[Bug 706880] sysmem: Alignment has to be kept when copying

[GStreamer (GNOME Bugzilla)]

https://bugzilla.gnome.org/show_bug.cgi?id=706880

Tim-Philipp Müller <t.i.m at zen.co.uk> changed:

           What    |Removed                     |Added
----------------------------------------------------------------------------
             Status|NEW                         |NEEDINFO

--- Comment #1 from Tim-Philipp Müller <t.i.m at zen.co.uk> ---
I am not sure if I really understand this bug. Jan said in comment #6 of bug
#705678 , which was the reason for cloning this aiui:

> Yes, the new memory block will be aligned. The data within it
> may not have the same alignment after the copy - for example,
> copy from a buffer containing uint32's with an offset of
> 3 - the new memory won't have aligned uint32s where the old
> buffer did.

Ok, so the second uint32 wouldn't be aligned any more, but why would one copy
data from offset 3 anyway if the data is processed in units of 4 bytes?

How does one determine what the correct alignment is in this case? e.g. if we
have 4096 uint32 and we copy data from offset 4094*4 + 3, what should be the
alignment of the new &data[1] ?

Conversely, often one may want to explicitly achieve a new clean alignment when
copying (and existing code might in fact rely on this behaviour). Example: a
demuxer or parser where we have 7 bytes of headers, and then follows some
payload, and we want the payload to be aligned nicely.

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