On Jul 4, 2012, at 10:41 PM, Nathan Day wrote: > It must if 64bits is read in that mean you have just read in two 32bit words. > So to put a 32bit word in a 64bit register some bit must be ditched, in some > way, and if the CPU is optimise to only work with 64bit word alignment (don't > know how intel does it), then to get 32 bit aligned words it must do some bit > shift.
Yes, but this happens in hardware way, way down inside the ALU, where I seriously doubt it takes any extra time at all. (It's not a 'shift' at that level so much as the way address lines are wired up to each other.) You'd have to consult the gory details of the timing diagrams in Intel's detailed instruction-set manuals to find out. In any case it has nothing to do with the current discussion. The CPU cache is a lot like virtual memory, just lower level. The 'page size' is called a 'cache line' and it's smaller, maybe 64 or 128 bytes. When the CPU needs to access an address it first looks in the cache to see if a cache line containing that address is available; if so it reads it directly from there (which is extremely fast since it's on-chip), otherwise it has to wait to load the entire cache line from RAM, then fetch the address. The cache _only_ deals in blocks of 128/256/whatever bytes, just like the disk controller only reads/writes entire sectors. It can't manage 4-byte or 8-byte values any more than the disk controller can. It has no idea whether the CPU wants a byte or a 32-bit word or a 64-bit one, it just knows how to read and write pages from off-chip RAM. http://en.wikipedia.org/wiki/Cache_line And the real point in this discussion: Just like with VM but on a smaller scale, the better your working set of data fits into the CPU cache, the faster your code will run. And the cache will, obviously, hold twice as many int32_t's as it will NSIntegers (in 64-bit mode.) So using smaller int sizes _can_ improve performance. —Jens _______________________________________________ Cocoa-dev mailing list (Cocoa-dev@lists.apple.com) Please do not post admin requests or moderator comments to the list. Contact the moderators at cocoa-dev-admins(at)lists.apple.com Help/Unsubscribe/Update your Subscription: https://lists.apple.com/mailman/options/cocoa-dev/archive%40mail-archive.com This email sent to arch...@mail-archive.com
