> 等于前者与所需（执行读 - 修改 - 写操作）。否则，将 obj 指向的实际值加载到 * expected （执行加载操作）。




我想这样做，但不是设置 * obj 值等于，我想要在大于另一个（假设我们在讨论有序类型）时设置它。

这是否以某种方式支持？

注意：一个CAS循环不会为我做，因为我比较的值可能会改变非原子操作。

解决方案

根据请求，这里是我的评论作为答案：

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我也希望这个存在，但它不，就我所知（当然不是为x86 / x64），除了概念上，和（潜在地）使用多于单个原子指令（其工作但不等待）的解决方案。

C++11 has a 'compare and exchange' operation for atomic variables.

The semantics are:

Atomically compares the value pointed to by obj with the value pointed to by expected, and if those are equal, replaces the former with desired (performs read-modify-write operation). Otherwise, loads the actual value pointed to by obj into *expected (performs load operation).

I want to do the same, but instead of setting *obj when the values are equal, I want it to be set when one is greater-than the other (assume we're talking about an ordered type).

Is this supported somehow? Achievable by some hack perhaps?

Note: A CAS loop will not do for me, since both the values I'm comparing might change between non-atomic operations.

解决方案

As requested, here's my comment as an answer:

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I, too, wish this existed, but it does not, as far as I know (certainly not for x86/x64), apart from conceptually, of course, and workarounds that (potentially) use more than a single atomic instruction (which work but are not wait-free).

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