I have recently discovered existence of standard fastest type, mainly int_fast32_t and int_fast64_t.
I was always told that, for normal use on mainstream architecture, one should better use classical int & long which should always fit to the processor default reading capacity and so avoid useless numeric conversions.
In the C99 Standard, it says in §7.18.1.3p2 :
"The typedef name int_fastN_t designates the fastest signed integer type with a width of at least N. The typedef name uint_fastN_t designates the fastest unsigned integer type with a width of at least N."
And there is also a quote about it in §7.18.1.3p1 :
"The designated type is not guaranteed to be fastest for all purposes; if the implementation has no clear grounds for choosing one type over another, it will simply pick some integer type satisfying the signedness and width requirements."
It's unclear to me what fastest really means. I do not understand when I should use this type and when I should not.
I have googled a little on this and found that some open source projects have changed some of their functions to it, but not all of them. They didn't really explain why they have changed a part, and only a part, of their code to it.
Do you know what are the specific cases/usages when int_fastXX_t are really faster than the classical ones ?