Debugging JITed Code With GDB

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Without special runtime support, debugging dynamically generated code with -GDB (as well as most debuggers) can be quite painful. Debuggers generally read -debug information from the object file of the code, but for JITed code, there is -no such file to look for. -

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In order to communicate the necessary debug info to GDB, an interface for -registering JITed code with debuggers has been designed and implemented for -GDB 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 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) marked noinline that GDB knows about. When -GDB attaches to a process, it puts a breakpoint in this function and loads all -of the object files in the global list. When MCJIT calls the registration -function, GDB catches the breakpoint signal, loads the new object file from -the inferior's memory, and resumes the execution. In this way, GDB can get the -necessary debug information. -

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 may be a better -option for debugging JIT-ed code on Mac OS X. -

Debugging MCJIT-ed code

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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.

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Note that lli has to be passed the -use-mcjit flag to JIT the code -with MCJIT instead of the old JIT.

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Example

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

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-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  }
-

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

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-$ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
-$ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5
-Reading symbols from $BINPATH/lli...done.
-(gdb) b showdebug.c:6
-No source file named showdebug.c.
-Make breakpoint pending on future shared library load? (y or [n]) y
-Breakpoint 1 (showdebug.c:6) pending.
-(gdb) r
-Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5
-[Thread debugging using libthread_db enabled]
-
-Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
-6	    int f = n;
-(gdb) p n
-$1 = 5
-(gdb) p f
-$2 = 0
-(gdb) n
-7	    while (--n > 1) 
-(gdb) p f
-$3 = 5
-(gdb) b showdebug.c:9
-Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
-(gdb) c
-Continuing.
-
-Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
-9	    return f;
-(gdb) p f
-$4 = 120
-(gdb) bt
-#0  compute_factorial (n=1) at showdebug.c:9
-#1  0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
-#2  0x3500000001652748 in ?? ()
-#3  0x00000000016677e0 in ?? ()
-#4  0x0000000000000002 in ?? ()
-#5  0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
-#6  0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
-    at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
-#7  0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
-(gdb) finish
-Run till exit from #0  compute_factorial (n=1) at showdebug.c:9
-0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
-18	    int result = compute_factorial(firstletter - '0');
-Value returned is $5 = 120
-(gdb) p result
-$6 = 23406408
-(gdb) n
-21	    return result;
-(gdb) p result
-$7 = 120
-(gdb) c
-Continuing.
-
-Program exited with code 0170.
-(gdb) 
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Debugging JIT-ed Code With GDB

Background

GDB Version

Debugging MCJIT-ed code

Example