1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <title>Debugging JITed Code With GDB</title> 7 <link rel="stylesheet" href="_static/llvm.css" type="text/css"> 8 </head> 9 <body> 10 11 <h1>Debugging JIT-ed Code With GDB</h1> 12 <ol> 13 <li><a href="#background">Background</a></li> 14 <li><a href="#gdbversion">GDB Version</a></li> 15 <li><a href="#mcjitdebug">Debugging MCJIT-ed code</a></li> 16 <ul> 17 <li><a href="#mcjitdebug_example">Example</a></li> 18 </ul> 19 </ol> 20 <div class="doc_author">Written by Reid Kleckner and Eli Bendersky</div> 21 22 <!--=========================================================================--> 23 <h2><a name="background">Background</a></h2> 24 <!--=========================================================================--> 25 <div> 26 27 <p>Without special runtime support, debugging dynamically generated code with 28 GDB (as well as most debuggers) can be quite painful. Debuggers generally read 29 debug information from the object file of the code, but for JITed code, there is 30 no such file to look for. 31 </p> 32 33 <p>In order to communicate the necessary debug info to GDB, an interface for 34 registering JITed code with debuggers has been designed and implemented for 35 GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code, 36 it does so in an in-memory object file that contains the debug information in 37 DWARF format. MCJIT then adds this in-memory object file to a global list of 38 dynamically generated object files and calls a special function 39 (<tt>__jit_debug_register_code</tt>) marked noinline that GDB knows about. When 40 GDB attaches to a process, it puts a breakpoint in this function and loads all 41 of the object files in the global list. When MCJIT calls the registration 42 function, GDB catches the breakpoint signal, loads the new object file from 43 the inferior's memory, and resumes the execution. In this way, GDB can get the 44 necessary debug information. 45 </p> 46 </div> 47 48 <!--=========================================================================--> 49 <h2><a name="gdbversion">GDB Version</a></h2> 50 <!--=========================================================================--> 51 52 <p>In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is 53 available on most modern distributions of Linux. The version of GDB that Apple 54 ships with Xcode has been frozen at 6.3 for a while. LLDB may be a better 55 option for debugging JIT-ed code on Mac OS X. 56 </p> 57 58 59 <!--=========================================================================--> 60 <h2><a name="mcjitdebug">Debugging MCJIT-ed code</a></h2> 61 <!--=========================================================================--> 62 <div> 63 64 <p>The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with 65 GDB. This is due to MCJIT's ability to use the MC emitter to provide full 66 DWARF debugging information to GDB.</p> 67 68 <p>Note that lli has to be passed the <tt>-use-mcjit</tt> flag to JIT the code 69 with MCJIT instead of the old JIT.</p> 70 71 <h3><a name="mcjitdebug_example">Example</a></h3> 72 73 <div> 74 75 <p>Consider the following C code (with line numbers added to make the example 76 easier to follow):</p> 77 78 <pre class="doc_code"> 79 1 int compute_factorial(int n) 80 2 { 81 3 if (n <= 1) 82 4 return 1; 83 5 84 6 int f = n; 85 7 while (--n > 1) 86 8 f *= n; 87 9 return f; 88 10 } 89 11 90 12 91 13 int main(int argc, char** argv) 92 14 { 93 15 if (argc < 2) 94 16 return -1; 95 17 char firstletter = argv[1][0]; 96 18 int result = compute_factorial(firstletter - '0'); 97 19 98 20 // Returned result is clipped at 255... 99 21 return result; 100 22 } 101 </pre> 102 103 <p>Here is a sample command line session that shows how to build and run this 104 code via lli inside GDB: 105 </p> 106 107 <pre class="doc_code"> 108 $ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c 109 $ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5 110 Reading symbols from $BINPATH/lli...done. 111 (gdb) b showdebug.c:6 112 No source file named showdebug.c. 113 Make breakpoint pending on future shared library load? (y or [n]) y 114 Breakpoint 1 (showdebug.c:6) pending. 115 (gdb) r 116 Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5 117 [Thread debugging using libthread_db enabled] 118 119 Breakpoint 1, compute_factorial (n=5) at showdebug.c:6 120 6 int f = n; 121 (gdb) p n 122 $1 = 5 123 (gdb) p f 124 $2 = 0 125 (gdb) n 126 7 while (--n > 1) 127 (gdb) p f 128 $3 = 5 129 (gdb) b showdebug.c:9 130 Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9. 131 (gdb) c 132 Continuing. 133 134 Breakpoint 2, compute_factorial (n=1) at showdebug.c:9 135 9 return f; 136 (gdb) p f 137 $4 = 120 138 (gdb) bt 139 #0 compute_factorial (n=1) at showdebug.c:9 140 #1 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18 141 #2 0x3500000001652748 in ?? () 142 #3 0x00000000016677e0 in ?? () 143 #4 0x0000000000000002 in ?? () 144 #5 0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161 145 #6 0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040) 146 at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397 147 #7 0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324 148 (gdb) finish 149 Run till exit from #0 compute_factorial (n=1) at showdebug.c:9 150 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18 151 18 int result = compute_factorial(firstletter - '0'); 152 Value returned is $5 = 120 153 (gdb) p result 154 $6 = 23406408 155 (gdb) n 156 21 return result; 157 (gdb) p result 158 $7 = 120 159 (gdb) c 160 Continuing. 161 162 Program exited with code 0170. 163 (gdb) 164 165 </pre> 166 167 </div> 168 </div> 169 170 171 <!-- *********************************************************************** --> 172 <hr> 173 <address> 174 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 175 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 176 <a href="http://validator.w3.org/check/referer"><img 177 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 178 <a href="mailto:reid.kleckner (a] gmail.com">Reid Kleckner</a>, 179 <a href="mailto:eliben (a] gmail.com">Eli Bendersky</a><br> 180 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 181 Last modified: $Date$ 182 </address> 183 </body> 184 </html> 185
