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<title>Debugging JITed Code With GDB</title>
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<link rel="stylesheet" href="llvm.css" type="text/css">
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</head>
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<body>
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<div class="doc_title">Debugging JITed Code With GDB</div>
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<ol>
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<li><a href="#introduction">Introduction</a></li>
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<li><a href="#quickstart">Quickstart</a></li>
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<li><a href="#example">Example with clang and lli</a></li>
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</ol>
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<div class="doc_author">Written by Reid Kleckner</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="introduction">Introduction</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>Without special runtime support, debugging dynamically generated code with
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GDB (as well as most debuggers) can be quite painful. Debuggers generally read
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debug information from the object file of the code, but for JITed code, there is
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no such file to look for.
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</p>
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<p>Depending on the architecture, this can impact the debugging experience in
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different ways. For example, on most 32-bit x86 architectures, you can simply
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compile with -fno-omit-framepointer for GCC and -fdisable-fp-elim for LLVM.
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When GDB creates a backtrace, it can properly unwind the stack, but the stack
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frames owned by JITed code have ??'s instead of the appropriate symbol name.
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However, on Linux x86_64 in particular, GDB relies on the DWARF CFA debug
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information to unwind the stack, so even if you compile your program to leave
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the frame pointer untouched, GDB will usually be unable to unwind the stack past
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any JITed code stack frames.
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</p>
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<p>In order to communicate the necessary debug info to GDB, an interface for
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registering JITed code with debuggers has been designed and implemented for
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GDB and LLVM. At a high level, whenever LLVM generates new machine code, it
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also generates an object file in memory containing the debug information. LLVM
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then adds the object file to the global list of object files and calls a special
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function (__jit_debug_register_code) marked noinline that GDB knows about. When
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GDB attaches to a process, it puts a breakpoint in this function and loads all
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of the object files in the global list. When LLVM calls the registration
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function, GDB catches the breakpoint signal, loads the new object file from
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LLVM's memory, and resumes the execution. In this way, GDB can get the
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necessary debug information.
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</p>
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<p>At the time of this writing, LLVM only supports architectures that use ELF
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object files and it only generates symbols and DWARF CFA information. However,
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it would be easy to add more information to the object file, so we don't need to
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coordinate with GDB to get better debug information.
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</p>
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</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="quickstart">Quickstart</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>In order to debug code JITed by LLVM, you need to install a recent version
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of GDB. The interface was added on 2009-08-19, so you need a snapshot of GDB
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more recent than that. Either download a snapshot of GDB or checkout CVS as
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instructed <a href="http://www.gnu.org/software/gdb/current/">here</a>. Here
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are the commands for doing a checkout and building the code:
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</p>
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<pre class="doc_code">
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$ cvs -z 3 -d :pserver:anoncvs@sourceware.org:/cvs/src co gdb
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$ mv src gdb # You probably don't want this checkout called "src".
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$ cd gdb
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$ ./configure --prefix="$GDB_INSTALL"
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$ make
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$ make install
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</pre>
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<p>You can then use -jit-emit-debug in the LLVM command line arguments to enable
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the interface.
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</p>
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</div>
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<!--=========================================================================-->
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<div class="doc_section"><a name="example">Example with clang and lli</a></div>
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<!--=========================================================================-->
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<div class="doc_text">
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<p>For example, consider debugging running lli on the following C code in
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foo.c:
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</p>
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<pre class="doc_code">
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#include <stdio.h>
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void foo() {
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printf("%d\n", *(int*)NULL); // Crash here
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}
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void bar() {
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foo();
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}
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void baz() {
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bar();
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}
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int main(int argc, char **argv) {
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baz();
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}
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</pre>
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<p>Here are the commands to run that application under GDB and print the stack
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trace at the crash:
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</p>
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<pre class="doc_code">
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# Compile foo.c to bitcode. You can use either clang or llvm-gcc with this
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# command line. Both require -fexceptions, or the calls are all marked
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# 'nounwind' which disables DWARF CFA info.
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$ clang foo.c -fexceptions -emit-llvm -c -o foo.bc
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# Run foo.bc under lli with -jit-emit-debug. If you built lli in debug mode,
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# -jit-emit-debug defaults to true.
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$ $GDB_INSTALL/gdb --args lli -jit-emit-debug foo.bc
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...
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# Run the code.
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(gdb) run
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Starting program: /tmp/gdb/lli -jit-emit-debug foo.bc
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[Thread debugging using libthread_db enabled]
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Program received signal SIGSEGV, Segmentation fault.
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0x00007ffff7f55164 in foo ()
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# Print the backtrace, this time with symbols instead of ??.
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(gdb) bt
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#0 0x00007ffff7f55164 in foo ()
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#1 0x00007ffff7f550f9 in bar ()
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#2 0x00007ffff7f55099 in baz ()
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#3 0x00007ffff7f5502a in main ()
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#4 0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*,
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std::vector<llvm::GenericValue,
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std::allocator<llvm::GenericValue> > const&) ()
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#5 0x00000000007d6d98 in
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llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
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std::vector<std::string,
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std::allocator<std::string> > const&, char const* const*) ()
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#6 0x00000000004dab76 in main ()
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</pre>
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</div>
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<p>As you can see, GDB can correctly unwind the stack and has the appropriate
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function names.
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</p>
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<!-- *********************************************************************** -->
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<hr>
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<address>
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<a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a><br>
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<a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
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Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
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</address>
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