尽管数据匹配期望值，但compare_exchange_strong仍失败 [英] compare_exchange_strong failing despite data matching expected value

问题描述

问题是 compare_exchange_strong 返回false，尽管基础数据等于预期。例如：

The issue is with a compare_exchange_strong returning false, despite the underlying data being equal to expected. e.g.:

std::atomic<data> ptr;
...
auto ptr_data = ptr.load();
bool cmp_result = memcmp(&ptr_data, &expected, sizeof(ptr_data));
bool cas_result = ptr.compare_exchange_strong(expected, desired);
assert(cas_result || !cmp_result);

data 是128位POD。 ptr.is_lock_free（）返回true。这以单线程方式进行了测试。 cas_result 始终为假， cmp_results 始终为真。

data is a 128-bit POD. ptr.is_lock_free() returns true. This is tested in a single-threaded fashion. cas_result is always false, cmp_results always true.

编译是使用Intel的C ++编译器（版本16更新2）完成的。在Linux上，libstdc ++版本5.3.1。 64位二进制文​​件。
与在Windows上编译时使用的完全相同的代码，具有相同的ICC16，但为32位代码。这使我相信这是stdlib实现的怪癖。

Compilation is done with Intel's C++ compiler, version 16 update 2. On Linux, libstdc++ version 5.3.1. 64-bit binary.
Exactly the same code used to work correctly when compiled on Windows, with same ICC16 but as 32-bit code. This makes me believe it is the stdlib implementation quirk.

谢谢

推荐答案

我在使用的结构上也遇到过这种情况。我认为这是由于对齐问题而插入填充位以及compare_exchange对两个值进行按位比较的结果。

I've had this situation as well on a struct I was using. I think it was caused by padding bits inserted due to alignment issues and the fact that compare_exchange does a bitwise comparison of the two values.

在我的机器上，字/指针的大小为8个字节而我拥有的结构是这样的：

On my machine, the word/pointer size is 8 bytes and the struct I had was something like this:

struct s {
    int i;
    void *ptr;

};

仅使用12个字节来表示其内容（ptr为8个字节，int为4），但其大小为16个字节（sizeof（s）== 16）。我所做的就是将结构更改为：

which used only 12 bytes to represent its contents (ptr was 8 bytes, int was 4), but it's size was of 16 bytes (sizeof(s) == 16). What I did was change the struct into:

struct s {
    long long i;
    void *ptr;
};

由于long long在我的机器上是8个字节，因此该版本的大小为16个字节，它们都用来表示实际值（无填充位）。

Because long long is 8 bytes on my machine, this version had a 16 bytes size and they were all used to represent the actual value (no padding bits).

在阅读此后，我看到了问题答案，可能会更详细。

I saw the problem after reading this answer, which might be more detailed.

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