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Reid Kleckner22884dd2009-09-21 02:34:59 +0000[diff] [blame]6 <title>Debugging JITed Code With GDB</title>
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Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]11<h1>Debugging JIT-ed Code With GDB</h1>
Reid Kleckner22884dd2009-09-21 02:34:59 +0000[diff] [blame]12<ol>
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]13 <li><a href="#background">Background</a></li>
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]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>
Reid Kleckner22884dd2009-09-21 02:34:59 +0000[diff] [blame]19</ol>
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]20<div class="doc_author">Written by Reid Kleckner and Eli Bendersky</div>
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]21
22<!--=========================================================================-->
NAKAMURA Takumi05d02652011-04-18 23:59:50 +0000[diff] [blame]23<h2><a name="background">Background</a></h2>
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]24<!--=========================================================================-->
NAKAMURA Takumif5af6ad2011-04-23 00:30:22 +0000[diff] [blame]25<div>
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]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
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]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
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]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
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]40GDB attaches to a process, it puts a breakpoint in this function and loads all
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]41of the object files in the global list. When MCJIT calls the registration
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]42function, GDB catches the breakpoint signal, loads the new object file from
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]43the inferior's memory, and resumes the execution. In this way, GDB can get the
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]44necessary debug information.
45</p>
Reid Kleckner89472822010-07-07 20:16:45 +0000[diff] [blame]46</div>
Reid Kleckner22884dd2009-09-21 02:34:59 +0000[diff] [blame]47
Eli Benderskyf8b30f92012-05-01 06:15:40 +0000[diff] [blame^]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
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Reid Kleckner22884dd2009-09-21 02:34:59 +0000[diff] [blame]171<!-- *********************************************************************** -->
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179 <a href="mailto:eliben@gmail.com">Eli Bendersky</a><br>
NAKAMURA Takumib9a33632011-04-09 02:13:37 +0000[diff] [blame]180 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
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