exceptions

	     Exception Implementation in the Mono Runtime
		 Dietmar Maurer (dietmar@ximian.com)
			(C) 2001 Ximian, Inc.

Exception implementation (jit):
===============================

Stack unwinding:
================

We record the code address (start_address, size) of all methods. That way it is
possible to map an instruction pointer (IP) to the method information needed
for unwinding the stack:

We also save a Last Managed Frame (LMF) structure at each call from managed to
unmanaged code. That way we can recover from exceptions inside unmanaged code.

void handle_exception ((struct sigcontext *ctx, gpointer obj)
{
        if (ctx->bp < mono_end_of_stack) {
	        /* unhandled exception */
	        abort ();
	}

	info = mono_jit_info_table_find (mono_jit_info_table, ctx->ip);

	if (info) { // we are inside managed code

		if (ch =  find_catch_handler ())
			execute_catch_handler (ch, ctx, obj); 
		
		execute_all_finally_handler ();

		// restore register, including IP and Frame pointer
		ctx = restore_caller_saved_registers_from_ctx (ji, ctx);

		// continue unwinding
		handle_exception (ctx, obj);

	} else {

	        lmf = get_last_managed_frame ();
		
		// restore register, including IP and Frame pointer
		ctx = restore_caller_saved_registers_from_lmf (ji, lmf);
		
		// continue unwinding
		handle_exception (ctx, obj);
	}
}


Code generation:
================

leave: is simply translated into a branch to the target. If the leave
instruction is inside a finally block (but not inside another handler)
we call the finally handler before we branch to the target.

finally/endfinally, filter/endfilter: is translated into subroutine ending with
a "return" statement. The subroutine does not save EBP, because we need access
to the local variables of the enclosing method. Its is possible that
instructions inside those handlers modify the stack pointer, thus we save the
stack pointer at the start of the handler, and restore it at the end. We have
to use a "call" instruction to execute such finally handlers. This makes it
also possible to execute them inside the stack unwinding code. The exception
object for filters is passed in a local variable (cfg->exvar).

throw: we first save all regs into a sigcontext struct and then call the stack
unwinding code.

catch handler: catch handlers are always called from the stack unwinding
code. The exception object is passed in a local variable (cfg->exvar).