Why are the fast integer types faster than the other integer types?

Question

In ISO/IEC 9899:2018 (C18), it is stated under 7.20.1.3:

7.20.1.3 Fastest minimum-width integer types

1 Each of the following types design

Answer 1

They aren't, at least not reliably.

The fast types are simply typedefs for regular types, however it is up to the implementation how to define them. They must be at least the size requested, but they can be larger.

It is true that on some architectures some integer types have better performance than others. For example, early ARM implementations had memory access instructions for 32-bit words and for unsigned bytes, but they did not have instructions for half-words or signed bytes. The half-word and signed-byte instructions were added later, but they still have less flexible addressing options, because they had to be shoehorned into the spare encoding space. Furthermore all the actual data processing instructions on ARM work on words, so in some cases it may be necessary to mask off smaller values after calculation to give correct results.

However, there is also the competing concern of cache pressure, even if it takes more instructions to load/store/process a smaller value. The smaller value may still perform better if it reduces the number of cache misses.

The definitions of the types on many common platforms do not seem to have been thought through. In particular, modern 64-bit platforms tend to have good support for 32-bit integers, yet the "fast" types are often unnecessarily 64-bit on these platforms.

Furthermore, types in C become part of the platform's ABI. So even if a platform vendor discovers they made dumb choices, it is difficult to change those dumb choices later.

Ignore the "fast" types. If you are really concerned about integer performance, benchmark your code with all the available sizes.