Re: Unaligned accesses in sha1dc

[Junio C Hamano <gitster@pobox.com>]

Lars Schneider <larsxschneider@gmail.com> writes:

>> On 01 Jun 2017, at 10:28, Andreas Schwab <schwab@suse.de> wrote:
>> 
>> The sh1dc implementation is making unaligned accesses, which will crash
>> on some architectures, others have to emulate them in software.
>
> Is SPARC an architecture that would run into this problem? I think
> there was a thread a few days ago about this...
>
> What architectures are affected by this? I think I read somewhere
> that ARM requires aligned accesses, too, right?
>
> I wonder if it makes sense to emulate SPARC/ARM/... with QEMU on TravisCI [1].
> Is this what you had in mind with "emulate" or do you see a better way?

I think Andreas's "emulate" is that on these architectures often the
kernel catches the hardware trap and makes the unaligned access
appear to "just work" to the userland software, just like in very
old days, i386 and i486 without 387/487 used software floating point
"emulation" to give illusion to the userland softare that the co
processor was available.

Having to trap and emulate of course costs cycles, and if the
userland is written in such a way not to do an unaligned access in
the first place.

Depending on the model of "ARM" (or "SPARC") emulated with QEMU, and
depending on the OS that runs on such an "ARM" or "SPARC", we may
not see this---if the emulated OS has the "software unaligned-access
emulation" our userland may not see a SIGBUS.