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Memory barrier cache coherence1/5/2024 ![]() ![]() Which should not be a surprise to anyone who does not expect Volatile glvalue are implementation-defined. 9.1.7.2p5 states that the semantics of an access through a.6.8.2.1p21 calls out volatile accesses as one of the.6.8.1p7 states that volatile accesses are side effects.Strictly according to the rules of the abstract machine.” 4.4.1p6.1 says “Accesses through volatile glvalues are evaluated.Should that ever emerge from the shadows, this paper will defer to it. This raises the question of what exactly the standard guarantees forĪ more detailed exposition on volatile is said to be in Match for the C and C++ memory_order_relaxed loads and READ_ONCE() and WRITE_ONCE() are a better Of course, this phase-out has the advantage that Macros are recommended for new code, in fact, ACCESS_ONCE() Note that use of the new READ_ONCE() and WRITE_ONCE() This loop could still be unrolled, but the read would also need to be The volatile qualifier prevents this transformation.įor example, consider the following loop: Value to be reloaded from a on each iteration, unrolled This would be unacceptable for real-time applications, which need the The compiler would be permitted to unroll it as follows: While (tmp = atomic_load_explicit(a, memory_order_relaxed)) To see this, consider that READ_ONCE() can be used to preventĬoncurrently modified accesses from being hoisted out of a loop or out Note that the volatile is absolutely required: The expectation that the compiler would emit a pair of 32-bitĪccesses, but otherwise respect volatility. ![]() WRITE_ONCE() to 64-bit variables on 32-bit systems with Or ACCESS_ONCE() when applied to oversized objects.Īnd there has been Linux-kernel use of READ_ONCE() and Linux-kernel developers would most certainly not be thankful to theĬompiler for adding locks to READ_ONCE(), WRITE_ONCE(), Is unlikely to do anything useful, and use of READ_ONCE()Īnd WRITE_ONCE() in this situation can result in load-tearing Only for properly aligned machine-word-sized variables.Īpplying ACCESS_ONCE() to a large array or structure However, please note that these macros are defined to work properly The now-obsolete ACCESS_ONCE() accesses may be modeled as a READ_ONCE(), WRITE_ONCE(), and the now-obsolete Loads from and stores to shared (but non-atomic) variables should be
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