When is assembly faster than C?

Question

One of the stated reasons for knowing assembler is that, on occasion, it can be employed to write code that will be more performant than writing that code in a higher-level

Answer 1

Without giving any specific example or profiler evidence, you can write better assembler than the compiler when you know more than the compiler.

In the general case, a modern C compiler knows much more about how to optimize the code in question: it knows how the processor pipeline works, it can try to reorder instructions quicker than a human can, and so on - it's basically the same as a computer being as good as or better than the best human player for boardgames, etc. simply because it can make searches within the problem space faster than most humans. Although you theoretically can perform as well as the computer in a specific case, you certainly can't do it at the same speed, making it infeasible for more than a few cases (i.e. the compiler will most certainly outperform you if you try to write more than a few routines in assembler).

On the other hand, there are cases where the compiler does not have as much information - I'd say primarily when working with different forms of external hardware, of which the compiler has no knowledge. The primary example probably being device drivers, where assembler combined with a human's intimate knowledge of the hardware in question can yield better results than a C compiler could do.

Others have mentioned special purpose instructions, which is what I'm talking in the paragraph above - instructions of which the compiler might have limited or no knowledge at all, making it possible for a human to write faster code.