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-<div class="doc_title">Debugging JITed Code With GDB</div>
+<h1>Debugging JIT-ed Code With GDB</h1>
<ol>
- <li><a href="#introduction">Introduction</a></li>
- <li><a href="#quickstart">Quickstart</a></li>
- <li><a href="#example">Example with clang and lli</a></li>
+ <li><a href="#background">Background</a></li>
+ <li><a href="#gdbversion">GDB Version</a></li>
+ <li><a href="#mcjitdebug">Debugging MCJIT-ed code</a></li>
+ <ul>
+ <li><a href="#mcjitdebug_example">Example</a></li>
+ </ul>
</ol>
-<div class="doc_author">Written by Reid Kleckner</div>
+<div class="doc_author">Written by Reid Kleckner and Eli Bendersky</div>
<!--=========================================================================-->
-<div class="doc_section"><a name="introduction">Introduction</a></div>
+<h2><a name="background">Background</a></h2>
<!--=========================================================================-->
-<div class="doc_text">
+<div>
<p>Without special runtime support, debugging dynamically generated code with
GDB (as well as most debuggers) can be quite painful. Debuggers generally read
no such file to look for.
</p>
-<p>Depending on the architecture, this can impact the debugging experience in
-different ways. For example, on most 32-bit x86 architectures, you can simply
-compile with -fno-omit-framepointer for GCC and -fdisable-fp-elim for LLVM.
-When GDB creates a backtrace, it can properly unwind the stack, but the stack
-frames owned by JITed code have ??'s instead of the appropriate symbol name.
-However, on Linux x86_64 in particular, GDB relies on the DWARF CFA debug
-information to unwind the stack, so even if you compile your program to leave
-the frame pointer untouched, GDB will usually be unable to unwind the stack past
-any JITed code stack frames.
-</p>
-
<p>In order to communicate the necessary debug info to GDB, an interface for
registering JITed code with debuggers has been designed and implemented for
-GDB and LLVM. At a high level, whenever LLVM generates new machine code, it
-also generates an object file in memory containing the debug information. LLVM
-then adds the object file to the global list of object files and calls a special
-function (__jit_debug_register_code) marked noinline that GDB knows about. When
+GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code,
+it does so in an in-memory object file that contains the debug information in
+DWARF format. MCJIT then adds this in-memory object file to a global list of
+dynamically generated object files and calls a special function
+(<tt>__jit_debug_register_code</tt>) marked noinline that GDB knows about. When
GDB attaches to a process, it puts a breakpoint in this function and loads all
-of the object files in the global list. When LLVM calls the registration
+of the object files in the global list. When MCJIT calls the registration
function, GDB catches the breakpoint signal, loads the new object file from
-LLVM's memory, and resumes the execution. In this way, GDB can get the
+the inferior's memory, and resumes the execution. In this way, GDB can get the
necessary debug information.
</p>
-
-<p>At the time of this writing, LLVM only supports architectures that use ELF
-object files and it only generates symbols and DWARF CFA information. However,
-it would be easy to add more information to the object file, so we don't need to
-coordinate with GDB to get better debug information.
-</p>
</div>
<!--=========================================================================-->
-<div class="doc_section"><a name="quickstart">Quickstart</a></div>
+<h2><a name="gdbversion">GDB Version</a></h2>
<!--=========================================================================-->
-<div class="doc_text">
-<p>In order to debug code JITed by LLVM, you need to install a recent version
-of GDB. The interface was added on 2009-08-19, so you need a snapshot of GDB
-more recent than that. Either download a snapshot of GDB or checkout CVS as
-instructed <a href="http://www.gnu.org/software/gdb/current/">here</a>. Here
-are the commands for doing a checkout and building the code:
+<p>In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is
+available on most modern distributions of Linux. The version of GDB that Apple
+ships with Xcode has been frozen at 6.3 for a while. LLDB may be a better
+option for debugging JIT-ed code on Mac OS X.
</p>
-<pre class="doc_code">
-$ cvs -z 3 -d :pserver:anoncvs@sourceware.org:/cvs/src co gdb
-$ mv src gdb # You probably don't want this checkout called "src".
-$ cd gdb
-$ ./configure --prefix="$GDB_INSTALL"
-$ make
-$ make install
-</pre>
-
-<p>You can then use -jit-emit-debug in the LLVM command line arguments to enable
-the interface.
