Re: [PATCH] sctp: Clean up type-punning in sctp_cmd_t union

On Fri, Oct 26, 2012 at 10:00:13AM +0100, David Laight wrote:

quoted

Subject: [PATCH] sctp: Clean up type-punning in sctp_cmd_t union

...

quoted

+#define SCTP_NULL_BYTE 0xAA
 static inline sctp_arg_t SCTP_NULL(void)
 {
-	sctp_arg_t retval; retval.ptr = NULL; return retval;
+	sctp_arg_t retval;
+	memset(&retval, SCTP_NULL_BYTE, sizeof(sctp_arg_t));
+	return retval;
 }

You really don't want to be taking the address of a local
variable that would normally be held in a register.
It stops the compiler doing a lot of optimisations.
In this case the structure is also being returned by value,
not nice except that all(?) modern ABI do pass small structures
in registers.

Thank you dave, I hadn't considered the implications of missed optimizations
when storing to a register here.

An assignment of some member of the union to NULL would
seem most appropriate.
OTOH the code that uses this must be someones 'bright idea (tm)'
that probably wasn't such a good idea after all.

nobody uses it currently, Its just that some state machine side effects don't
require an argument, but you still have to pass one in, so I was trying to
enhance this to make it obvious in a backtrace should someone inadvertently use
it in the future.  I'm going to revert it though, a big fat zero is probably
just as clear.
Neil

	David