Idiom recognition. Some compilers had idiom recognition before C existed, because in some of the early languages the literal interpretation of the common idioms is incredibly expensive to perform, but a compiler can recognise that actually it can take massive shortcuts.

For example, sort this list of numbers into ascending order, then, take that sorted list, get the last number from it. With idiom recognition the compiler can say oh, I'll walk the list once, tracking the maximum as I go, that's much faster. In C this seems like, well, if I meant that I'd write that but I think APL is an example where the list expression is idiomatic and so that's what programmers will write but they want the fast thing.