1.. _debugging-jited-code:
2
3==============================
4Debugging JIT-ed Code With GDB
5==============================
6
7.. sectionauthor:: Reid Kleckner and Eli Bendersky
8
9Background
10==========
11
12Without special runtime support, debugging dynamically generated code with
13GDB (as well as most debuggers) can be quite painful.  Debuggers generally
14read debug information from the object file of the code, but for JITed
15code, there is no such file to look for.
16
17In order to communicate the necessary debug info to GDB, an interface for
18registering JITed code with debuggers has been designed and implemented for
19GDB and LLVM MCJIT.  At a high level, whenever MCJIT generates new machine code,
20it does so in an in-memory object file that contains the debug information in
21DWARF format.  MCJIT then adds this in-memory object file to a global list of
22dynamically generated object files and calls a special function
23(``__jit_debug_register_code``) marked noinline that GDB knows about.  When
24GDB attaches to a process, it puts a breakpoint in this function and loads all
25of the object files in the global list.  When MCJIT calls the registration
26function, GDB catches the breakpoint signal, loads the new object file from
27the inferior's memory, and resumes the execution.  In this way, GDB can get the
28necessary debug information.
29
30GDB Version
31===========
32
33In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is
34available on most modern distributions of Linux.  The version of GDB that
35Apple ships with Xcode has been frozen at 6.3 for a while.  LLDB may be a
36better option for debugging JIT-ed code on Mac OS X.
37
38
39Debugging MCJIT-ed code
40=======================
41
42The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with
43GDB.  This is due to MCJIT's ability to use the MC emitter to provide full
44DWARF debugging information to GDB.
45
46Note that lli has to be passed the ``-use-mcjit`` flag to JIT the code with
47MCJIT instead of the old JIT.
48
49Example
50-------
51
52Consider the following C code (with line numbers added to make the example
53easier to follow):
54
55..
56   FIXME:
57   Sphinx has the ability to automatically number these lines by adding
58   :linenos: on the line immediately following the `.. code-block:: c`, but
59   it looks like garbage; the line numbers don't even line up with the
60   lines. Is this a Sphinx bug, or is it a CSS problem?
61
62.. code-block:: c
63
64   1   int compute_factorial(int n)
65   2   {
66   3       if (n <= 1)
67   4           return 1;
68   5
69   6       int f = n;
70   7       while (--n > 1)
71   8           f *= n;
72   9       return f;
73   10  }
74   11
75   12
76   13  int main(int argc, char** argv)
77   14  {
78   15      if (argc < 2)
79   16          return -1;
80   17      char firstletter = argv[1][0];
81   18      int result = compute_factorial(firstletter - '0');
82   19
83   20      // Returned result is clipped at 255...
84   21      return result;
85   22  }
86
87Here is a sample command line session that shows how to build and run this
88code via ``lli`` inside GDB:
89
90.. code-block:: bash
91
92   $ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
93   $ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5
94   Reading symbols from $BINPATH/lli...done.
95   (gdb) b showdebug.c:6
96   No source file named showdebug.c.
97   Make breakpoint pending on future shared library load? (y or [n]) y
98   Breakpoint 1 (showdebug.c:6) pending.
99   (gdb) r
100   Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5
101   [Thread debugging using libthread_db enabled]
102
103   Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
104   6	    int f = n;
105   (gdb) p n
106   $1 = 5
107   (gdb) p f
108   $2 = 0
109   (gdb) n
110   7	    while (--n > 1)
111   (gdb) p f
112   $3 = 5
113   (gdb) b showdebug.c:9
114   Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
115   (gdb) c
116   Continuing.
117
118   Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
119   9	    return f;
120   (gdb) p f
121   $4 = 120
122   (gdb) bt
123   #0  compute_factorial (n=1) at showdebug.c:9
124   #1  0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
125   #2  0x3500000001652748 in ?? ()
126   #3  0x00000000016677e0 in ?? ()
127   #4  0x0000000000000002 in ?? ()
128   #5  0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
129   #6  0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
130       at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
131   #7  0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
132   (gdb) finish
133   Run till exit from #0  compute_factorial (n=1) at showdebug.c:9
134   0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
135   18	    int result = compute_factorial(firstletter - '0');
136   Value returned is $5 = 120
137   (gdb) p result
138   $6 = 23406408
139   (gdb) n
140   21	    return result;
141   (gdb) p result
142   $7 = 120
143   (gdb) c
144   Continuing.
145
146   Program exited with code 0170.
147   (gdb)
148