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A general description is here.
‘R CMD INSTALL’ and ‘R CMD SHLIB’ make use of the flag ‘--use-LTO’ when the ‘LTO_OPT’ make macro is set in file ‘etc/${R_ARCH}/Makeconf’ - it is not by default but a setting in ‘MkRules.local’ will be imported when R is built.
There is a new text file ‘src/gnuwin32/README.compilation’, which outlines how C/Fortran code compilation is organized and documents new features:
• R can be built with Link-Time Optimization with a suitable compiler - doing so with GCC 9.2 showed several inconsistencies which have been corrected.
• There is support for cross-compiling the C and Fortran code in R and standard packages on suitable (Linux) platforms. This is mainly intended to allow developers to test later versions of compilers - for example using GCC 9.2 has detected issues that GCC 8.3 in Rtools40 does not.
• There is experimental support for cross-building R packages with C, C++ and/or Fortran code.