docs/DebuggingJITedCode.html

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   5   <title>Debugging JITed Code With GDB</title>
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  10 <div class="doc_title">Debugging JITed Code With GDB</div>
  11 <ol>
  12   <li><a href="#introduction">Introduction</a></li>
  13   <li><a href="#quickstart">Quickstart</a></li>
  14   <li><a href="#example">Example with clang and lli</a></li>
  15 </ol>
  16 <div class="doc_author">Written by Reid Kleckner</div>
  17
  18 <!--=========================================================================-->
  19 <div class="doc_section"><a name="introduction">Introduction</a></div>
  20 <!--=========================================================================-->
  21 <div class="doc_text">
  22
  23 <p>Without special runtime support, debugging dynamically generated code with
  24 GDB (as well as most debuggers) can be quite painful.  Debuggers generally read
  25 debug information from the object file of the code, but for JITed code, there is
  26 no such file to look for.
  27 </p>
  28
  29 <p>Depending on the architecture, this can impact the debugging experience in
  30 different ways.  For example, on most 32-bit x86 architectures, you can simply
  31 compile with -fno-omit-framepointer for GCC and -fdisable-fp-elim for LLVM.
  32 When GDB creates a backtrace, it can properly unwind the stack, but the stack
  33 frames owned by JITed code have ??'s instead of the appropriate symbol name.
  34 However, on Linux x86_64 in particular, GDB relies on the DWARF CFA debug
  35 information to unwind the stack, so even if you compile your program to leave
  36 the frame pointer untouched, GDB will usually be unable to unwind the stack past
  37 any JITed code stack frames.
  38 </p>
  39
  40 <p>In order to communicate the necessary debug info to GDB, an interface for
  41 registering JITed code with debuggers has been designed and implemented for
  42 GDB and LLVM.  At a high level, whenever LLVM generates new machine code, it
  43 also generates an object file in memory containing the debug information.  LLVM
  44 then adds the object file to the global list of object files and calls a special
  45 function (__jit_debug_register_code) marked noinline that GDB knows about.  When
  46 GDB attaches to a process, it puts a breakpoint in this function and loads all
  47 of the object files in the global list.  When LLVM calls the registration
  48 function, GDB catches the breakpoint signal, loads the new object file from
  49 LLVM's memory, and resumes the execution.  In this way, GDB can get the
  50 necessary debug information.
  51 </p>
  52
  53 <p>At the time of this writing, LLVM only supports architectures that use ELF
  54 object files and it only generates symbols and DWARF CFA information.  However,
  55 it would be easy to add more information to the object file, so we don't need to
  56 coordinate with GDB to get better debug information.
  57 </p>
  58 </div>
  59
  60 <!--=========================================================================-->
  61 <div class="doc_section"><a name="quickstart">Quickstart</a></div>
  62 <!--=========================================================================-->
  63 <div class="doc_text">
  64
  65 <p>In order to debug code JITed by LLVM, you need to install a recent version
  66 of GDB.  The interface was added on 2009-08-19, so you need a snapshot of GDB
  67 more recent than that.  Either download a snapshot of GDB or checkout CVS as
  68 instructed <a href="http://www.gnu.org/software/gdb/current/">here</a>.  Here
  69 are the commands for doing a checkout and building the code:
  70 </p>
  71
  72 <pre class="doc_code">
  73 $ cvs -z 3 -d :pserver:anoncvs@sourceware.org:/cvs/src co gdb
  74 $ mv src gdb   # You probably don't want this checkout called "src".
  75 $ cd gdb
  76 $ ./configure --prefix="$GDB_INSTALL"
  77 $ make
  78 $ make install
  79 </pre>
  80
  81 <p>You can then use -jit-emit-debug in the LLVM command line arguments to enable
  82 the interface.
  83 </p>
  84 </div>
  85
  86 <!--=========================================================================-->
  87 <div class="doc_section"><a name="example">Example with clang and lli</a></div>
  88 <!--=========================================================================-->
  89 <div class="doc_text">
  90
  91 <p>For example, consider debugging running lli on the following C code in
  92 foo.c:
  93 </p>
  94
  95 <pre class="doc_code">
  96 #include &lt;stdio.h&gt;
  97
  98 void foo() {
  99     printf("%d\n", *(int*)NULL);  // Crash here
 100 }
 101
 102 void bar() {
 103     foo();
 104 }
 105
 106 void baz() {
 107     bar();
 108 }
 109
 110 int main(int argc, char **argv) {
 111     baz();
 112 }
 113 </pre>
 114
 115 <p>Here are the commands to run that application under GDB and print the stack
 116 trace at the crash:
 117 </p>
 118
 119 <pre class="doc_code">
 120 # Compile foo.c to bitcode.  You can use either clang or llvm-gcc with this
 121 # command line.  Both require -fexceptions, or the calls are all marked
 122 # 'nounwind' which disables DWARF CFA info.
 123 $ clang foo.c -fexceptions -emit-llvm -c -o foo.bc
 124
 125 # Run foo.bc under lli with -jit-emit-debug.  If you built lli in debug mode,
 126 # -jit-emit-debug defaults to true.
 127 $ $GDB_INSTALL/gdb --args lli -jit-emit-debug foo.bc
 128 ...
 129
 130 # Run the code.
 131 (gdb) run
 132 Starting program: /tmp/gdb/lli -jit-emit-debug foo.bc
 133 [Thread debugging using libthread_db enabled]
 134
 135 Program received signal SIGSEGV, Segmentation fault.
 136 0x00007ffff7f55164 in foo ()
 137
 138 # Print the backtrace, this time with symbols instead of ??.
 139 (gdb) bt
 140 #0  0x00007ffff7f55164 in foo ()
 141 #1  0x00007ffff7f550f9 in bar ()
 142 #2  0x00007ffff7f55099 in baz ()
 143 #3  0x00007ffff7f5502a in main ()
 144 #4  0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*,
 145     std::vector&lt;llvm::GenericValue,
 146     std::allocator&lt;llvm::GenericValue&gt; &gt; const&) ()
 147 #5  0x00000000007d6d98 in
 148     llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
 149     std::vector&lt;std::string,
 150     std::allocator&lt;std::string&gt; &gt; const&, char const* const*) ()
 151 #6  0x00000000004dab76 in main ()
 152 </pre>
 153 </div>
 154
 155 <p>As you can see, GDB can correctly unwind the stack and has the appropriate
 156 function names.
 157 </p>
 158
 159 <!-- *********************************************************************** -->
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 166   <a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a><br>
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 168   Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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