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A general description is here.
The supported toolchain has changed since the one used for R 2.12.0 to
2.14.x: see the‘R Installation and Administration manual’. Compiled
code (except DLLs) may be incompatible with previous toolchains (and
compiled C++ code almost certainly will be).
This is a ‘multilib’ toolchain: there is a single set of tools in the ‘bin’ directory and which sub-architecture is selected by a flag: in most cases ‘-m32’ _vs_ ‘-m64’.
There is a new macro (aka make variable) ‘SHLIB_PTHREAD_FLAGS’. This
is set to ‘-pthread’ on builds which support it, and should be included
in both ‘PKG_CPPFLAGS’ (or the Fortran or F9x equivalents) and
‘PKG_LIBS’.
As the current toolchain uses only 32-bit executables, it is possible to compile a package for both architectures on 32-bit Windows provided both architectures of R have been installed: use ‘R CMD INSTALL --compile-both’.
Using a prompt of more than 80 characters in ‘readline()’ could cause a
buffer overflow in Rterm. (Reported by Henrik Bengtsson.)
The preferred toolchain has changed since the one used for R 2.12.0 to 2.14.1: see the‘R Installation and Administration manual’. Compiled code (except DLLs) may be incompatible with previous toolchains (and compiled C++ code almost certainly will be: users of ‘Rcpp’ take care).
Ensure that the settings in ‘MkRules.local’ are appropriate to the toolchain you use.
There is a new macro (aka make variable) ‘SHLIB_PTHREAD_FLAGS’. This is set to ‘-pthread’ on builds using toolchains which support it, and should be included in both ‘PKG_CPPFLAGS’ (or the Fortran or F9x equivalents) and ‘PKG_LIBS’.
The sources (and packages) can now be compiled using the multilib toolchain developed for R 2.15.x: see the ‘MULTI’ macro in ‘MkRules.dist’. This toolchain is used for the CRAN binary distribution.
‘merge()’ is no longer allowed to create a data frame with duplicate column names (which confused PR#14786).
The internal ‘untar()’ (as used by default by ‘R CMD INSTALL’) now knows about some ‘pax’ headers which ‘bsdtar’ (e.g., the default ‘tar’ for Mac OS >= 10.6) can incorrectly include in ‘tar’ files, and will skip them with a warning.
PCRE has been upgraded to version 8.21: as well as bug fixes and greater Perl compatibility, this adds a JIT pattern compiler, about which PCRE's news says ‘large performance benefits can be had in many situations’.