perlasm/x86_64-xlate.pl: clarify SEH coding guidelines.

#	movq	-16(%rcx),%rbx

#	movq	-8(%rcx),%r15

#	movq	%rcx,%rsp	# restore original rsp

# magic_epilogue:

#	ret

# .size function,.-function

#

# EXCEPTION_DISPOSITION handler (EXCEPTION_RECORD *rec,ULONG64 frame,

#		CONTEXT *context,DISPATCHER_CONTEXT *disp)

# {	ULONG64 *rsp = (ULONG64 *)context->Rax;

#	if (context->Rip >= magic_point)

#	{   rsp = ((ULONG64 **)context->Rsp)[0];

#	ULONG64  rip = context->Rip;

#

#	if (rip >= magic_point)

#	{   rsp = (ULONG64 *)context->Rsp;

#	    if (rip < magic_epilogue)

#	    {	rsp = (ULONG64 *)rsp[0];

#		context->Rbp = rsp[-3];

#		context->Rbx = rsp[-2];

#		context->R15 = rsp[-1];

#	    }

#	}

#	context->Rsp = (ULONG64)rsp;

#	context->Rdi = rsp[1];

#	context->Rsi = rsp[2];

# instruction and reflecting it in finer grade unwind logic in handler.

# After all, isn't it why it's called *language-specific* handler...

#

# Attentive reader can notice that exceptions would be mishandled in

# auto-generated "gear" epilogue. Well, exception effectively can't

# occur there, because if memory area used by it was subject to

# segmentation violation, then it would be raised upon call to the

# function (and as already mentioned be accounted to caller, which is

# not a problem). If you're still not comfortable, then define tail

# "magic point" just prior ret instruction and have handler treat it...

# SE handlers are also involved in unwinding stack when executable is

# profiled or debugged. Profiling implies additional limitations that

# are too subtle to discuss here. For now it's sufficient to say that

# in order to simplify handlers one should either a) offload original

# %rsp to stack (like discussed above); or b) if you have a register to

# spare for frame pointer, choose volatile one.

#

# (*)	Note that we're talking about run-time, not debug-time. Lack of

#	unwind information makes debugging hard on both Windows and