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A general description is here.
‘sample.int()’ has some support for n >= 2^31: see its help for the limitations.
A different algorithm is used for ‘n, size, replace = FALSE, prob = NULL)’ for ‘n > 1e7’ and ‘size <= n/2’. This is much faster and uses less memory, but does give different results.
More use has been made of R objects representing registered entry points, which is more efficient as the address is provided by the loader once only when the package is loaded.
This has been done for packages ‘base’, ‘methods’, ‘splines’ and ‘tcltk’: it was already in place for the other standard packages.
Since these entry points are always accessed by the R entry points they do not need to be in the load table which can be searched faster. This does mean that ‘.C’ / ‘.Fortran’ / ‘.Call’ calls copied from earlier versions of R may no longer work - but they were never part of the API.