144 lines
4.9 KiB
ReStructuredText
144 lines
4.9 KiB
ReStructuredText
==============================
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Debugging JIT-ed Code With GDB
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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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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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Debugging MCJIT-ed code
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=======================
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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 ``-use-mcjit`` flag to JIT the code with
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MCJIT instead of the old JIT.
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Example
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-------
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Consider the following C code (with line numbers added to make the example
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easier to follow):
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..
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FIXME:
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Sphinx has the ability to automatically number these lines by adding
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:linenos: on the line immediately following the `.. code-block:: c`, but
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it looks like garbage; the line numbers don't even line up with the
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lines. Is this a Sphinx bug, or is it a CSS problem?
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.. code-block:: c
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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
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6 int f = n;
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7 while (--n > 1)
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8 f *= n;
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9 return f;
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10 }
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11
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12
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13 int main(int argc, char** argv)
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14 {
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15 if (argc < 2)
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16 return -1;
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17 char firstletter = argv[1][0];
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18 int result = compute_factorial(firstletter - '0');
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19
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20 // Returned result is clipped at 255...
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21 return result;
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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-block:: bash
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$ $BINPATH/clang -cc1 -O0 -g -emit-llvm showdebug.c
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$ gdb --quiet --args $BINPATH/lli -use-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 -use-mcjit showdebug.ll 5
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[Thread debugging using libthread_db enabled]
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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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