[R-package] replaced gendef.exe with R code to create R.def (fixes #3064)

[jameslamb]

Experimental fix for microsoft#3064.

Short description

Building the LightGBM R package on Windows with Visual Studio compilers requires gendef.exe. That was bundled in previous versions of RTools but isn't part of RTools 4.0.

This PR replaces it with an R script that uses objdump.exe, software bundled in old and new distributions of RTools.

How you can test this

In a Windows environment with R 4.0, RTools 4.0, CMake, and Visual Studio, run the following:

git clone https://github.com/jameslamb/LightGBM.git
cd LightGBM
git fetch origin fix/r-4.0
git checkout fix/r-4.0

# install LightGBM
Rscript build_r.R

# test that it worked
cd R-package/tests
Rscript testthat.R

Long Description

Here's how building the R package with Visual Studio works today:

1. The LightGBM R package has to be linked to R.dll to use R-provided C/C++ functions such as Rprintf for printing.
2. Linking with Visual Studio compilers requires a .lib file (https://docs.microsoft.com/en-us/cpp/build/reference/link-input-files?redirectedfrom=MSDN&view=vs-2019)
3. To create a .lib file from R.dll on Windows, we create a definition file (.def) and then use that to make a library file (.lib).
   • R.dll ships with R
   • A file R.def is created from R.dll, using gendef.exe
   • A file R.lib is created from R.def using dlltool.exe

The issue: gendefe.exe is not part of RTools 4.0.

My solution in this PR is to use objdump.exe + an R script to generate R.def. objdump creates output intended for humans not machines...it uses indentation and special chracters to make the file visually appealing. So the R script is used to run objdump, but more importantly to read in the text it produces and do some simple cleaning to get it into a machine-friendly format that can be re-written as a .def file.

From the "How to create a def file from a dll" section of this amazing MinGW documentation.

If the previous options don't work, you can still try to create a def file using the output from the objdump program (from the mingw distribution).
Here's an example.
objdump -p file.dll > dll.fil

Search for [Ordinal/Name Pointer] Table in dll.fil and use the list of functions following it to create your def file.

References

A lot of the content for this PR was totally new to me. Here are some links I found very helpful.

• .def file: text file that contains the names of objections exported from a DLL
  • "Exporting from a DLL using def files"
• .lib file: a library file, basically a specific format that describe DLL so a linker understands how to link to it
  • StackOverflow answer explaining .lib vs .dll
  • ".lib file as linker input"
• R uses the MinGW toolchain for R
  • "MinGW FAQ" is an AMAZING resource on how to work with libraries using MinGW tools
• RTools
  • official CRAN docs on RTools 4.0
  • more detailed docs on what is new in RTools 4.0 (work in progress)

[github-actions]

This pull request has been automatically locked since there has not been any recent activity since it was closed. To start a new related discussion, open a new issue at https://github.com/microsoft/LightGBM/issues including a reference to this.