On 18/08/11 11:47, Johan Tibell wrote:
On Thu, Aug 18, 2011 at 12:43 PM, Alexander Kjeldaas
<alexander.kjeld...@gmail.com> wrote:
Unaligned word-sized loads work fine on x86, and this would be x86-64 only,
or even Nehalem (and later) only. Or, from a cost perspective, it could
be interesting for non-Nehalem as well, as RAM is (usually) the most
expensive component when running a server. (But anyways, developing
features for pre-Nehalem is slightly pointless IMHO).
So I guess the alignment would be per-architecture or a flag. If it is
currently hardcoded, it would have to be configurable.
Alexander
I think problem is that there's code that's shared between all
platforms that rely on the alignment. This code would have to be
parameterized by the platform. It's potentially a very big task.
Right, it's a big job. The word-sized unit of allocation is fairly
deeply wired in.
Still, it's an interesting idea, and not impossible. Objects would have
to start on a word boundary due to pointer tagging as Ben points out,
but within an object smaller pointers could be used. But isn't it
48-bit pointers, not 40-bit? (48-bit pointers aren't quite so good,
because a cons cell spills over into 3 words rather than squeezing into
2). And pointer loads would be slower, as we'd have to mask the upper bits.
Cheers,
Simon
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