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diff --git a/docs/main/DebuggingJITedCode.html b/docs/main/DebuggingJITedCode.html new file mode 100644 index 0000000..92570f4 --- /dev/null +++ b/docs/main/DebuggingJITedCode.html @@ -0,0 +1,171 @@ +<!DOCTYPE HTML PUBLIC "-//W3C//DTD HTML 4.01//EN" + "http://www.w3.org/TR/html4/strict.dtd"> +<html> +<head> + <title>Debugging JITed Code With GDB</title> + <link rel="stylesheet" href="llvm.css" type="text/css"> +</head> +<body> + +<div class="doc_title">Debugging JITed Code With GDB</div> +<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> +</ol> +<div class="doc_author">Written by Reid Kleckner</div> + +<!--=========================================================================--> +<div class="doc_section"><a name="introduction">Introduction</a></div> +<!--=========================================================================--> +<div class="doc_text"> + +<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-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 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> + +<!--=========================================================================--> +<div class="doc_section"><a name="quickstart">Quickstart</a></div> +<!--=========================================================================--> +<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> + +<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> +<!--=========================================================================--> +<div class="doc_text"> + +<p>For example, consider debugging running lli on the following C code in +foo.c: +</p> + +<pre class="doc_code"> +#include <stdio.h> + +void foo() { + printf("%d\n", *(int*)NULL); // Crash here +} + +void bar() { + foo(); +} + +void baz() { + bar(); +} + +int main(int argc, char **argv) { + baz(); +} +</pre> + +<p>Here are the commands to run that application under GDB and print the stack +trace at the crash: +</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 +[Thread debugging using libthread_db enabled] + +Program received signal SIGSEGV, Segmentation fault. +0x00007ffff7f55164 in foo () + +# Print the backtrace, this time with symbols instead of ??. +(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 () +</pre> +</div> + +<p>As you can see, GDB can correctly unwind the stack and has the appropriate +function names. +</p> + +<!-- *********************************************************************** --> +<hr> +<address> + <a href="http://jigsaw.w3.org/css-validator/check/referer"><img + 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="http://llvm.org">The LLVM Compiler Infrastructure</a><br> + Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $ +</address> +</body> +</html> |