On Fri, Mar 2, 2012 at 4:17 PM, <david@xxxxxxx> wrote:
>
> or did some CPUs have efficient char access, but inefficient unaligned word
> access?
Tons of CPU's have efficient char accesses but horrible unaligned word
accesses. Some are even outright buggy (ie at least some ARM cores)
and load crap. Others take a fault.
They just aren't x86, because x86 has traditionally had code with a
fair amount of unaligned loads and stores (and not just for historical
reasons either: even modern code replaces constant-sized memcpy() etc
with direct loads and stores)
For some other architectures, we could just use "get_unaligned()",
which fixes things up for them. I could have made that explicit, even
if it doesn't matter on x86.
So the bigger portability problem to some degree is the fact that it
is limited to little-endian, so even if you have a CPU with good
unaligned accesses (some POWER chips do ok, for example, although not
all), you'd have to also do something with the mask generation (which
currently uses the "(x-1)&~x" trick that means that it generates the
mask of the *low bits* - and then assumes that "low bits" means "first
bytes" - ie little endian).
Linus
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