问题描述
我发现 LDREX 和 STREX 可能是要使用的。但是它们是两个 指令（因此不提供 xchgl 的原子性）。我想要原子交换的值是一个32位值。
Can LDREX 和 STREX 可用于提供32位值的原子交换或他们是否有其他方法来实现它（假设它在armv7l或更高版本上工作）？
I found LDREX and STREX might be the ones to use. But they are two instructions (and thus not provide the atomicity of xchgl). The value I want to exchange atomically is a 32-bit value.Can LDREX and STREX be used in a way that provides atomic exchange of a 32-bit value or are they other ways to achieve it (provided it works on armv7l or higher)?
通常，我会使用gcc的或更新的（与C ++ 11相当的）
用于这种原子操作。但在这种情况下，我必须在C中使用内联汇编（以将基于x86的futex实现移植到ARM体系结构中）。谢谢！
Normally, I'd the gcc's atomic builtins or the more recent (C++11 equivalent) builtin functionsfor such atomic operations. But in this case, I have to use inline assembly in C (to port an x86-based futex implementation to ARM architecture). Thanks!
推荐答案
在ARM指令集中，没有原子交换指令。相反，您可以使用 ldrex 和 strex 以及如下代码：
In the ARM instruction set, there is no atomic exchange instruction. Instead you use ldrex and strex and code like this:
@ exchange r0 and [r1]
ldrex r2,[r1]
strex r3, r0,[r1]
mov r0,r2
当 [r1] 在 ldrex 和 strex 或者由于其他原因无法保证交换是原子的，1返回在 r3 并且不执行存储。如果序列是原子序列，则返回0。因此，通过在循环中执行此代码片段，直到获得零 r3 ，您最终可以实现原子交换操作。实际上，gcc和clang是如何实现相应的内在的;将 -S 传递给编译器以观察它的作用。
When [r1] is modified between ldrex and strex or the exchange cannot be guaranteed to be atomic for some other reason, 1 is returned in r3 and the store isn't performed. If the sequence is atomic, 0 is returned. Thus, by executing this snippet in a loop until you get a zero r3 you can eventually reach an atomic exchange operation. That's actually how gcc and clang implement the corresponding intrinsic; pass -S to the compiler to observe what it does.
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