Hi, I am trying to speed up some C code and find that I need to know details about compilation and assembler that I cannot seem to find either via FAQs or Google. 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". At the moment, I am using a bit manipulation trick as follows: #define IF2BITS(x) x &= x-1; if (x && !(x&(x-1))) (this relies on the fact that x&(x-1) strips off the least significant bit that is set) In order to use 64-bit words, I am using the type "long long". 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.... Is there something important that I am missing, or is this to be expected? Can I check that it *is* actually using the G5 in native 64-bit mode? 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? Thanks in advance.. Gordon -- Dr. Gordon Royle, http://www.csse.uwa.edu.au, gordon(AT)csse.uwa.edu.au --
