[docs] Update DebuggingJITedCode page after fix in LLDB
Generalize the documentation to include both, GDB and LLDB. Add a link to the interface definition. Make a note on MCJIT's restriction to ELF. Mention the regression and bugfix in LLDB as well as the jit-loader setting for macOS. Update the command line session to use LLDB instead of GDB. Reviewed By: lhames Differential Revision: https://reviews.llvm.org/D90789
This commit is contained in:
Stefan Gränitz
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@ -1,35 +1,48 @@
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==============================
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Debugging JIT-ed Code With GDB
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==============================
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=====================
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Debugging JIT-ed Code
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=====================
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Background
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==========
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Without special runtime support, debugging dynamically generated code with
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GDB (as well as most debuggers) can be quite painful. Debuggers generally
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read debug information from the object file of the code, but for JITed
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code, there is no such file to look for.
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Without special runtime support, debugging dynamically generated code can be
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quite painful. Debuggers generally read debug information from object files on
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disk, but for JITed code there is no such file to look for.
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In order to hand over the necessary debug info, `GDB established an
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interface <https://sourceware.org/gdb/current/onlinedocs/gdb/JIT-Interface.html>`_
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for registering JITed code with debuggers. LLDB implements it in the
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JITLoaderGDB plugin. On the JIT side, LLVM MCJIT does implement the interface
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for ELF object files.
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At a high level, whenever MCJIT generates new machine code, it does so in an
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in-memory object file that contains the debug information in DWARF format.
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MCJIT then adds this in-memory object file to a global list of dynamically
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generated object files and calls a special function
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``__jit_debug_register_code`` that the debugger knows about. When the debugger
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attaches to a process, it puts a breakpoint in this function and associates a
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special handler with it. Once MCJIT calls the registration function, the
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debugger catches the breakpoint signal, loads the new object file from the
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inferior's memory and resumes execution. This way it can obtain debug
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information for pure in-memory object files.
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In order to communicate the necessary debug info to GDB, an interface for
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registering JITed code with debuggers has been designed and implemented for
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GDB and LLVM MCJIT. At a high level, whenever MCJIT generates new machine code,
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it does so in an in-memory object file that contains the debug information in
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DWARF format. MCJIT then adds this in-memory object file to a global list of
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dynamically generated object files and calls a special function
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(``__jit_debug_register_code``) marked noinline that GDB knows about. When
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GDB attaches to a process, it puts a breakpoint in this function and loads all
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of the object files in the global list. When MCJIT calls the registration
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function, GDB catches the breakpoint signal, loads the new object file from
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the inferior's memory, and resumes the execution. In this way, GDB can get the
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necessary debug information.
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GDB Version
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===========
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In order to debug code JIT-ed by LLVM, you need GDB 7.0 or newer, which is
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available on most modern distributions of Linux. The version of GDB that
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Apple ships with Xcode has been frozen at 6.3 for a while. LLDB may be a
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better option for debugging JIT-ed code on macOS.
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Apple ships with Xcode has been frozen at 6.3 for a while.
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LLDB Version
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============
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Due to a regression in release 6.0, LLDB didn't support JITed code debugging for
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a while. The bug was fixed in mainline recently, so that debugging JITed ELF
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objects should be possible again from the upcoming release 12.0 on. On macOS the
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feature must be enabled explicitly using the ``plugin.jit-loader.gdb.enable``
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setting.
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Debugging MCJIT-ed code
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@ -39,8 +52,8 @@ The emerging MCJIT component of LLVM allows full debugging of JIT-ed code with
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GDB. This is due to MCJIT's ability to use the MC emitter to provide full
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DWARF debugging information to GDB.
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Note that lli has to be passed the ``-jit-kind=mcjit`` flag to JIT the code with
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MCJIT instead of the old JIT.
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Note that lli has to be passed the ``--jit-kind=mcjit`` flag to JIT the code
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with MCJIT instead of the newer ORC JIT.
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Example
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-------
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@ -81,63 +94,89 @@ easier to follow):
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22 }
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Here is a sample command line session that shows how to build and run this
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code via ``lli`` inside GDB:
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code via ``lli`` inside LLDB:
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.. code-block:: bash
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$ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
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$ gdb --quiet --args $BINPATH/lli -jit-kind=mcjit showdebug.ll 5
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Reading symbols from $BINPATH/lli...done.
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(gdb) b showdebug.c:6
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No source file named showdebug.c.
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Make breakpoint pending on future shared library load? (y or [n]) y
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Breakpoint 1 (showdebug.c:6) pending.
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(gdb) r
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Starting program: $BINPATH/lli -jit-kind=mcjit showdebug.ll 5
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[Thread debugging using libthread_db enabled]
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> export BINPATH=/workspaces/llvm-project/build/bin
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> $BINPATH/clang -g -S -emit-llvm --target=x86_64-unknown-unknown-elf showdebug.c
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> lldb $BINPATH/lli
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(lldb) target create "/workspaces/llvm-project/build/bin/lli"
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Current executable set to '/workspaces/llvm-project/build/bin/lli' (x86_64).
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(lldb) settings set plugin.jit-loader.gdb.enable on
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(lldb) b compute_factorial
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Breakpoint 1: no locations (pending).
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WARNING: Unable to resolve breakpoint to any actual locations.
