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

Many years ago I was teaching someone to program in C. The exercise was to rotate a graphic through 90 degrees. He came back with a solution that took several minutes to complete, mainly because he was using multiplies and divides etc.

I showed him how to recast the problem using bit shifts, and the time to process came down to about 30 seconds on the non-optimizing compiler he had.

I had just got an optimizing compiler and the same code rotated the graphic in < 5 seconds. I looked at the assembly code that the compiler was generating, and from what I saw decided there and then that my days of writing assembler were over.