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A general description is here.
‘R CMD INSTALL’ preserves the package-directory modification time when it restores an earlier install of the package.
The ‘Update packages ...’ menu item now runs ‘update.packages(ask="graphics", checkBuilt=TRUE)’.
Compilation of C and Fortran code now uses the optimization flag ‘-mtune=core2’: this will improve performance a few percent on recent CPUs at the expense of those which are several years old. Its effect is particularly evident on 64-bit builds.
This can be overridden when building from the sources: see the ‘EOPTS’ macro defined in file ‘MkRules.dist’.