[llvm-commits] [llvm] r107808 - /llvm/trunk/docs/DebuggingJITedCode.html
Reid Kleckner
reid at kleckner.net
Wed Jul 7 13:16:45 PDT 2010
Author: rnk
Date: Wed Jul 7 15:16:45 2010
New Revision: 107808
URL: http://llvm.org/viewvc/llvm-project?rev=107808&view=rev
Log:
Update the docs for debugging JITed code with GDB.
Modified:
llvm/trunk/docs/DebuggingJITedCode.html (contents, props changed)
Modified: llvm/trunk/docs/DebuggingJITedCode.html
URL: http://llvm.org/viewvc/llvm-project/llvm/trunk/docs/DebuggingJITedCode.html?rev=107808&r1=107807&r2=107808&view=diff
==============================================================================
--- llvm/trunk/docs/DebuggingJITedCode.html (original)
+++ llvm/trunk/docs/DebuggingJITedCode.html Wed Jul 7 15:16:45 2010
@@ -9,87 +9,24 @@
<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>
+ <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="introduction">Introduction</a></div>
+<div class="doc_section"><a name="example">Example usage</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 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>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>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 at 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>Consider debugging the following code compiled with clang and run through
+lli:
</p>
<pre class="doc_code">
@@ -119,7 +56,9 @@
<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.
+# '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,
@@ -143,18 +82,60 @@
#3 0x00007ffff7f5502a in main ()
#4 0x00000000007c0225 in llvm::JIT::runFunction(llvm::Function*,
std::vector<llvm::GenericValue,
- std::allocator<llvm::GenericValue> > const&) ()
+ 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*) ()
+ 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>
+</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>
@@ -165,7 +146,7 @@
src="http://www.w3.org/Icons/valid-html401-blue" alt="Valid HTML 4.01"></a>
<a href="mailto:reid.kleckner at 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) $
+ Last modified: $Date$
</address>
</body>
</html>
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