[Bug libc/28432] Aarch64 memcpy used on device-memory

["jon@solid-run.com" <sourceware-bugzilla@sourceware.org>]

https://sourceware.org/bugzilla/show_bug.cgi?id=28432

--- Comment #3 from Jon Nettleton <jon@solid-run.com> ---
(In reply to Wilco from comment #2)
> (In reply to Jon Nettleton from comment #0)
> 
> > This does fix the issue I was trying to solve in both my specific test case
> > as well as a few real world rendering failures.  Since we are only
> > re-ordering stp's there should be no functional or performance regressions,
> > and all the glibc test's do pass.
> 
> It's not clear what the underlying issue is, but I don't believe reordering
> stores would be sufficient - it might depend on alignment, state of the
> writebuffer, cache miss/hit and many other factors that software can't
> control.
> 
> So my first question is whether this works around the issue 100%, ie. for
> all possible combinations of src and dst alignment, overlap (for memmove)
> and sizes?

This fixes a singular issue that we were seeing and was reproducible with this
modified version of a previous test program that was done when the problem was
first detected on the Macchiatobin. 
https://gist.github.com/jnettlet/80f8d09d01c0dc0ffc0122f36ed78de6

The issue addressed here is purely overlapping writes to device memory of sizes
97-110 would have bytes starting or immediately after 64 bytes that would be
either zero'd or unchanged.

This behaviour could be successfully worked around by simple breaking up
memcpy's of that size range into smaller copies. 
https://gist.github.com/jnettlet/80f8d09d01c0dc0ffc0122f36ed78de6#file-fb_test-c-L98

This does not solve all GPU rendering issues, I have another patch for that but
this does resolve the issues in my test program, as well as rendering errors
that were seen in glmark2 -b terrain as well as the game minetest that still
existed with my mesa patch.  Because these issues still existing with my
patched mesa I believe this is a separate bug.

This is another developer that reported the issues to me testing my glibc with
this patch included. https://twitter.com/sahajsarup/status/1445813643744985093

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