Can a float volatile asm be used as optimization barrier?

[Parmenides]

> This is exactly what the manual says.  The asm has no side-effects
> except to change __old.  And __old is not used, which means that any
> statements which assign a value to __old are unnecessary and may be
> removed.  Since the only effect of the asm is to change __old, it is
> removed.

I really have gotten a misunstanding about the manual. ;-)
Thanks a lot.