[rootcling] `genreflex --cxxmodule` generates a truncated .pcm file

[jalopezg-git]

• Checked for duplicates

Describe the bug

genreflex --cxxmodule ... generates a truncated xxx_rflx.pcm file (0 bytes). The equivalent invocation of rootcling --cxxmodule using a linkdef.h instead of a XML file works as expected.

Expected behavior

genreflex should generate a valid cxxmodule file.

To Reproduce

Run genreflex/rootcling as follows. The contents of the required files are provided below.

$ genreflex --cxxmodule -s classes.xml classes.h
$ # or
$ rootcling --cxxmodule -f out.cpp -s ./libclasses.so classes.h classes_linkdef.h

classes.h:

#ifndef CLASSES_H
#define CLASSES_H

struct Foo {
   Foo(int _i) : i(_i) {}
   int i = 0;
};

#endif

classes_linkdef.h:

#ifdef __CINT__
#pragma link off all globals;
#pragma link off all classes;
#pragma link off all functions;

#pragma link C++ class Foo+;
#endif

classes.xml:

<lcgdict>
  <class name="Foo"/>
</lcgdict>

Setup

ROOT master, Arch Linux x86_64

[Axel-Naumann]

IIRC that's expected (@vgvassilev might correct me): genreflex embeds the header as string in the dictionary. As such we don't need an actual PCM file; we provide an empty one to regularize the build systems.

[pcanal]

humm ... why? Is that much much slower (parsing) than a binary pcm or do you mean that it is a binary blob? Either way why is the option --cxxmodule behaving differently for genreflex than rootcling?

[vgvassilev]

On my machine both commands produce a 0-sized module. Eg:

rootcling --cxxmodule -f out.cpp -s ./libclasses.so classes.h classes_linkdef.h
Error in <CheckModuleValid>: Couldn't find module with name 'classes' in modulemap!
du classes.pcm 
0	classes.pcm

That is because you need to write a modulemap (normally the build system or aclic does it for you).

echo "module classes { export * header \"classes.h\" }" > classes.modulemap
rootcling -cxxmodule -moduleMapFile=classes.modulemap  -f out.cpp -s ./libclasses.so  classes.h classes_linkdef.h 
vvassilev@vv-nuc ~/workspace/builds/root_master $ du -hs classes.pcm 
400K	classes.pcm

If you want to use the selection xml file you could still do it like:

rootcling -reflex -cxxmodule -moduleMapFile=classes.modulemap  -f out.cpp -s ./libclasses.so  classes.h classes.xml 
du classes.pcm 
404	classes.pcm

We never really tried the -cxxmodule option under genreflex. Maybe we should just drop it from there. We use rootcling -reflex .... See here https://github.com/cms-sw/cmssw-config/blob/41730a3bd862bb50a68d52279d096f1794d8874e/SCRAM/GMake/Makefile.cxxmodule#L112
Using rootcling -reflex would not follow the genreflex convention to add _rflx prefix to the files but so far we have not seen issues due to that.

A general note is that rootcling is a superset of genreflex. The argument parsing of rootcling is more helpful and we run extra checks in the context of -cxxmodule. I'd warmly recommend to use rootcling in favor of genreflex in the context of modules. Additionally, this gives a potential way to deprecate genreflex once/if people move to modules.