[llvm] 332e220 - [docs] Update DebuggingJITedCode page after fix in LLDB
Stefan Gränitz via llvm-commits
llvm-commits at lists.llvm.org
Thu Jan 14 06:45:29 PST 2021
Author: Stefan Gränitz
Date: 2021-01-14T15:43:11+01:00
New Revision: 332e220ef42b92df4e080cfa5b91c9accbe9731b
URL: https://github.com/llvm/llvm-project/commit/332e220ef42b92df4e080cfa5b91c9accbe9731b
DIFF: https://github.com/llvm/llvm-project/commit/332e220ef42b92df4e080cfa5b91c9accbe9731b.diff
LOG: [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
Added:
Modified:
llvm/docs/DebuggingJITedCode.rst
Removed:
################################################################################
diff --git a/llvm/docs/DebuggingJITedCode.rst b/llvm/docs/DebuggingJITedCode.rst
index e4885e42fa10..e1583648978a 100644
--- a/llvm/docs/DebuggingJITedCode.rst
+++ b/llvm/docs/DebuggingJITedCode.rst
@@ -1,35 +1,48 @@
-==============================
-Debugging JIT-ed Code With GDB
-==============================
+=====================
+Debugging JIT-ed Code
+=====================
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.
+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 may be a
-better option for debugging JIT-ed code on macOS.
+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
@@ -39,8 +52,8 @@ 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.
+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
-------
@@ -81,63 +94,89 @@ easier to follow):
22 }
Here is a sample command line session that shows how to build and run this
-code via ``lli`` inside GDB:
+code via ``lli`` inside LLDB:
.. 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)
+ > 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
More information about the llvm-commits
mailing list