I would recommend changing it to 16-byte aligned. Lots of SSE instructions won't work or won't work efficiently at 8-byte aligned addresses. Even without SSE, this makes array access more cache-friendly, and is likely to help. If we're only talking about arrays larger than page-size, then it's not too much memory overhead.
For less-than-page-sized arrays (or performance-tight code if you DON'T make the change), you'd have to use something like std.c.stdlib or std.<system> _aligned_alloc() to get around this. Might be worth verifying this is actually available and works (maybe a unit test?. ) Jason ----- Original Message ---- > From: Steve Schveighoffer <[email protected]> > To: Phobos <[email protected]> > Sent: Mon, June 28, 2010 11:36:25 AM > Subject: [phobos] byte alignment for arrays > > Recently, this bug has surfaced: > http://d.puremagic.com/issues/show_bug.cgi?id=4400 In a nutshell, > sometimes the byte alignment of arrays is 8 bytes instead 16 bytes. This > was caused by my array append patch, because in large arrays, I store the > length > at the front of the array. With some queries before I created my patch, I > was told that 8 byte alignment was fine. However, the alignment is easy to > change since it's a couple specific functions that determine the padding and > alignment. So changing to 16 bytes is not an issue technically, and > functionally, this is only on PAGE sized arrays and larger, so 16 bytes vs. 8 > bytes isn't likely to cause problems. Bearophile's main argument stems > from this. I am not a processor or assembly expert, so I have no idea > about this at all: ----------------- The 16 bytes alignment was > introduced because instructions like the SSE2 movapd need 16 byte > alignment: http://en.wikipedia.org/wiki/MOVAPD I have recently used > it > here: http://www.digitalmars.com/webnews/newsgroups.php?art_group=digitalmars.D&article_id=112670 > And some other SSE* instructions work with 8 byte alignment too, but they > are slower (future CPUs can remove such alignment requirements, some of it > has being removed already, so in that future the GC can go back giving 8 > bytes aligned memory). ----------------- So should I change > it? -Steve > _______________________________________________ phobos mailing list > ymailto="mailto:[email protected]"; > href="mailto:[email protected]";>[email protected] http://lists.puremagic.com/mailman/listinfo/phobos _______________________________________________ phobos mailing list [email protected] http://lists.puremagic.com/mailman/listinfo/phobos
