Difference between int32_t and int_fast32_t
What is the difference between the two? I know that int32_t is exactly 32 bits regardless of the environment but, as its name suggests that it's fast, how much faster can int_fast32_t really be compared to int32_t? And if it's significantly faster, then why so?
Answer
C is specified in terms of an idealized, abstract machine. But real-world hardware has behavioural characteristics that are not captured by the language standard. The _fast types are type aliases that allow each platform to specify types which are "convenient" for the hardware.
For example, if you had an array of 8-bit integers and wanted to mutate each one individually, this would be rather inefficient on contemporary desktop machines, because their load operations usually want to fill an entire processor register, which is either 32 or 64 bit wide (a "machine word"). So lots of loaded data ends up wasted, and more importantly, you cannot parallelize the loading and storing of two adjacent array elements, because they live in the same machine word and thus need to be load-modify-stored sequentially.
The _fast types are usually as wide as a machine word, if that's feasible. That is, they may be wider than you need and thus consume more memory (and thus are harder to cache!), but your hardware may be able to access them faster. It all depends on the usage pattern, though. (E.g. an array of int_fast8_t would probably be an array of machine words, and a tight loop modifying such an array may well benefit significantly.)
The only way to find out whether it makes any difference is to compare!