The C++ Programming Language 4th edition, page 225 reads: A compiler may reorder code to improve performance as long as the result is identical to that of the simple order of execution. Some compilers, e.g. Visual C++ in release mode, will reorder this code:
#include <time.h>
...
auto t0 = clock();
auto r = veryLongComputation();
auto t1 = clock();
std::cout << r << " time: " << t1-t0 << endl;
into this form:
auto t0 = clock();
auto t1 = clock();
auto r = veryLongComputation();
std::cout << r << " time: " << t1-t0 << endl;
which guarantees different result than original code (zero vs. greater than zero time reported). See my other question for detailed example. Is this behavior compliant with the C++ standard?
The compiler cannot exchange the two clock calls. t1 must be set after t0. Both calls are observable side effects. The compiler may reorder anything between those observable effects, and even over an observable side effect, as long as the observations are consistent with possible observations of an abstract machine.
Since the C++ abstract machine is not formally restricted to finite speeds, it could execute veryLongComputation() in zero time. Execution time itself is not defined as an observable effect. Real implementations may match that.
Mind you, a lot of this answer depends on the C++ standard not imposing restrictions on compilers.