Reid Kleckner | 22884dd | 2009-09-21 02:34:59 +0000 | [diff] [blame] | 1 | <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" |
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| 4 | <head> |
| 5 | <title>Debugging JITed Code With GDB</title> |
| 6 | <link rel="stylesheet" href="llvm.css" type="text/css"> |
| 7 | </head> |
| 8 | <body> |
| 9 | |
| 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 <stdio.h> |
| 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<llvm::GenericValue, |
| 146 | std::allocator<llvm::GenericValue> > const&) () |
| 147 | #5 0x00000000007d6d98 in |
| 148 | llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, |
| 149 | std::vector<std::string, |
| 150 | std::allocator<std::string> > 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 | <!-- *********************************************************************** --> |
| 160 | <hr> |
| 161 | <address> |
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| 166 | <a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a><br> |
| 167 | <a href="http://llvm.org">The LLVM Compiler Infrastructure</a><br> |
| 168 | Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $ |
| 169 | </address> |
| 170 | </body> |
| 171 | </html> |