How Can I Prevent Statement Reordering in Optimized C Code?

Enforcing Statement Order in C

Question:

How can one prevent the reordering of statements in C code, even with optimizations enabled?

Answer:

Despite the common assumption, C does not inherently enforce statement ordering, especially under optimizations. Certain statements may appear out-of-order in optimized code, which can be problematic for scenarios like time-sensitive operations.

Solutions:

Fundamentally, manipulating the operational semantics of fundamental operations like addition to achieve strict ordering is not feasible. The compiler and hardware design optimizations are premised on the assumption that these operations do not have observable timing properties.

However, effective techniques have emerged for micro-benchmarking:

Opaque Data Manipulation:

• Mark input and output data as "unoptimizable" around the computation in question.
• This ensures that the computation remains bounded by the timings while allowing optimization within its boundaries.

Library Support:

• Libraries like Google Benchmark provide the DoNotOptimize function for marking data as unoptimizable.
• Example usage:

template <class T>
__attribute__((always_inline)) inline void DoNotOptimize(const T &value) {
  asm volatile("" : "+m"(const_cast<T &&>(value)));
}

Compiler Directives:

• Some compilers offer directives like "__attribute__((optimize("no-reorder")))" or "__restrict__" to prevent reordering.
• However, these directives may not always be sufficient or portable.

Note:

The C standards committee is exploring the standardization of APIs like DoNotOptimize to further facilitate enforcing statement order.

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