-</p>
-</div>
<!--=========================================================================-->
-<div class="doc_section"><a name="example">Example with clang and lli</a></div>
+<h2><a name="mcjitdebug">Debugging MCJIT-ed code</a></h2>
<!--=========================================================================-->
-<div class="doc_text">
+<div>
-<p>For example, consider debugging running lli on the following C code in
-foo.c:
-</p>
+<p>The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with
+GDB. This is due to MCJIT's ability to use the MC emitter to provide full
+DWARF debugging information to GDB.</p>
-<pre class="doc_code">
-#include <stdio.h>
+<p>Note that lli has to be passed the <tt>-use-mcjit</tt> flag to JIT the code
+with MCJIT instead of the old JIT.</p>
-void foo() {
- printf("%d\n", *(int*)NULL); // Crash here
-}
+<h3><a name="mcjitdebug_example">Example</a></h3>
-void bar() {
- foo();
-}
+<div>
-void baz() {
- bar();
-}
+<p>Consider the following C code (with line numbers added to make the example
+easier to follow):</p>
-int main(int argc, char **argv) {
- baz();
-}
+<pre class="doc_code">
+1 int compute_factorial(int n)
+2 {
+3 if (n <= 1)
+4 return 1;
+5
+6 int f = n;
+7 while (--n > 1)
+8 f *= n;
+9 return f;
+10 }
+11
+12
+13 int main(int argc, char** argv)
+14 {
+15 if (argc < 2)
+16 return -1;
+17 char firstletter = argv[1][0];
+18 int result = compute_factorial(firstletter - '0');
+19
+20 // Returned result is clipped at 255...
+21 return result;
+22 }
</pre>
-<p>Here are the commands to run that application under GDB and print the stack
-trace at the crash:
+<p>Here is a sample command line session that shows how to build and run this
+code via lli inside GDB:
</p>
<pre class="doc_code">
-# Compile foo.c to bitcode. You can use either clang or llvm-gcc with this
-# command line. Both require -fexceptions, or the calls are all marked
-# 'nounwind' which disables DWARF CFA info.
-$ clang foo.c -fexceptions -emit-llvm -c -o foo.bc
-
-# Run foo.bc under lli with -jit-emit-debug. If you built lli in debug mode,
-# -jit-emit-debug defaults to true.
-$ $GDB_INSTALL/gdb --args lli -jit-emit-debug foo.bc
-...
-
-# Run the code.
-(gdb) run
-Starting program: /tmp/gdb/lli -jit-emit-debug foo.bc
+$ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
+$ gdb --quiet --args $BINPATH/lli -use-mcjit showdebug.ll 5
+Reading symbols from $BINPATH/lli...done.
+(gdb) b showdebug.c:6
+No source file named showdebug.c.
+Make breakpoint pending on future shared library load? (y or [n]) y
+Breakpoint 1 (showdebug.c:6) pending.
+(gdb) r
+Starting program: $BINPATH/lli -use-mcjit showdebug.ll 5
[Thread debugging using libthread_db enabled]
-Program received signal SIGSEGV, Segmentation fault.
-0x00007ffff7f55164 in foo ()
-
-# Print the backtrace, this time with symbols instead of ??.
+Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
+6 int f = n;
+(gdb) p n
+$1 = 5
+(gdb) p f
+$2 = 0
+(gdb) n
+7 while (--n > 1)
+(gdb) p f
+$3 = 5
+(gdb) b showdebug.c:9
+Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
+(gdb) c
+Continuing.
+
+Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
+9 return f;
+(gdb) p f
+$4 = 120
(gdb) bt
-#0 0x00007ffff7f55164 in foo ()
-#1 0x00007ffff7f550f9 in bar ()
-#2 0x00007ffff7f55099 in baz ()
-#3 0x00007ffff7f5502a in main ()
-#4 0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*,
- std::vector<llvm::GenericValue,
- std::allocator<llvm::GenericValue> > const&) ()
-#5 0x00000000007d6d98 in
- llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*,
- std::vector<std::string,
- std::allocator<std::string> > const&, char const* const*) ()
-#6 0x00000000004dab76 in main ()
+#0 compute_factorial (n=1) at showdebug.c:9
+#1 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
+#2 0x3500000001652748 in ?? ()
+#3 0x00000000016677e0 in ?? ()
+#4 0x0000000000000002 in ?? ()
+#5 0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
+#6 0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
+ at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
+#7 0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
+(gdb) finish
+Run till exit from #0 compute_factorial (n=1) at showdebug.c:9
+0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
+18 int result = compute_factorial(firstletter - '0');
+Value returned is $5 = 120
+(gdb) p result
+$6 = 23406408
+(gdb) n
+21 return result;
+(gdb) p result
+$7 = 120
+(gdb) c
+Continuing.
+
+Program exited with code 0170.
+(gdb)
+
</pre>
+
+</div>
</div>
-<p>As you can see, GDB can correctly unwind the stack and has the appropriate
-function names.
-</p>
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- <a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a><br>
- <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br>
- Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $
+ <a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a>,
+ <a href="mailto:eliben@gmail.com">Eli Bendersky</a><br>
+ <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br>
+ Last modified: $Date$
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