How Can I Guarantee Statement Execution Order in C for Accurate Timing Measurements?

Enforcing Statement Order in C

Reordering Concerns

In C code, using optimization flags can lead to potential reordering of statements, raising concerns about accurate execution order. It's essential to understand the compiler's optimizations and how they may affect statement sequence.

Enforcing Statement Order with Barriers

Unfortunately, C lacks built-in mechanisms to enforce statement order directly. Compilers can freely reorder instructions during optimization, considering their established operational semantics and the absence of observable effects from operations like integer addition.

Alternative Techniques for Timing Measurements

For precise timing measurements, it's recommended to use specialized techniques such as:

• Data Pincering: Prevent compiler optimizations by encasing the code to be timed with opaque data markers.
• Micro-Benchmarking Libraries: Utilize libraries like Google's Benchmark that employ these techniques to provide reliable timing measurements.

Data Pincering Example

Consider the following example where the intent is to measure the execution time of function foo:

using Clock = std::chrono::high_resolution_clock;

auto t1 = Clock::now();         // Statement 1
auto output = foo(input);       // Statement 2
auto t2 = Clock::now();         // Statement 3

auto elapsedTime = t2 - t1;

Using data pincering techniques, the code can be altered to ensure that the specific computation remains within the measured time interval:

auto input = 42;

auto t1 = Clock::now();         // Statement 1
DoNotOptimize(input);
auto output = foo(input);       // Statement 2
DoNotOptimize(output);
auto t2 = Clock::now();         // Statement 3

return t2 - t1;

Here, DoNotOptimize marks input and output data as un-optimizable, preventing their removal or reordering by the compiler. This guarantees accurate timing of the desired computation, despite compiler optimizations.

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