Recursive Function Inlining

It's a common misconception that recursive functions cannot be inlined. However, compilers can indeed inline recursive functions, albeit with certain considerations.

Inline Qualifier vs. Compiler Optimization

The inline specifier on a function is merely a hint to the compiler. The compiler has the final say whether to inline the function or not, regardless of the inline qualifier.

Compiler's Inlining Decision

A compiler decides whether to inline a function based on factors such as:

• Optimization level: Higher optimization levels tend to favor inlining.
• Function size and complexity: Smaller and less complex functions are more likely to be inlined.
• Availability of optimization techniques: The compiler might use loop unrolling or tail call optimization to make inlining possible.
• Recursive limit: Some compilers have a limit on the number of times a recursive function can be inlined.

Example Inlining Optimization

Consider the following recursive factorial function:

inline int factorial(int n) {
    if (n 
An optimizing compiler could potentially inline this function to a certain level, as seen in the following optimized code:
int factorial(int n) {
    if (n 
In this case, the compiler has unrolled the factorial function three times, effectively inlining a portion of the recursive calls.