perlasm/x86_64-xlate.pl: clarify SEH coding guidelines.
@@ -1072,6 +1072,7 @@ close STDOUT;
# movq -16(%rcx),%rbx
# movq -8(%rcx),%r15
# movq %rcx,%rsp # restore original rsp
+# magic_epilogue:
# ret
# .size function,.-function
#
@@ -1084,12 +1085,17 @@ close STDOUT;
# 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];
@@ -1180,13 +1186,12 @@ close STDOUT;
# 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
