This only affects arrays that are PAGE size or larger. A question then -- let's say you have an array of doubles, which are 8 bytes wide, and you want to use these SSE instructions. Even if the first one is aligned on a 16-byte boundary, wouldn't every other double be misaligned?
-Steve ----- Original Message ---- > From: Jason Spencer <[email protected]> > To: Discuss the phobos library for D <[email protected]> > Sent: Mon, June 28, 2010 3:00:10 PM > Subject: Re: [phobos] byte alignment for arrays > > 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 < > href="mailto:[email protected]";>[email protected]> > To: Phobos > < > href="mailto:[email protected]";>[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: > href="mailto:[email protected]";>[email protected]" > > href="mailto: > href="mailto:[email protected]";>[email protected]"> > ymailto="mailto:[email protected]"; > href="mailto:[email protected]";>[email protected] http://lists.puremagic.com/mailman/listinfo/phobos _______________________________________________ phobos > mailing list > href="mailto:[email protected]";>[email protected] > href="http://lists.puremagic.com/mailman/listinfo/phobos"; target=_blank > >http://lists.puremagic.com/mailman/listinfo/phobos _______________________________________________ phobos mailing list [email protected] http://lists.puremagic.com/mailman/listinfo/phobos
