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diff --git a/docs/DebuggingJITedCode.html b/docs/DebuggingJITedCode.html index c318846..4e4b7b8 100644 --- a/docs/DebuggingJITedCode.html +++ b/docs/DebuggingJITedCode.html @@ -8,136 +8,166 @@ </head> <body> -<h1>Debugging JITed Code With GDB</h1> +<h1>Debugging JIT-ed Code With GDB</h1> <ol> - <li><a href="#example">Example usage</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> <!--=========================================================================--> -<h2><a name="example">Example usage</a></h2> +<h2><a name="background">Background</a></h2> <!--=========================================================================--> <div> -<p>In order to debug code JITed by LLVM, you need GDB 7.0 or newer, which is +<p>Without special runtime support, debugging dynamically generated code with +GDB (as well as most debuggers) can be quite painful. Debuggers generally read +debug information from the object file of the code, but for JITed code, there is +no such file to look for. +</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 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 MCJIT calls the registration +function, GDB catches the breakpoint signal, loads the new object file from +the inferior's memory, and resumes the execution. In this way, GDB can get the +necessary debug information. +</p> +</div> + +<!--=========================================================================--> +<h2><a name="gdbversion">GDB Version</a></h2> +<!--=========================================================================--> + +<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 JITed code on Mac OS X. +option for debugging JIT-ed code on Mac OS X. </p> -<p>Consider debugging the following code compiled with clang and run through -lli: -</p> -<pre class="doc_code"> -#include <stdio.h> +<!--=========================================================================--> +<h2><a name="mcjitdebug">Debugging MCJIT-ed code</a></h2> +<!--=========================================================================--> +<div> + +<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> -void foo() { - printf("%d\n", *(int*)NULL); // Crash here -} +<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 bar() { - foo(); -} +<h3><a name="mcjitdebug_example">Example</a></h3> -void baz() { - bar(); -} +<div> + +<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 exception handling info. Custom frontends -# should avoid adding this attribute to JITed code, since it interferes with -# DWARF CFA generation at the moment. -$ 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> -<p>As you can see, GDB can correctly unwind the stack and has the appropriate -function names. -</p> </div> - -<!--=========================================================================--> -<h2><a name="background">Background</a></h2> -<!--=========================================================================--> -<div> - -<p>Without special runtime support, debugging dynamically generated code with -GDB (as well as most debuggers) can be quite painful. Debuggers generally read -debug information from the object file of the code, but for JITed code, there is -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-frame-pointer for GCC and -disable-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 call frame -address (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 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 -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 -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> + <!-- *********************************************************************** --> <hr> <address> @@ -145,7 +175,8 @@ coordinate with GDB to get better debug information. src="http://jigsaw.w3.org/css-validator/images/vcss-blue" alt="Valid CSS"></a> <a href="http://validator.w3.org/check/referer"><img src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a> - <a href="mailto:reid.kleckner@gmail.com">Reid Kleckner</a><br> + <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: $Date$ </address> |