Re: PowerPC G5, 64-bit long long, bit counting and inline assembler...

[Falk Hueffner <falk dot hueffner at student dot uni-tuebingen dot de>]

Gordon Royle <gordon@csse.uwa.edu.au> writes:

> I have a program that represents sets using the bit pattern in a 64-bit
> word, and I repeatedly have to answer the question "Does this word have
> exactly 2 bits set".
> 
> Now I recently gained access to a Mac G5, with the 64-bit PowerPC G5
> chip, and so I naively expected a substantial increase in
> performance over my existing testing machine (in case it is relevant
> the chips are 2.667GHz Xeon vs 2GHz G5) because presumably gcc must
> "simulate" the 64-bit behaviour of "long long" on a 32-bit machine
> while it can do it natively on a G5.
> 
> But in practice, I only gained about 20% - even when I did a test
> program that *only* did a few billion operations like &, |, ^ and the
> IF2BITS macro above.... 

Are you sure you used the right compiler switches to activate 64 bit
code generation? I don't know about powerpc, but usually you have to
give extra switches.

> Can I check that it *is* actually using the G5 in native 64-bit
> mode?

You could look at the assembly of, say, a+b with a and b long longs,
and see whether it is a single instruction.

> In addition, is there a faster way to check if a word has exactly
> 2-bits set? Would there be any gain from trying to inline assembler
> to do just this operation?

I don't think so. Powerpc does not have a population count intruction
(at least none is defined in its machine description in gcc).

-- 
	Falk