Debugging JIT-ed Code — LLVM 12 documentation

# Debugging JIT-ed Code

## Background

Without special runtime support, debugging dynamically generated code can be quite painful. Debuggers generally read debug information from object files on disk, but for JITed code there is no such file to look for.

In order to hand over the necessary debug info, GDB established an interface for registering JITed code with debuggers. LLDB implements it in the JITLoaderGDB plugin. On the JIT side, LLVM MCJIT does implement the interface for ELF object files.

At a high level, whenever MCJIT generates new machine code, it does so in an in-memory object file that contains the debug information in DWARF format. MCJIT then adds this in-memory object file to a global list of dynamically generated object files and calls a special function `__jit_debug_register_code` that the debugger knows about. When the debugger attaches to a process, it puts a breakpoint in this function and associates a special handler with it. Once MCJIT calls the registration function, the debugger catches the breakpoint signal, loads the new object file from the inferior’s memory and resumes execution. This way it can obtain debug information for pure in-memory object files.

## GDB Version

In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is available on most modern distributions of Linux. The version of GDB that Apple ships with Xcode has been frozen at 6.3 for a while.

## LLDB Version

Due to a regression in release 6.0, LLDB didn’t support JITed code debugging for a while. The bug was fixed in mainline recently, so that debugging JITed ELF objects should be possible again from the upcoming release 12.0 on. On macOS the feature must be enabled explicitly using the `plugin.jit-loader.gdb.enable` setting.

## Debugging MCJIT-ed code

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

Note that lli has to be passed the `--jit-kind=mcjit` flag to JIT the code with MCJIT instead of the newer ORC JIT.

### Example

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

    1   int compute_factorial(int n)
    2   {
    3       if (n <= 1)
    4           return 1;
    5
    6       int f = n;
    7       while (--n > 1)
    8           f *= n;
    9       return f;
    10  }
    11
    12
    13  int main(int argc, char** argv)
    14  {
    15      if (argc < 2)
    16          return -1;
    17      char firstletter = argv[1][0];
    18      int result = compute_factorial(firstletter - '0');
    19
    20      // Returned result is clipped at 255...
    21      return result;
    22  }

Here is a sample command line session that shows how to build and run this code via `lli` inside LLDB:

    > export BINPATH=/workspaces/llvm-project/build/bin
    > $BINPATH/clang -g -S -emit-llvm --target=x86_64-unknown-unknown-elf showdebug.c
    > lldb $BINPATH/lli
    (lldb) target create "/workspaces/llvm-project/build/bin/lli"
    Current executable set to '/workspaces/llvm-project/build/bin/lli' (x86_64).
    (lldb) settings set plugin.jit-loader.gdb.enable on
    (lldb) b compute_factorial
    Breakpoint 1: no locations (pending).
    WARNING:  Unable to resolve breakpoint to any actual locations.
    (lldb) run --jit-kind=mcjit showdebug.ll 5
    1 location added to breakpoint 1
    Process 21340 stopped
    * thread #1, name = 'lli', stop reason = breakpoint 1.1
       frame #0: 0x00007ffff7fd0007 JIT(0x45c2cb0)`compute_factorial(n=5) at showdebug.c:3:11
       1    int compute_factorial(int n)
       2    {
    -> 3        if (n <= 1)
       4            return 1;
       5        int f = n;
       6        while (--n > 1)
       7            f *= n;
    (lldb) p n
    (int) $0 = 5
    (lldb) b showdebug.c:9
    Breakpoint 2: where = JIT(0x45c2cb0)`compute_factorial + 60 at showdebug.c:9:1, address = 0x00007ffff7fd003c
    (lldb) c
    Process 21340 resuming
    Process 21340 stopped
    * thread #1, name = 'lli', stop reason = breakpoint 2.1
       frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
       6        while (--n > 1)
       7            f *= n;
       8        return f;
    -> 9    }
       10
       11   int main(int argc, char** argv)
       12   {
    (lldb) p f
    (int) $1 = 120
    (lldb) bt
    * thread #1, name = 'lli', stop reason = breakpoint 2.1
    * frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
       frame #1: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:18
       frame #2: 0x0000000002a8306e lli`llvm::MCJIT::runFunction(this=0x000000000458ed10, F=0x0000000004589ff8, ArgValues=ArrayRef<llvm::GenericValue> @ 0x00007fffffffc798) at MCJIT.cpp:554:31
       frame #3: 0x00000000029bdb45 lli`llvm::ExecutionEngine::runFunctionAsMain(this=0x000000000458ed10, Fn=0x0000000004589ff8, argv=size=0, envp=0x00007fffffffe140) at ExecutionEngine.cpp:467:10
       frame #4: 0x0000000001f2fc2f lli`main(argc=4, argv=0x00007fffffffe118, envp=0x00007fffffffe140) at lli.cpp:643:18
       frame #5: 0x00007ffff788c09b libc.so.6`__libc_start_main(main=(lli`main at lli.cpp:387), argc=4, argv=0x00007fffffffe118, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, stack_end=0x00007fffffffe108) at libc-start.c:308:16
       frame #6: 0x0000000001f2dc7a lli`_start + 42
    (lldb) finish
    Process 21340 stopped
    * thread #1, name = 'lli', stop reason = step out
    Return value: (int) $2 = 120

       frame #0: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:9
       13       if (argc < 2)
       14           return -1;
       15       char firstletter = argv[1][0];
    -> 16       int result = compute_factorial(firstletter - '0');
       17
       18       // Returned result is clipped at 255...
       19       return result;
    (lldb) p result
    (int) $3 = 73670648
    (lldb) n
    Process 21340 stopped
    * thread #1, name = 'lli', stop reason = step over
       frame #0: 0x00007ffff7fd0098 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:19:12
       16       int result = compute_factorial(firstletter - '0');
       17
       18       // Returned result is clipped at 255...
    -> 19       return result;
       20   }
    (lldb) p result
    (int) $4 = 120
    (lldb) expr result=42
    (int) $5 = 42
    (lldb) p result
    (int) $6 = 42
    (lldb) c
    Process 21340 resuming
    Process 21340 exited with status = 42 (0x0000002a)
    (lldb) exit