"inline" and "register" are two very known examples. Other examples are function attributes in gcc: https://gcc.gnu.org/onlinedocs/gcc/Function-Attributes.html
And most current wisdom is that "inline" no longer serves as a useful performance suggestion to the compiler, but instead should only be used as a linkage directive to allow multiple declarations of the same function: http://stackoverflow.com/questions/29796264/is-there-still-a...
But those are still 2 well known features that were added intentionally to help improve performance.