GCModuleInfo and MCJIT

Hi all,

So, MCJIT is working with vmkit, that's great. However, I need to
retrieve the GCModuleInfo and the MachineCodeEmitter used during the
compilation process. And I don't know how I should proceed? I'm trying
to understand how they are preserved with the old jit, but I have to
say that I'm a little bit lost because I haven't a global view of the
code... So, any help would be greatly appreciated!

See you,


I have now a more precise question, I hope that this time, someone
will be able to help me :slight_smile:

I'm now able to find the gc meta data. To find this data, I simply
register a GCMetadataPrinter (just like during an AOT compilation). A
GCMetadataprinter is called at the end of the compilation of a module
after having assigned the slots to the live roots. I have almost
everything except that at this step, a collection point (aka safepoint
in the GC world) is describe by:
* a MCSymbol, which indicates the location of a safepoint (basically
the instruction pointer that follows the safepoint)
* a list of live roots, each one is described by an offset relative to
the frame pointer.

Now, I would like to find the concrete address of the MCSymbol. And I
don't know how I could find this information. When I generate the
assembly, I don't have the actual location of the function (I
manipulate the AsmPrinter but it's not directly the object file). And
after having loaded the object file, I don't know how I could find the
concrete address of the MCSymbol. When I call
mcjit->getGlobalValueAddress("safepoint-symb"), mcjit is unable to
find it in the object file.

Any idea?


I have the solution to my problem :). And the llvm gc infrastructure
can work with the current MCJIT without any change (good news).

I will explain what I have done to find the garbage collector metadata
generated by llvm with MCJIT, I'm pretty sure that it could help

And it's very easy. You just have to activate a static gcprinter for
the assembly. A very good example is given in
OcamlGCPrinter.cpp/OCamlGC.cpp. The only problem with the hook of
OcamlGCPrinter::finishAssembly() is that it changes the current
section to emit the gc metadata in the data section. And it does not
work with MCJIT. I don't know why, probably because somewhere, an
AsmPrinter or MCJIT supposes that another section is active after the
call to finishAssembly. To solve the problem, you just have to add

    const MCSection* sec = AP.getCurrentSection();

et the beginning of finishAssembly, and


at the end.

See you,

Hi all,

I continue my post because I made a mistake: there is no need to
preserve and to restore the section in finishAssembly (I faced a
really ugly bug elsewhere caused by random values in floatting point
registers, which was hidden by preserving the section).

So, basically, if you want to use gcroot with mcjit, it's easy: just
generate the stack maps in the data section, just like in
ErlangGCPrinter.cpp or OcamlGCPrinter.cpp, and then, you can find this
stack map in mcjit by using mcjit::getGlobalValueAddress.

See you,