constexpr functions vs inline functions

constexpr functions vs inline functions

A lot of the same value in terms of optimization can be had by making a function just inline rather than constexpr. Note that you can use both, but officially constexpr for functions implies inline on the function as well.

Is constexpr any better than just inline? If you pass a constant argument to a small inline function, then the expansion of the function body will trigger lots of constant propagation optimizations, effectively evaluating most of it at compile-time, which is almost the same as constexpr.

constexpr is supposed to be more formal in guaranteeing that the result of a function is a compile-time constant, and the compiler is honor-bound to do “compile-time function evaluation” to get the constant return value. Also, a constexpr function is more officially usable as a compile-time constant, so that you can use an expression with a constexpr function's return value in various places where C++ needs a constant (e.g. an array size declaration, some template situations, etc.).

An inline function is also supposed to be optimized at run-time for non-constant arguments, and constexpr functions are implicitly inline functions. The code generation requirements of dynamic inlining are often more advanced that constant expression evaluation.

Also, the limitations on how a constexpr function can be structured are a lot easier to code than the unrestricted nature of an inline function body. However, as a practical matter, the compile-time evaluation of expressions and the code generation for inlined expressions have a lot of overlap, so I expect C++ compilers will mostly try to do both on every type of function.

The inline keyword also serves a weird secondary purpose, by guaranteeing that there's only one copy of the function. This means we can include header files with the full definition of that inline function anywhere we like, without getting a compiler error at link-time about multiple definitions. But this isn't a performance optimization, and the linker feature of inline is almost the opposite of what we want in making a function inline, because we don't want a real function to be called at all.

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