Hi!
I am using LLVM to create a JIT – I have followed the excellent Kaleidoscope tutorial, and am now starting to build off of it.
Down the line, I plan to use this JIT for my research into computer architecture, which means that I will need to run the JIT itself within a simulated CPU and bare-bones linux system. To make that research easier, I am trying to statically compile my JIT. Everything works with the normal compilation (clang … -rdynamic) but when switched to static (clang … -static), I can no longer execute externally defined C functions. I suspect this is something to do with the symbol loader not being properly set up for static linking, but I don’t know much about that.
Here are some more details:
In the normally-compiled kaleidoscope JIT, I can run something like this:
extern printd(x);
printd(10);
10.000000
printd was one of the C functions defined within the main program:
/// printd - printf that takes a double prints it as “%f\n”, returning 0.
extern “C” DLLEXPORT double printd(double X) {
fprintf(stderr, “%f\n”, X);
return X;
}
However, in the static linked version, the JIT cannot find the symbol:
extern printd(x);
printd(1);
kaleidoscope error: Symbols not found: { printd }
But the symbol is there:
$ llvm-nm kld_s | grep printd
0000000000497e30 T printd
$ llvm-nm kld | grep printd
00000000006b1820 T printd
(kld_s is the static version)
How can I begin to debug this issue? Does something different happen to the symbol loader when I compile statically?
My two make scripts look like
$ make kld
clang++ -v kld.cpp libkld.o /home/eli/sproj-git/passes/test/libTestPass.o llvm-config --cxxflags --ldflags --system-libs --libs core orcjit native -fuse-ld=gold -g3 -o kld -rdynamic
$ make kld_s
clang++ -v kld.cpp libkld.o /home/eli/sproj-git/passes/test/libTestPass.o llvm-config --cxxflags --ldflags --system-libs --libs core orcjit native -fuse-ld=gold -g3 -o kld_s -static
The only difference is in the final flag (bold). libkld.o and libTestPass.o are unrelated object files I am linking from other parts of my project.
If the clang option -static behaves like gcc’s, the man page does say that it “… prevents linking with the shared libraries”, which I suppose is not precisely what I want.
I previously emailed llvm-dev a few months ago about a similar issue, where extern functions were not working at all (even dynamically). I resolved that issue by downgrading to LLVM 6, and recently, by upgrading to LLVM 9. Now the issue has returned, I suppose, in slightly different form.
If anyone has any ideas, please let me know!
Thank you so much,
Eli Baum