On Wed, Aug 24, 2005 at 03:34:41PM -0400, mark@mark.mielke.cc wrote:
>It isn't an urban myth that 64-bit math on a 64-bit processor is
>faster, at least if done using registers. It definately is faster.
>It may be an urban myth, though, that most applications perform
>a sufficient amount of 64-bit arithmetic to warrant the upgrade.
The mjor problem is that the definition of "64bit processor" is fuzzy.
The major slowdown of "64bitness" is the necessity of carting around
64 bit pointers. It's not, however, necessary to do 64bit pointers to
get 64bit registers & fast 64 bit ops. E.g., sgi has "n32" & "n64" abi's
which can access exactly the same instruction set & registers, the
difference between them is the size of pointers and whether a "long" is
the same as a "long long". Any discussion of "64 bit processors" is
doomed from the start because people tend to start making implementation
assumptions on top of an already vague concept. Current & future
discussions are tinged by the fact that amd64 *doubles* the number
of registers in 64 bit mode, potentially providing a major speedup--but
one that doesn't really have anything to do with being "64bit".
Pretty much any discussion of 64 bit mode really needs to be a
discussion of a particular abi on a particular processor; talking about
"64 bit processors" abstractly is a waste of time.
Mike Stone
