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A general description is here.
(Windows) An experimental build of R based on ‘gcc’ 4.9.3 is now available on CRAN on the ‘bin/windows/base/rdevel.html’ snapshot page. This build will require recompilation of at least those packages that include C++ code, and possibly others. The CRAN binary packages still use the older toolchain, so package installs default to ‘type = "source"’.
During the transition, the environment variable ‘R_COMPILED_BY’ will be defined to indicate which toolchain was used to compile R (and hence, which should be used to compile code in packages).
The API call ‘logspace_sum’ introduced in R 3.2.0 is now remapped as an entry point to ‘Rf_logspace_sum’, and its first argument has gained a ‘const’ qualifier.
Code using it will need to be reinstalled. (PR#16470)
Similarly, entry point ‘log1pexp’ also defined in ‘Rmath.h’ is remapped there to ‘Rf_log1pexp’