1 <!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" 2 "http://www.w3.org/TR/html4/strict.dtd"> 3 <html> 4 <head> 5 <meta http-equiv="Content-Type" content="text/html; charset=utf-8"> 6 <title>Debugging JITed Code With GDB</title> 7 <link rel="stylesheet" href="llvm.css" type="text/css"> 8 </head> 9 <body> 10 11 <h1>Debugging JITed Code With GDB</h1> 12 <ol> 13 <li><a href="#example">Example usage</a></li> 14 <li><a href="#background">Background</a></li> 15 </ol> 16 <div class="doc_author">Written by Reid Kleckner</div> 17 18 <!--=========================================================================--> 19 <h2><a name="example">Example usage</a></h2> 20 <!--=========================================================================--> 21 <div> 22 23 <p>In order to debug code JITed by LLVM, you need GDB 7.0 or newer, which is 24 available on most modern distributions of Linux. The version of GDB that Apple 25 ships with XCode has been frozen at 6.3 for a while. LLDB may be a better 26 option for debugging JITed code on Mac OS X. 27 </p> 28 29 <p>Consider debugging the following code compiled with clang and run through 30 lli: 31 </p> 32 33 <pre class="doc_code"> 34 #include <stdio.h> 35 36 void foo() { 37 printf("%d\n", *(int*)NULL); // Crash here 38 } 39 40 void bar() { 41 foo(); 42 } 43 44 void baz() { 45 bar(); 46 } 47 48 int main(int argc, char **argv) { 49 baz(); 50 } 51 </pre> 52 53 <p>Here are the commands to run that application under GDB and print the stack 54 trace at the crash: 55 </p> 56 57 <pre class="doc_code"> 58 # Compile foo.c to bitcode. You can use either clang or llvm-gcc with this 59 # command line. Both require -fexceptions, or the calls are all marked 60 # 'nounwind' which disables DWARF exception handling info. Custom frontends 61 # should avoid adding this attribute to JITed code, since it interferes with 62 # DWARF CFA generation at the moment. 63 $ clang foo.c -fexceptions -emit-llvm -c -o foo.bc 64 65 # Run foo.bc under lli with -jit-emit-debug. If you built lli in debug mode, 66 # -jit-emit-debug defaults to true. 67 $ $GDB_INSTALL/gdb --args lli -jit-emit-debug foo.bc 68 ... 69 70 # Run the code. 71 (gdb) run 72 Starting program: /tmp/gdb/lli -jit-emit-debug foo.bc 73 [Thread debugging using libthread_db enabled] 74 75 Program received signal SIGSEGV, Segmentation fault. 76 0x00007ffff7f55164 in foo () 77 78 # Print the backtrace, this time with symbols instead of ??. 79 (gdb) bt 80 #0 0x00007ffff7f55164 in foo () 81 #1 0x00007ffff7f550f9 in bar () 82 #2 0x00007ffff7f55099 in baz () 83 #3 0x00007ffff7f5502a in main () 84 #4 0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*, 85 std::vector<llvm::GenericValue, 86 std::allocator<llvm::GenericValue> > const&) () 87 #5 0x00000000007d6d98 in 88 llvm::ExecutionEngine::runFunctionAsMain(llvm::Function*, 89 std::vector<std::string, 90 std::allocator<std::string> > const&, char const* const*) () 91 #6 0x00000000004dab76 in main () 92 </pre> 93 94 <p>As you can see, GDB can correctly unwind the stack and has the appropriate 95 function names. 96 </p> 97 </div> 98 99 <!--=========================================================================--> 100 <h2><a name="background">Background</a></h2> 101 <!--=========================================================================--> 102 <div> 103 104 <p>Without special runtime support, debugging dynamically generated code with 105 GDB (as well as most debuggers) can be quite painful. Debuggers generally read 106 debug information from the object file of the code, but for JITed code, there is 107 no such file to look for. 108 </p> 109 110 <p>Depending on the architecture, this can impact the debugging experience in 111 different ways. For example, on most 32-bit x86 architectures, you can simply 112 compile with -fno-omit-frame-pointer for GCC and -disable-fp-elim for LLVM. 113 When GDB creates a backtrace, it can properly unwind the stack, but the stack 114 frames owned by JITed code have ??'s instead of the appropriate symbol name. 115 However, on Linux x86_64 in particular, GDB relies on the DWARF call frame 116 address (CFA) debug information to unwind the stack, so even if you compile 117 your program to leave the frame pointer untouched, GDB will usually be unable 118 to unwind the stack past any JITed code stack frames. 119 </p> 120 121 <p>In order to communicate the necessary debug info to GDB, an interface for 122 registering JITed code with debuggers has been designed and implemented for 123 GDB and LLVM. At a high level, whenever LLVM generates new machine code, it 124 also generates an object file in memory containing the debug information. LLVM 125 then adds the object file to the global list of object files and calls a special 126 function (__jit_debug_register_code) marked noinline that GDB knows about. When 127 GDB attaches to a process, it puts a breakpoint in this function and loads all 128 of the object files in the global list. When LLVM calls the registration 129 function, GDB catches the breakpoint signal, loads the new object file from 130 LLVM's memory, and resumes the execution. In this way, GDB can get the 131 necessary debug information. 132 </p> 133 134 <p>At the time of this writing, LLVM only supports architectures that use ELF 135 object files and it only generates symbols and DWARF CFA information. However, 136 it would be easy to add more information to the object file, so we don't need to 137 coordinate with GDB to get better debug information. 138 </p> 139 </div> 140 141 <!-- *********************************************************************** --> 142 <hr> 143 <address> 144 <a href="http://jigsaw.w3.org/css-validator/check/referer"><img 145 src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> 146 <a href="http://validator.w3.org/check/referer"><img 147 src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> 148 <a href="mailto:reid.kleckner (a] gmail.com">Reid Kleckner</a><br> 149 <a href="http://llvm.org/">The LLVM Compiler Infrastructure</a><br> 150 Last modified: $Date$ 151 </address> 152 </body> 153 </html> 154
