Debugging JIT-ed Code With GDB

    In order to communicate the necessary debug info to GDB, an interface forregistering JITed code with debuggers has been designed and implemented forGDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code,it does so in an in-memory object file that contains the debug information inDWARF format. MCJIT then adds this in-memory object file to a global list ofdynamically generated object files and calls a special function() marked noinline that GDB knows about. WhenGDB attaches to a process, it puts a breakpoint in this function and loads allof the object files in the global list. When MCJIT calls the registrationfunction, GDB catches the breakpoint signal, loads the new object file fromthe inferior’s memory, and resumes the execution. In this way, GDB can get thenecessary debug information.

    The emerging MCJIT component of LLVM allows full debugging of JIT-ed code withGDB. This is due to MCJIT’s ability to use the MC emitter to provide fullDWARF debugging information to GDB.

    Consider the following C code (with line numbers added to make the exampleeasier to follow):

    1. $ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
    2. $ gdb --quiet --args $BINPATH/lli -jit-kind=mcjit showdebug.ll 5
    3. Reading symbols from $BINPATH/lli...done.
    4. (gdb) b showdebug.c:6
    5. No source file named showdebug.c.
    6. Make breakpoint pending on future shared library load? (y or [n]) y
    7. Breakpoint 1 (showdebug.c:6) pending.
    8. (gdb) r
    9. Starting program: $BINPATH/lli -jit-kind=mcjit showdebug.ll 5
    10. [Thread debugging using libthread_db enabled]
    11.  
    12. Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
    13. 6        int f = n;
    14. (gdb) p n
    15. $1 = 5
    16. $2 = 0
    17. (gdb) n
    18. 7        while (--n > 1)
    19. (gdb) p f
    20. $3 = 5
    21. (gdb) b showdebug.c:9
    22. Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
    23. (gdb) c
    24. Continuing.
    25.  
    26. Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
    27. 9        return f;
    28. (gdb) p f
    29. $4 = 120
    30. (gdb) bt
    31. #0  compute_factorial (n=1) at showdebug.c:9
    32. #1  0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
    33. #2  0x3500000001652748 in ?? ()
    34. #3  0x00000000016677e0 in ?? ()
    35. #5  0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
    36. #6  0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
    37.     at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
    38. #7  0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
    39. (gdb) finish
    40. Run till exit from #0  compute_factorial (n=1) at showdebug.c:9
    41. 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
    42. 18       int result = compute_factorial(firstletter - '0');
    43. Value returned is $5 = 120
    44. (gdb) p result
    45. $6 = 23406408
    46. (gdb) n
    47. 21       return result;
    48. (gdb) p result
    49. $7 = 120
    50. (gdb) c
    51. Continuing.
    52.  
    53. Program exited with code 0170.