mirror of
https://github.com/c64scene-ar/llvm-6502.git
synced 2025-10-30 16:17:05 +00:00
894728211d
git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@107808 91177308-0d34-0410-b5e6-96231b3b80d8
153 lines
5.7 KiB
HTML
153 lines
5.7 KiB
HTML
<!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="#example">Example usage</a></li>
|
|
<li><a href="#background">Background</a></li>
|
|
</ol>
|
|
<div class="doc_author">Written by Reid Kleckner</div>
|
|
|
|
<!--=========================================================================-->
|
|
<div class="doc_section"><a name="example">Example usage</a></div>
|
|
<!--=========================================================================-->
|
|
<div class="doc_text">
|
|
|
|
<p>In order to debug code JITed 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.
|
|
</p>
|
|
|
|
<p>Consider debugging the following code compiled with clang and run through
|
|
lli:
|
|
</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 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
|
|
[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>
|
|
|
|
<p>As you can see, GDB can correctly unwind the stack and has the appropriate
|
|
function names.
|
|
</p>
|
|
</div>
|
|
|
|
<!--=========================================================================-->
|
|
<div class="doc_section"><a name="background">Background</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-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>
|
|
<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: $Date$
|
|
</address>
|
|
</body>
|
|
</html>
|