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A general description is here.
The ‘Rterm’ or ‘Rgui’ title mentions 32-bit as well as 64-bit.
The desktop icons are labelled ‘R i386’ and ‘R x64’.
‘R CMD INSTALL’ for a package with compiled code will fail if compilation fails for any of the installed sub-architectures: use ‘--no-multiarch’ to override this.
Optimization level ‘-O3’ is now used by default on 64-bit builds as
well as 32-bit ones.