Thanks, this information helps a lot! I will make the change to 16-byte aligned. I'm already using 8 bytes for a 4 byte length. Using 16 bytes isn't much different, especially when the block size is 4096+ bytes.
One final question -- I currently use sizeof(size_t) * 2, which could now be sizeof(size_t) * 4, but of course, this changes to 32 bytes on 64-bit dmd. Would it make sense to just use 16 instead of some multiple of size_t? -Steve ----- Original Message ---- > From: Jason Spencer <[email protected]> > To: Discuss the phobos library for D <[email protected]> > Sent: Mon, June 28, 2010 4:09:01 PM > Subject: Re: [phobos] byte alignment for arrays > > Sorry, I forgot to address the every-other-one concern. The MMX registers > are 64-bits, so you can only do 1 double at a time. Those instructions > only require 8-byte aligned memory. The SSE instructions use 128-bit > registers, so they take 2 doubles at a time. As long as the first one is > 16-byte aligned, you can iterate through on 16-byte (128 bits) chunks, and > you'll be good. That's why element 0 should be 128-aligned. If it's > not, the processor will either have an alignment fault (in the instruction > requires alignment) or will do a bunch of split-loads across cache lines, > which > kill performance. One other thought: If you wanted to be > tricky, you could do a general, 4-byte allocation and based on the address > you > get, assign your storage pointer to the next 128-aligned address. But > you're offloading to run-time lot's of housekeeping. Again, maybe > tolerable for just these large arrays. But it starts to add a lot of > corner cases. Walter might have some good suggestions > here. Jason ----- Original Message ---- > From: > Steve Schveighoffer < > 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? _______________________________________________ phobos mailing > list > 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
