[R-SIG-Win] mingw-w64 toolchain status update

[Jeroen Ooms]

Welcome to the new r-sig-windows mailing list! The initial purpose is
to announce updates of the mingw-w64 tool chain (aka Rtools) and
discuss problems related to building R and R packages on windows. Feel
free to invite other people that might be interested in these
developments.

## Building mingw-w64

Several people have been working on and off on this over the past
year. Below a summary of the current status to best of my knowledge.
Please correct me where I am wrong.

There are various collections of build-scripts available that produce
a mingw-w64 tool-chain. Some cross compile from Linux, others are
compile natively on Windows. In my experience, best results have been
achieved using the "mingw-builds" [1] scripts, which build from the
msys2 environment on windows. The "mingw-builds" scripts are pretty
complete and well maintained. The mingw-w64 project itself uses these
same scripts for publishing builds on their sourceforge website [2].

Because we strive for maximum compatibility and minimal disruption, we
target the same settings as the current rtools:

 - i686 host (win32)
 - sjlj exceptions
 - win32 threads (not posix)
 - static gcc (i.e. no separate libstdc++-6.dll and libgcc_s_sjlj-1.dll files)
 - mingw-w64 runtime v3 (needed for c++11, current rtools has v2)

When using sjlj exceptions, the mingw-builds defaults to a multilib
toolchain. This means that the same compiler can target both win32 and
win64 (using the -m32 and -m64 C flags). The current rtools uses a
multilib tool chain as well, so R already passes the correct flags to
the compiler, which makes the migration a little easier.

I used the following command in msys2 to produce a gcc-4.9.2 build:

  ./build --mode=gcc-4.9.2 --rt-version=v3 --arch=i686
--exceptions=sjlj --static-gcc --threads=win32
--enable-languages=c,c++,fortran

Building took about 3 hours. A zip file with the binaries is available
here [3]. The zip file is pretty large because the scripts also build
a lot of extra utilities that we probably don't need in the final
rtools distribution.

## Numeric precision bug

One outstanding bug [4] in the mingw-w64 runtime can cause tiny
numeric differences on 64bit windows. KK has found a simple workaround
is to change R_pow (in src/main/arithmetic.c) to use powl() instead of
pow().

  sed -i "s/pow(x, y)/powl(x, y)/g" %R_HOME%/src/main/arithmetic.c

I have confirmed that with this fix, R passes make check on win64.
Hopefully base R can adopt this fix so that we don't need special
patching for windows.

## Rf_error problem

The mingw-w64 runtime version 3.3.0 had a bug [5] for sjlj exceptions
that caused Rf_error() to crash. This problem has since then been
fixed both the v3 and v4 versions of mingw-w64. Because the
"mingw-builds" scripts always uses the latest v3.x version of
mingw-w64, we do not need special patching anymore.

In case we want to build a tool chain using the v3.3.0 version of the
mingw runtime, KK has extracted the required patch:
http://pastebin.com/k4dLndkw.

## Crashing V8

There was a problem with the V8 package which would crash with the new
tool chain. It looks like this was related to some ABI change in the
mingw-w64 runtime. Rebuilding the libv8 library for i386 has solved
the problem.

## Static Libraries

I was able to build R-devel with the new tool chain using the same
static libraries as for R 3.2.2 (libicu, libcairo, libcurl, Tcl, and
the "extsoft" stuff in local320.zip, see [6]). However perhaps it
couldn't hurt to rebuild some of these with more recent versions.

## To do

There is still a lot of work to do. We need to do more more testing of
R itself, in particular GUI features that are not covered by make
check. Kevin already found a problem with tab completion in RGui that
we need to investigate.

Then we need to test if all CRAN packages work with the new tool
chain. In particular packages that link to external C/C++ libraries,
or those that use exotic c++ features might require attention.

I will try to provide builds of the windows installer via [3] until
the new tool chain is ready for CRAN. This installer contains both x32
and x64 builds of R-devel, as well as a copy of the new tool chain
itself.

Thanks everyone for you help and patience!


[1] https://github.com/niXman/mingw-builds/tree/develop
[2] http://sourceforge.net/projects/mingw-w64/files/Toolchains%20targetting%20Win32/Personal%20Builds/mingw-builds/
[3] http://www.stat.ucla.edu/~jeroen/mingw-w64/
[4] http://sourceforge.net/p/mingw-w64/bugs/466/
[5] http://sourceforge.net/p/mingw-w64/bugs/465/
[6] http://www.stats.ox.ac.uk/pub/Rtools