mirror of
https://github.com/RPCS3/llvm-mirror.git
synced 2024-11-22 10:42:39 +01:00
bc7e42d8b9
The -use-mcjit option was replaced with -jit-kind=mcjit a while back. This patch updates the docs to reflect that. Patch by Yu Jian. Thanks Jian!
144 lines
4.9 KiB
ReStructuredText
144 lines
4.9 KiB
ReStructuredText
==============================
|
|
Debugging JIT-ed Code With GDB
|
|
==============================
|
|
|
|
Background
|
|
==========
|
|
|
|
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.
|
|
|
|
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
|
|
(``__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 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.
|
|
|
|
GDB Version
|
|
===========
|
|
|
|
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 JIT-ed code on macOS.
|
|
|
|
|
|
Debugging MCJIT-ed code
|
|
=======================
|
|
|
|
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.
|
|
|
|
Note that lli has to be passed the ``-jit-kind=mcjit`` flag to JIT the code with
|
|
MCJIT instead of the old JIT.
|
|
|
|
Example
|
|
-------
|
|
|
|
Consider the following C code (with line numbers added to make the example
|
|
easier to follow):
|
|
|
|
..
|
|
FIXME:
|
|
Sphinx has the ability to automatically number these lines by adding
|
|
:linenos: on the line immediately following the `.. code-block:: c`, but
|
|
it looks like garbage; the line numbers don't even line up with the
|
|
lines. Is this a Sphinx bug, or is it a CSS problem?
|
|
|
|
.. code-block:: c
|
|
|
|
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 }
|
|
|
|
Here is a sample command line session that shows how to build and run this
|
|
code via ``lli`` inside GDB:
|
|
|
|
.. code-block:: bash
|
|
|
|
$ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
|
|
$ gdb --quiet --args $BINPATH/lli -jit-kind=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 -jit-kind=mcjit showdebug.ll 5
|
|
[Thread debugging using libthread_db enabled]
|
|
|
|
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 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)
|