John MacFrenz <macfr...@yandex.com> wrote on 10/14/2014 05:33:56 AM:
>
> > allocRailZeroed does an allocate followed by a memset
> >
> > allocRailUninitialized does an allocate without the memset.
> >
> > clearRail does a memset of a portion of a Rail.
>
> Thanks for explanation
>
>
> > We tend to use allocRailUninitialized as a performance
> optimization when we know that we are "just about" to overwrite the
> allocated memory with initial values. So, no point in first writing
> zeros, then writing some other value.
> >
> > In GrowableRail, you are seeing some accommodations for advanced
> features of X10's type system. It is possible to define a type in
> X10 that does not include zero (or null). We'd still like to be
> able to create GrowableRails that contain values of these types, but
> the > type doesn't have a zero/null value for us to use to fill in
> the unused part of the Rail. So we use methods in Unsafe like
> clearRail to allow us to write a zero bytes into the Rail to clear a
> removed element even if that isn't legal at the level of the X10 type
system.
>
> I phrased my initial question rather badly. The actual question is,
> why do you ever have to zero any elements of the rail, because as
> far as I can see no element of GrowableRail should should ever be
> accessed without first setting it with add, insert etc. Does this
> have something to do with Rail storing references instead of objects
> and gargage collection?
>
Yes. If an element is removed from the GrowableRail, we want to clear the
reference to it to avoid retaining garbage (if the removed element was the
last reference to a large data structure). In Java, one would simply write
a null into the array after removing the element. Because of X10 allowing
non-null types, we have to cheat and use clearRail to step outside the type
system and zero the storage that had the removed element to get the same
effect.
--dave
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