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 28GDB (as well as most debuggers) can be quite painful. Debuggers generally read 29debug information from the object file of the code, but for JITed code, there is 30no such file to look for. 31</p> 32 33<p>In order to communicate the necessary debug info to GDB, an interface for 34registering JITed code with debuggers has been designed and implemented for 35GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code, 36it does so in an in-memory object file that contains the debug information in 37DWARF format. MCJIT then adds this in-memory object file to a global list of 38dynamically generated object files and calls a special function 39(<tt>__jit_debug_register_code</tt>) marked noinline that GDB knows about. When 40GDB attaches to a process, it puts a breakpoint in this function and loads all 41of the object files in the global list. When MCJIT calls the registration 42function, GDB catches the breakpoint signal, loads the new object file from 43the inferior's memory, and resumes the execution. In this way, GDB can get the 44necessary 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 53available on most modern distributions of Linux. The version of GDB that Apple 54ships with Xcode has been frozen at 6.3 for a while. LLDB may be a better 55option 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 65GDB. This is due to MCJIT's ability to use the MC emitter to provide full 66DWARF 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 69with 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 76easier to follow):</p> 77 78<pre class="doc_code"> 791 int compute_factorial(int n) 802 { 813 if (n <= 1) 824 return 1; 835 846 int f = n; 857 while (--n > 1) 868 f *= n; 879 return f; 8810 } 8911 9012 9113 int main(int argc, char** argv) 9214 { 9315 if (argc < 2) 9416 return -1; 9517 char firstletter = argv[1][0]; 9618 int result = compute_factorial(firstletter - '0'); 9719 9820 // Returned result is clipped at 255... 9921 return result; 10022 } 101</pre> 102 103<p>Here is a sample command line session that shows how to build and run this 104code 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 110Reading symbols from $BINPATH/lli...done. 111(gdb) b showdebug.c:6 112No source file named showdebug.c. 113Make breakpoint pending on future shared library load? (y or [n]) y 114Breakpoint 1 (showdebug.c:6) pending. 115(gdb) r 116Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5 117[Thread debugging using libthread_db enabled] 118 119Breakpoint 1, compute_factorial (n=5) at showdebug.c:6 1206 int f = n; 121(gdb) p n 122$1 = 5 123(gdb) p f 124$2 = 0 125(gdb) n 1267 while (--n > 1) 127(gdb) p f 128$3 = 5 129(gdb) b showdebug.c:9 130Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9. 131(gdb) c 132Continuing. 133 134Breakpoint 2, compute_factorial (n=1) at showdebug.c:9 1359 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 149Run till exit from #0 compute_factorial (n=1) at showdebug.c:9 1500x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18 15118 int result = compute_factorial(firstletter - '0'); 152Value returned is $5 = 120 153(gdb) p result 154$6 = 23406408 155(gdb) n 15621 return result; 157(gdb) p result 158$7 = 120 159(gdb) c 160Continuing. 161 162Program 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@gmail.com">Reid Kleckner</a>, 179 <a href="mailto:eliben@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