forked from OSchip/llvm-project
183 lines
6.9 KiB
ReStructuredText
183 lines
6.9 KiB
ReStructuredText
=====================
|
|
Debugging JIT-ed Code
|
|
=====================
|
|
|
|
Background
|
|
==========
|
|
|
|
Without special runtime support, debugging dynamically generated code can be
|
|
quite painful. Debuggers generally read debug information from object files on
|
|
disk, but for JITed code there is no such file to look for.
|
|
|
|
In order to hand over the necessary debug info, `GDB established an
|
|
interface <https://sourceware.org/gdb/current/onlinedocs/gdb/JIT-Interface.html>`_
|
|
for registering JITed code with debuggers. LLDB implements it in the
|
|
JITLoaderGDB plugin. On the JIT side, LLVM MCJIT does implement the interface
|
|
for ELF object files.
|
|
|
|
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`` that the debugger knows about. When the debugger
|
|
attaches to a process, it puts a breakpoint in this function and associates a
|
|
special handler with it. Once MCJIT calls the registration function, the
|
|
debugger catches the breakpoint signal, loads the new object file from the
|
|
inferior's memory and resumes execution. This way it can obtain debug
|
|
information for pure in-memory object files.
|
|
|
|
|
|
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 Version
|
|
============
|
|
|
|
Due to a regression in release 6.0, LLDB didn't support JITed code debugging for
|
|
a while. The bug was fixed in mainline recently, so that debugging JITed ELF
|
|
objects should be possible again from the upcoming release 12.0 on. On macOS the
|
|
feature must be enabled explicitly using the ``plugin.jit-loader.gdb.enable``
|
|
setting.
|
|
|
|
|
|
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 newer ORC 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 LLDB:
|
|
|
|
.. code-block:: bash
|
|
|
|
> export BINPATH=/workspaces/llvm-project/build/bin
|
|
> $BINPATH/clang -g -S -emit-llvm --target=x86_64-unknown-unknown-elf showdebug.c
|
|
> lldb $BINPATH/lli
|
|
(lldb) target create "/workspaces/llvm-project/build/bin/lli"
|
|
Current executable set to '/workspaces/llvm-project/build/bin/lli' (x86_64).
|
|
(lldb) settings set plugin.jit-loader.gdb.enable on
|
|
(lldb) b compute_factorial
|
|
Breakpoint 1: no locations (pending).
|
|
WARNING: Unable to resolve breakpoint to any actual locations.
|
|
(lldb) run --jit-kind=mcjit showdebug.ll 5
|
|
1 location added to breakpoint 1
|
|
Process 21340 stopped
|
|
* thread #1, name = 'lli', stop reason = breakpoint 1.1
|
|
frame #0: 0x00007ffff7fd0007 JIT(0x45c2cb0)`compute_factorial(n=5) at showdebug.c:3:11
|
|
1 int compute_factorial(int n)
|
|
2 {
|
|
-> 3 if (n <= 1)
|
|
4 return 1;
|
|
5 int f = n;
|
|
6 while (--n > 1)
|
|
7 f *= n;
|
|
(lldb) p n
|
|
(int) $0 = 5
|
|
(lldb) b showdebug.c:9
|
|
Breakpoint 2: where = JIT(0x45c2cb0)`compute_factorial + 60 at showdebug.c:9:1, address = 0x00007ffff7fd003c
|
|
(lldb) c
|
|
Process 21340 resuming
|
|
Process 21340 stopped
|
|
* thread #1, name = 'lli', stop reason = breakpoint 2.1
|
|
frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
|
|
6 while (--n > 1)
|
|
7 f *= n;
|
|
8 return f;
|
|
-> 9 }
|
|
10
|
|
11 int main(int argc, char** argv)
|
|
12 {
|
|
(lldb) p f
|
|
(int) $1 = 120
|
|
(lldb) bt
|
|
* thread #1, name = 'lli', stop reason = breakpoint 2.1
|
|
* frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
|
|
frame #1: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:18
|
|
frame #2: 0x0000000002a8306e lli`llvm::MCJIT::runFunction(this=0x000000000458ed10, F=0x0000000004589ff8, ArgValues=ArrayRef<llvm::GenericValue> @ 0x00007fffffffc798) at MCJIT.cpp:554:31
|
|
frame #3: 0x00000000029bdb45 lli`llvm::ExecutionEngine::runFunctionAsMain(this=0x000000000458ed10, Fn=0x0000000004589ff8, argv=size=0, envp=0x00007fffffffe140) at ExecutionEngine.cpp:467:10
|
|
frame #4: 0x0000000001f2fc2f lli`main(argc=4, argv=0x00007fffffffe118, envp=0x00007fffffffe140) at lli.cpp:643:18
|
|
frame #5: 0x00007ffff788c09b libc.so.6`__libc_start_main(main=(lli`main at lli.cpp:387), argc=4, argv=0x00007fffffffe118, init=<unavailable>, fini=<unavailable>, rtld_fini=<unavailable>, stack_end=0x00007fffffffe108) at libc-start.c:308:16
|
|
frame #6: 0x0000000001f2dc7a lli`_start + 42
|
|
(lldb) finish
|
|
Process 21340 stopped
|
|
* thread #1, name = 'lli', stop reason = step out
|
|
Return value: (int) $2 = 120
|
|
|
|
frame #0: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:9
|
|
13 if (argc < 2)
|
|
14 return -1;
|
|
15 char firstletter = argv[1][0];
|
|
-> 16 int result = compute_factorial(firstletter - '0');
|
|
17
|
|
18 // Returned result is clipped at 255...
|
|
19 return result;
|
|
(lldb) p result
|
|
(int) $3 = 73670648
|
|
(lldb) n
|
|
Process 21340 stopped
|
|
* thread #1, name = 'lli', stop reason = step over
|
|
frame #0: 0x00007ffff7fd0098 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:19:12
|
|
16 int result = compute_factorial(firstletter - '0');
|
|
17
|
|
18 // Returned result is clipped at 255...
|
|
-> 19 return result;
|
|
20 }
|
|
(lldb) p result
|
|
(int) $4 = 120
|
|
(lldb) expr result=42
|
|
(int) $5 = 42
|
|
(lldb) p result
|
|
(int) $6 = 42
|
|
(lldb) c
|
|
Process 21340 resuming
|
|
Process 21340 exited with status = 42 (0x0000002a)
|
|
(lldb) exit
|