Constant Expression Optimization: Can String Length be Determined at Compile Time?

In the quest to optimize code, a developer attempts to compute the length of a string literal at compile time using a recursive function. This function evaluates the string character by character and returns the length.

Initial Observations:

The function appears to work as intended, returning the correct lengths at runtime and generating assembly code that suggests the computation occurs at compile time. This raises the question:

Is it guaranteed that the length function will execute at compile time?

The Standard's Perspective:

Constant expressions are not explicitly mandated to be evaluated at compile time by the C standard. However, a non-normative note suggests this possibility.

Enforcing Compile-Time Execution:

To ensure compile-time evaluation, the function can be assigned to a constexpr variable. According to C 11 guidelines, marking a variable constexpr necessitates its evaluation at compile time.

Additional Considerations:

Bjarne Stroustrup, the creator of C , summarizes the conditions for compile-time evaluation as:

1. Constant expression requirement: When used in locations that demand constant expressions (e.g., array bounds, case labels).
2. Initialization of a constexpr: When the function is used to initialize a constexpr variable.

Conclusion:

While not explicitly stated in the standard, compile-time evaluation of constexpr functions can be guaranteed by utilizing constant expressions or initializing constexpr variables. This opens up possibilities for optimizing string and other computations at compile time, enabling more efficient code execution.