Why Don't C Compilers Optimize Repeated Atomic Variable Writes?

Optimization Challenges in Atomic Variable Writes

Despite the "as-if" rule in the C standard allowing for the coalescence of repeated writes to the same atomic variable, compilers typically do not perform this optimization.

Reason for Compiler Restraint

The primary concern is the potential disruption of expected behavior. Programmers expect atomic stores to occur individually, preserving the visibility of intermediate values for potential race conditions. Coalescing stores could violate this expectation, particularly in scenarios like progress bars, where immediate updates are desired.

Standard Implications

The C standards allow for the folding of multiple writes to a single atomic variable, even if different values are stored. This suggests that compilers could coalesce multiple writes to improve performance. However, the "as-if" nature of atomic operations leaves it open to implementation decisions, and compilers have opted for a cautious approach to avoid unexpected behavior.

Cases for Optimization

Despite the potential pitfalls, there are legitimate use cases for optimizing atomic writes. For instance, reducing unnecessary ref count manipulation in loops could enhance performance. To address this, the C working group is considering extending the atomic API to provide explicit control over optimization.

Volatile Atomic Variables

Using volatile atomic variables can prevent the coalescing of writes. While volatile does not explicitly prohibit coalescing, it restricts optimization and maintains the consistency of individual writes. However, it may also introduce unwanted overhead and is discouraged as a primary means of preventing coalescing.

Ongoing Discussions

Discussions within the C working group are exploring ways to enable compilers to optimize atomic writes safely when appropriate. This includes proposals for opt-in syntax or annotations to indicate when optimization is desired and when it should be avoided.

Until a consensus is reached and implemented in the standard, compilers will likely continue to avoid coalescing atomic writes to maintain predictability and conform to programmer expectations.

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