The R for Windows Build Process
===============================

from the point of view of a Unix installer of R.

Compilation of C/Fortran code is done in two pathways.

1) standard and add-on packages, the most similar to Unix.

The primary Makefiles are
$(R_HOME)/etc${R_ARCH}/Makeconf
share/make/winshlib.mk

included in that order.  Some settings in Makeconf are substituted in
etc/Makeconf by fixed/Makefile, taken from MkRules.


2) Everything else, where the primary Makefile is MkRules which is
where all the customization is done.  This is included by all
Makefile.win files in directories above this one and

./Makefile
./cran/Makefile
./fixed/Makefile
./front-ends/Makefile
./getline/Makefile
./installer/Makefile

and replaces Unix's top-level Makeconf.


The largest difference from Unix (even a R-shlib build) is the use of
DLLs, which require the exported symbols to be listed (via nm) in a
.def file and fed to the compiler with the object files for linking.

In recent years DLLs are linked to directly as libraries, but there
remains provision for import libraries via lib%.dll.a: %.def rules.

Making executables has a number of Windows-specific features, done in
front-ends/Makefile:

- A resource compiler is used to compile in version information, an
  icon and a manifest.  Also done for version information in all the
  other DLLs under bin and modules (R.dll Rblas.dll Rgraphapp.dll
  Riconv.dll Rlapack.dll internet.dll lapack.dll).

- LINKFLAGS includes flags for the stack size and to allow addressing
  over 2GB for 32-bit executables.

A resource compiler is used to add version information to the DLLs
under directories bin and modules.

The default is to compile without debug info: this can be enabled by
using 'make DEBUG=T' from this directory, or install add-on packages
with Rcmd INSTALL --debug.  Otherwise DLLs and executables are
stripped.


Link-Time Optimization
======================

To enable LTO for building R set the LTO macro in MkRules.local, to
-flto or e.g. -flto=8 to use 8 threads.

Checking of packages with LTO can be performed by setting the LTO_OPT
macro to -flto in MkRules.local and using Rcmd INSTALL --use-LTO.

For an installed version of R it should suffice to edit the value of
LTO_OPT in the etc/*/Makeconf files or to set it in a personal or site
Makevars file.

[Unlike a Unix-alike, LTO/LTO_OPT are used for Fortran as well as C/C++.]


Cross-compilation
=================

Linux distributions including Fedora, Debian and Ubuntu provide
cross-compilers and many cross-compiled libraries for both 32- and
64-bit Windows.  For example, on Fedora 32 for a 64-bit build one
might install the RPMs

mingw64-binutils
mingw64-bzip2
mingw64-cairo
mingw64-cpp
mingw64-crt
mingw64-filesystem
mingw64-gcc
mingw64-gcc-c++ (not needed for R, but needed for cairo, icu and libtiff)
mingw64-gcc-gfortran
mingw64-headers
mingw64-libgomp
mingw64-libpng
mingw64-libjpeg-turbo
mingw64-libtiff
mingw64-pcre2
mingw64-pkg-config
mingw64-readline
mingw64-tcl
mingw64-termcap
mingw64-tk
mingw64-winpthreads
mingw64-xz-libs
mingw64-zlib

or install the mingw32- variants for a 32-bit build.  At the time of
writing this used GCC 9.2.1.

To build all the compiled code, set BINPREF in MkRules.local, e.g. to

WIN = 64
BINPREF64 = /usr/bin/x86_64-w64-mingw32-

or

WIN = 32
BINPREF = /usr/bin/i686-w64-mingw32-

Setting LTO is supported.

Then in this directory

make MkRules rbuild rpackages-cross

To make the cairo devices, set something like

USE_CAIRO = TRUE
CAIRO_CPPFLAGS = -I/usr/x86_64-w64-mingw32/sys-root/mingw/include/cairo
CAIRO_LIBS = "-lcairo -lpixman-1 -lpng -lz -lgdi32 -lmsimg32"

in MkRules.local and run

make cairodevices


This will link to the DLLs of external software and for libgomp and
pthreads (for OpenMP).

For a more static build, install

mingw64-bzip2-static
mingw64-libpng-static
mingw64-libjpeg-turbo-static
mingw64-libtiff-static
mingw64-pcre2-static
mingw64-winpthreads-static
mingw64-xz-libs-static
mingw64-zlib-static

and remove the corresponding libfoo.dll.a files from
/usr/x86_64-w64-mingw32/sys-root/mingw/lib, and libgomp.dll.a.  (The
build still uses DLLs for libgcc_s_seh and Tcl/Tk).  For a static
build of the Cairo devices one would need other RPMs like

mingw64-cairo-static
mingw64-fontconfig-static
mingw64-freetype-static
mingw64-pixman-static

Rather than remove the .dll.a files, one could experiment with -Wl,-Bstatic.

There are curl and ICU libraries for optional features.  Settings for
Fedora's libs (DLL versions):

USE_LIBCURL = YES
CURL_LIBS = -lcurl

USE_ICU = YES
ICU_LIBS = -licuuc -licui18n -licudata

There is a static version for curl but not ICU.


Cross-building packages
=======================

There is experimental minimal support for cross-building packages.

Copy {bin,etc}/{i386,x64} from this build to a standard Linux build.

Edit the copy of etc/*/Makeconf to have

R_WIN = /path/to/top/directory/of/this/build
R_XTRA_CPPFLAGS = -I"$(R_WIN)/include" -DNDEBUG

Set the environment variable R_CROSS_BUILD to i386 or x64.

Use R CMD INSTALL as usual. (This does not do a staged install nor
test loading.)  --use-LTO is supported if LTO_OPT was set.

R CMD INSTALL --build will produce a zipped installation.

Should a package require external libraries, these can be placed under
LOCAL_SOFT as defined in etc/*/Makeconf in the copy.