Effectively this means the speedup comes from optimizations that assume the code in question is only ever run in that context. When the code is inline this is

Effectively this means the speedup comes from optimizations that assume the code in question is only ever run in that context. When the code is inline this is an easy call to make. For a function it's trickier. I would hope some compilers make a function to handle arbitrary contexts but try inlining on individual cases to see if significant gains such as this can be made. It's another hurdle for the sufficiently smart compiler though. You need to know how the program will be run to know which is the better form. Once you get into making code-size Vs speed things get murky with instruction caches etc.