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