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A general description is here.
Optimization level ‘-O3’ is now used by default on 64-bit builds as well as 32-bit ones.
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).