Re: long long performance

[Rob Willis <rob at e-critical dot com>]

Thanks for the info.  Assuming the longlong multiply isnt too much more
complex, it still does not explain the 13x slower performance over that
of just a normal long mulitply.  The timing on my P3/600 gave the
following (i've already subtracted out the overhead for the loop and
variable assignment):

All times are for 1 billion operations to complete.

long long mult:  33 secs
long mult:       2.4 secs

Which just seems wrong.  And even worse is casting from a double to a
long or casting from a double to a long long, both give me the following
(again - i subtracted out the overhead times):

double cast to longlong:  87 secs
double cast to long:      87 secs
longlong cast to double:   3 secs

Something has to be broken here...

Has anyone else tried running the test case i attached in my original
email?

-Rob



"Josh M. Osborne" wrote:
> 
> I didn't see any answers to this, so I'll take a stab at it.
> 
> A 64 bit add can be built with two 32 bit adds, and a conditonal
> branch over an incrment.
> 
> A 32 by 64 bit multiply can be done with two 32 bit multiplies and
> a 64 bit add.
> 
> A 64 bit multiply is not as simple.  I don't know the best algo for it
> off the top of my head (I havn't had to code an extened multiply for
> almost 10 years).
> 
> >I was considering taking advantage of the "long long" type, but now i'm
> >not sure if that is a good idea.
> 
> If you need 64 bit values, you probbably won't do any faster then the
> compiler.