As we've mentioned before, Julia code consists of many small functions. Unlike most other language implementations, some of the core primitives in the base library are also implemented in Julia. This means that the function call overhead has the potential to be a bottleneck in a Julia program. This is mitigated using some aggressive inlining performed by the Julia compiler.
Inlining is an optimization performed by a compiler, where the contents of a function or method is inserted directly into the body of the caller of that function. Thus, instead of making a function call, execution continues directly by executing the operations of the callee within the caller's body.
In addition, many compiler optimization techniques work within the body of a single function. Inlining, therefore, allows many more optimizations to be effective within the program.