Debugging JITed Code With GDB

Date: Thu, 6 May 2010 23:45:43 +0000 Subject: Overhauled llvm/clang docs builds. Closes PR6613. NOTE: 2nd part changeset for cfe trunk to follow. *** PRE-PATCH ISSUES ADDRESSED - clang api docs fail build from objdir - clang/llvm api docs collide in install PREFIX/ - clang/llvm main docs collide in install - clang/llvm main docs have full of hard coded destination assumptions and make use of absolute root in static html files; namely CommandGuide tools hard codes a website destination for cross references and some html cross references assume website root paths *** IMPROVEMENTS - bumped Doxygen from 1.4.x -> 1.6.3 - splits llvm/clang docs into 'main' and 'api' (doxygen) build trees - provide consistent, reliable doc builds for both main+api docs - support buid vs. install vs. website intentions - support objdir builds - document targets with 'make help' - correct clean and uninstall operations - use recursive dir delete only where absolutely necessary - added call function fn.RMRF which safeguards against botched 'rm -rf'; if any target (or any variable is evaluated) which attempts to remove any dirs which match a hard-coded 'safelist', a verbose error will be printed and make will error-stop. git-svn-id: https://llvm.org/svn/llvm-project/llvm/trunk@103213 91177308-0d34-0410-b5e6-96231b3b80d8 --- docs/DebuggingJITedCode.html | 171 ------------------------------------------- 1 file changed, 171 deletions(-) delete mode 100644 docs/DebuggingJITedCode.html (limited to 'docs/DebuggingJITedCode.html') diff --git a/docs/DebuggingJITedCode.html b/docs/DebuggingJITedCode.html deleted file mode 100644 index 92570f4..0000000 --- a/docs/DebuggingJITedCode.html +++ /dev/null @@ -1,171 +0,0 @@ - - - - Debugging JITed Code With GDB - - - - -

Introduction
Quickstart
Example with clang and lli

Written by Reid Kleckner

- - -

Introduction

- -

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. -

- -

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. -

- -

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. -

- -

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. -

- - -

Quickstart

- -

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 here. Here -are the commands for doing a checkout and building the 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
-

- -

You can then use -jit-emit-debug in the LLVM command line arguments to enable -the interface. -

- - -

Example with clang and lli

- -

For example, consider debugging running lli on the following C code in -foo.c: -

- -

-#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();
-}
-

- -

Here are the commands to run that application under GDB and print the stack -trace at the crash: -

- -

-# 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 ()
-

- -

As you can see, GDB can correctly unwind the stack and has the appropriate -function names. -

- - -

- Reid Kleckner
- The LLVM Compiler Infrastructure
- Last modified: $Date: 2009-01-01 23:10:51 -0800 (Thu, 01 Jan 2009) $ -

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