[lldb-dev] Debugging JIT-compiled code with LLVM

Simon Ask Ulsnes simon at ulsnes.dk
Wed Nov 17 08:32:40 PST 2010

Hey list,

I know similar questions have been asked before, but I have been
unable to find a clear answer to what approach would be recommended
for this use case (the LLDB documentation is sparse, to put it

My situation: The scripting language is JIT-compiled by LLVM. As is
the language runtime, written in C and precompiled to bitcode with
Clang. The runtime is hosted by a driver application that loads and
initializes LLVM, then hands over responsibilities to the runtime.

Currently, this is hard to debug, because neither gdb nor LLDB seem to
know about the symbols and debug information emitted by the JIT
compiler. The driver application sets llvm::JITEmitDebugInfo = true at

According to the LLVM documentation, gdb 7.0 and later should support
debugging of JIT'ed code, but this does not appear to be true
(backtraces from JIT'ed functions are nonsensical, the eh_frame looks
wrong, and of course breakpoints don't work). What surprises me is
that LLDB doesn't seem to catch the JIT'ed debug information either.

The previously proposed solution of compiling code to dylibs and using
dlopen()/dladdr() to call functions, instead of using the on-the-fly
JIT compiler in LLVM, is not only pretty inconvenient, it's also
incorrect, since my code and language takes advantage of quite a few
opportunities for run-time-optimization (and so the code used for
debugging would poorly reflect the actual code).

I have been attaching LLDB/gdb to the main driver executable, but
perhaps this is the wrong way to go — would it be more advisable to
host the debugger within my own driver process using LLDB.framework?

Please advise. :-)

Platform: Mac OS X 10.6.5, GCC 4.5, gdb 7.1, LLVM 2.8, LLDB from SVN trunk.


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