c++ - Why is inlining considered faster than a function call?

Question

Now, I know it's because there's not the overhead of calling a function, but is the overhead of calling a function really that heavy (and worth the bloat of having it inlined) ?

From what I can remember, when a function is called, say f(x,y), x and y are pushed onto the stack, and the stack pointer jumps to an empty block, and begins execution. I know this is a bit of an oversimplification, but am I missing something? A few pushes and a jump to call a function, is there really that much overhead?

Let me know if I'm forgetting something, thanks!

Answer 1

Aside from the fact that there's no call (and therefore no associated expenses, like parameter preparation before the call and cleanup after the call), there's another significant advantage of inlining. When the function body is inlined, it's body can be re-interpreted in the specific context of the caller. This might immediately allow the compiler to further reduce and optimize the code.

For one simple example, this function

void foo(bool b) {
  if (b) {
    // something
  }
  else {
    // something else
  }
}

will require actual branching if called as a non-inlined function

foo(true);
...
foo(false);

However, if the above calls are inlined, the compiler will immediately be able to eliminate the branching. Essentially, in the above case inlining allows the compiler to interpret the function argument as a compile-time constant (if the parameter is a compile-time constant) - something that is generally not possible with non-inlined functions.

However, it is not even remotely limited to that. In general, the optimization opportunities enabled of inlining are significantly more far-reaching. For another example, when the function body is inlined into the specific caller's context, the compiler in general case will be able to propagate the known aliasing-related relationships present in the calling code into the inlined function code, thus making it possible to optimize the function's code better.

Again, the possible examples are numerous, all of them stemming from the basic fact that inlined calls are immersed into the specific caller's context, thus enabling various inter-context optimizations, which would not be possible with non-inlined calles. With inlining you basically get many individual versions of your original function, each version is tailored and optimized individually for each specific caller context. The price of that is, obviously, the potential danger of code bloat, but if used correctly, it can provide noticeable performance benefits.