Inline
An inline function is one for which the compiler copies the code from the function definition directly into the code of the calling function rather than creating a separate set of instructions in memory. This eliminates call-linkage overhead and can expose significant optimization opportunities. Using the "inline" specifier is only a suggestion to the compiler that an inline expansion can be performed; the compiler is free to ignore the suggestion.
Call overhead is the time it takes to call a function. This includes the time it takes to push the arguments onto the stack, the time it takes to push the return address onto the stack, and the time it takes to jump to the function. This overhead is usually small, but it can be significant for small functions that are called many times. Inlining a function eliminates this overhead.