Getting the Debugging JIT-ed Code with GDB example to work

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1

I’m trying to run the example described at:

http://llvm.org/docs/DebuggingJITedCode.html

I followed the sample command line session (below, with versions numbers for everything), but gdb doesn’t stop at the breakpoints as described. Any idea what is wrong?

Thanks,

Zach

2

I haven’t tried that in a while. It’s possible that it has been broken. I thought there was a test for it, but I’m not seeing it right now.

A couple of suggestions to help you troubleshoot the problem:

  1. Enable verbose output in GDB.

  2. Set a breakpoint in MCJIT.cpp where it calls loadedObject->registerWithDebugger. Step through that and verify that it gets as far as the call to __jit_debug_register_code in GDBRegistrar.cpp.

When __jit_debug_register_code is called, you should get a message saying that GDB has loaded symbols for the dynamic code (or possibly that it couldn’t find symbols).

Let me know what happens when you do that, and I might be able to make a reasonable suggestion as to what to do next.

-Andy

3

Actually, put aside what I said in my previous reply for a second.

I just noticed in your output below that GDB didn’t have symbols for lli. That’s probably the heart of your problem. GDB relies on hooking a known function to get the JITed code registration to work. Since it didn’t find symbols for lli, it won’t find the hook function.

You’ll need to use a build of lli with symbols to make this work.

-Andy

4

I tested it with the symbols enabled for lli and it still doesn’t work. Below is the assembly for GDBJITRegistrar::registerObject containing the inlined body of NotifyDebugger. It looks like the call to __jit_debug_register_code was inlined away despite being marked with LLVM_ATTRIBUTE_NOINLINE. LLVM was compiled with the following GCC:

zdevito@derp:~/terra/tests$ gcc --version
gcc (Ubuntu/Linaro 4.6.3-1ubuntu5) 4.6.3
Copyright (C) 2011 Free Software Foundation, Inc.
This is free software; see the source for copying conditions. There is NO
warranty; not even for MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.

Maybe the LLVM_ATTRIBUTE_NOINLINE macro is defined wrong, or the compiler is ignoring it?

Zach

5

That bug was fixed this January in r199350, so you may need to update LLVM.