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="#example">Example usage</a></li>
  13   <li><a href="#background">Background</a></li>
  14 </ol>
  15 <div class="doc_author">Written by Reid Kleckner</div>
  16
  17 <!--=========================================================================-->
  18 <div class="doc_section"><a name="example">Example usage</a></div>
  19 <!--=========================================================================-->
  20 <div class="doc_text">
  21
  22 <p>In order to debug code JITed by LLVM, you need GDB 7.0 or newer, which is
  23 available on most modern distributions of Linux.  The version of GDB that Apple
  24 ships with XCode has been frozen at 6.3 for a while.  LLDB may be a better
  25 option for debugging JITed code on Mac OS X.
  26 </p>
  27
  28 <p>Consider debugging the following code compiled with clang and run through
  29 lli:
  30 </p>
  31
  32 <pre class="doc_code">
  33 #include &lt;stdio.h&gt;
  34
  35 void foo() {
  36     printf("%d\n", *(int*)NULL);  // Crash here
  37 }
  38
  39 void bar() {
  40     foo();
  41 }
  42
  43 void baz() {
  44     bar();
  45 }
  46
  47 int main(int argc, char **argv) {
  48     baz();
  49 }
  50 </pre>
  51
  52 <p>Here are the commands to run that application under GDB and print the stack
  53 trace at the crash:
  54 </p>
  55
  56 <pre class="doc_code">
  57 # Compile foo.c to bitcode.  You can use either clang or llvm-gcc with this
  58 # command line.  Both require -fexceptions, or the calls are all marked
  59 # 'nounwind' which disables DWARF exception handling info.  Custom frontends
  60 # should avoid adding this attribute to JITed code, since it interferes with
  61 # DWARF CFA generation at the moment.
  62 $ clang foo.c -fexceptions -emit-llvm -c -o foo.bc
  63
  64 # Run foo.bc under lli with -jit-emit-debug.  If you built lli in debug mode,
  65 # -jit-emit-debug defaults to true.
  66 $ $GDB_INSTALL/gdb --args lli -jit-emit-debug foo.bc
  67 ...
  68
  69 # Run the code.
  70 (gdb) run
  71 Starting program: /tmp/gdb/lli -jit-emit-debug foo.bc
  72 [Thread debugging using libthread_db enabled]
  73
  74 Program received signal SIGSEGV, Segmentation fault.
  75 0x00007ffff7f55164 in foo ()
  76
  77 # Print the backtrace, this time with symbols instead of ??.
  78 (gdb) bt
  79 #0  0x00007ffff7f55164 in foo ()
  80 #1  0x00007ffff7f550f9 in bar ()
  81 #2  0x00007ffff7f55099 in baz ()
  82 #3  0x00007ffff7f5502a in main ()
  83 #4  0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*,
  84     std::vector&lt;llvm::GenericValue,
  85     std::allocator&lt;llvm::GenericValue&gt; &gt; const&amp;) ()
  86 #5  0x00000000007d6d98 in
  87     llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
  88     std::vector&lt;std::string,
  89     std::allocator&lt;std::string&gt; &gt; const&amp;, char const* const*) ()
  90 #6  0x00000000004dab76 in main ()
  91 </pre>
  92
  93 <p>As you can see, GDB can correctly unwind the stack and has the appropriate
  94 function names.
  95 </p>
  96 </div>
  97
  98 <!--=========================================================================-->
  99 <div class="doc_section"><a name="background">Background</a></div>
 100 <!--=========================================================================-->
 101 <div class="doc_text">
 102
 103 <p>Without special runtime support, debugging dynamically generated code with
 104 GDB (as well as most debuggers) can be quite painful.  Debuggers generally read
 105 debug information from the object file of the code, but for JITed code, there is
 106 no such file to look for.
 107 </p>
 108
 109 <p>Depending on the architecture, this can impact the debugging experience in
 110 different ways.  For example, on most 32-bit x86 architectures, you can simply
 111 compile with -fno-omit-frame-pointer for GCC and -disable-fp-elim for LLVM.
 112 When GDB creates a backtrace, it can properly unwind the stack, but the stack
 113 frames owned by JITed code have ??'s instead of the appropriate symbol name.
 114 However, on Linux x86_64 in particular, GDB relies on the DWARF call frame
 115 address (CFA) debug information to unwind the stack, so even if you compile
 116 your program to leave the frame pointer untouched, GDB will usually be unable
 117 to unwind the stack past any JITed code stack frames.
 118 </p>
 119
 120 <p>In order to communicate the necessary debug info to GDB, an interface for
 121 registering JITed code with debuggers has been designed and implemented for
 122 GDB and LLVM.  At a high level, whenever LLVM generates new machine code, it
 123 also generates an object file in memory containing the debug information.  LLVM
 124 then adds the object file to the global list of object files and calls a special
 125 function (__jit_debug_register_code) marked noinline that GDB knows about.  When
 126 GDB attaches to a process, it puts a breakpoint in this function and loads all
 127 of the object files in the global list.  When LLVM calls the registration
 128 function, GDB catches the breakpoint signal, loads the new object file from
 129 LLVM's memory, and resumes the execution.  In this way, GDB can get the
 130 necessary debug information.
 131 </p>
 132
 133 <p>At the time of this writing, LLVM only supports architectures that use ELF
 134 object files and it only generates symbols and DWARF CFA information.  However,
 135 it would be easy to add more information to the object file, so we don't need to
 136 coordinate with GDB to get better debug information.
 137 </p>
 138 </div>
 139
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