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(lldb) run --jit-kind=mcjit showdebug.ll 5
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1 location added to breakpoint 1
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Process 21340 stopped
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* thread #1, name = 'lli', stop reason = breakpoint 1.1
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frame #0: 0x00007ffff7fd0007 JIT(0x45c2cb0)`compute_factorial(n=5) at showdebug.c:3:11
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1 int compute_factorial(int n)
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2 {
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-> 3 if (n <= 1)
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4 return 1;
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5 int f = n;
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6 while (--n > 1)
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7 f *= n;
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(lldb) p n
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(int) $0 = 5
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(lldb) b showdebug.c:9
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Breakpoint 2: where = JIT(0x45c2cb0)`compute_factorial + 60 at showdebug.c:9:1, address = 0x00007ffff7fd003c
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(lldb) c
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Process 21340 resuming
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Process 21340 stopped
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* thread #1, name = 'lli', stop reason = breakpoint 2.1
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frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
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6 while (--n > 1)
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7 f *= n;
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8 return f;
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-> 9 }
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10
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11 int main(int argc, char** argv)
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12 {
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(lldb) p f
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(int) $1 = 120
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(lldb) bt
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* thread #1, name = 'lli', stop reason = breakpoint 2.1
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* frame #0: 0x00007ffff7fd003c JIT(0x45c2cb0)`compute_factorial(n=1) at showdebug.c:9:1
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frame #1: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:18
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frame #2: 0x0000000002a8306e lli`llvm::MCJIT::runFunction(this=0x000000000458ed10, F=0x0000000004589ff8, ArgValues=ArrayRef<llvm::GenericValue> @ 0x00007fffffffc798) at MCJIT.cpp:554:31
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frame #3: 0x00000000029bdb45 lli`llvm::ExecutionEngine::runFunctionAsMain(this=0x000000000458ed10, Fn=0x0000000004589ff8, argv=size=0, envp=0x00007fffffffe140) at ExecutionEngine.cpp:467:10
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frame #4: 0x0000000001f2fc2f lli`main(argc=4, argv=0x00007fffffffe118, envp=0x00007fffffffe140) at lli.cpp:643:18
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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
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frame #6: 0x0000000001f2dc7a lli`_start + 42
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(lldb) finish
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Process 21340 stopped
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* thread #1, name = 'lli', stop reason = step out
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Return value: (int) $2 = 120
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Breakpoint 1, compute_factorial (n=5) at showdebug.c:6
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6 int f = n;
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(gdb) p n
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$1 = 5
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(gdb) p f
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$2 = 0
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(gdb) n
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7 while (--n > 1)
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(gdb) p f
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$3 = 5
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(gdb) b showdebug.c:9
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Breakpoint 2 at 0x7ffff7ed404c: file showdebug.c, line 9.
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(gdb) c
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Continuing.
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Breakpoint 2, compute_factorial (n=1) at showdebug.c:9
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9 return f;
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(gdb) p f
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$4 = 120
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(gdb) bt
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#0 compute_factorial (n=1) at showdebug.c:9
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#1 0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
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#2 0x3500000001652748 in ?? ()
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#3 0x00000000016677e0 in ?? ()
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#4 0x0000000000000002 in ?? ()
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#5 0x0000000000d953b3 in llvm::MCJIT::runFunction (this=0x16151f0, F=0x1603020, ArgValues=...) at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/MCJIT/MCJIT.cpp:161
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#6 0x0000000000dc8872 in llvm::ExecutionEngine::runFunctionAsMain (this=0x16151f0, Fn=0x1603020, argv=..., envp=0x7fffffffe040)
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at /home/ebenders_test/llvm_svn_rw/lib/ExecutionEngine/ExecutionEngine.cpp:397
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#7 0x000000000059c583 in main (argc=4, argv=0x7fffffffe018, envp=0x7fffffffe040) at /home/ebenders_test/llvm_svn_rw/tools/lli/lli.cpp:324
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(gdb) finish
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Run till exit from #0 compute_factorial (n=1) at showdebug.c:9
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0x00007ffff7ed40a9 in main (argc=2, argv=0x16677e0) at showdebug.c:18
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18 int result = compute_factorial(firstletter - '0');
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Value returned is $5 = 120
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(gdb) p result
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$6 = 23406408
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(gdb) n
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21 return result;
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(gdb) p result
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$7 = 120
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(gdb) c
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Continuing.
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Program exited with code 0170.
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(gdb)
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frame #0: 0x00007ffff7fd0095 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:16:9
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13 if (argc < 2)
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14 return -1;
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15 char firstletter = argv[1][0];
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-> 16 int result = compute_factorial(firstletter - '0');
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17
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18 // Returned result is clipped at 255...
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19 return result;
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(lldb) p result
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(int) $3 = 73670648
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(lldb) n
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Process 21340 stopped
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* thread #1, name = 'lli', stop reason = step over
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frame #0: 0x00007ffff7fd0098 JIT(0x45c2cb0)`main(argc=2, argv=0x00000000046122f0) at showdebug.c:19:12
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16 int result = compute_factorial(firstletter - '0');
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17
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18 // Returned result is clipped at 255...
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-> 19 return result;
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20 }
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(lldb) p result
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(int) $4 = 120
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(lldb) expr result=42
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(int) $5 = 42
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(lldb) p result
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(int) $6 = 42
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(lldb) c
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Process 21340 resuming
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Process 21340 exited with status = 42 (0x0000002a)
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(lldb) exit
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