Document zend_vm_gen.php --with-lines flag (#142)

Author: iluuu1994

Book/php7/debugging.rst

@@ -11,3 +11,12 @@ Debug symbols
 GDB requires debug symbols to map the memory addresses in your binary to the original position in your source code. To
 generate debug symbols you need to pass the ``--enable-debug`` flag to the ``./configure`` script. To get even more
 debugging information you may add the ``CFLAGS="-ggdb3"`` flag which will add support for macros.
+
+Debugging the VM
+----------------
+
+The VM lives in the big ``zend_vm_execute.h`` file that is generated by ``zend_vm_gen.php`` from ``zend_vm_def.h``.
+Debugging this file can be tedious because it is very large, and it's often not obvious which specialized handler
+will run. Luckily, it is possible to debug ``zend_vm_def.h`` directly by generating the VM using the
+``php Zend/zend_vm_gen.php --with-lines`` command. This will annotate the code with the ``#line`` preprocessor directive
+to allow the debugger to know the origin of the generated instructions.