Thanks all. You're right, "just how fast DOES this code need to be?" And the answer is I should know, but I don't. I don't want to waste the customer's cycles. I am smart enough to know that I am too dumb to know how fast it needs to be. The right answer lies in profiling, and some other task has always been just a little higher priority than profiling.
Thanks! Great link! The De Bruijn thing is amazing. I was a math minor but I hated it. I am very weak on the higher math relevant to programming. Charles -----Original Message----- From: IBM Mainframe Discussion List [mailto:IBM-MAIN@LISTSERV.UA.EDU] On Behalf Of Andrew Rowley Sent: Wednesday, July 24, 2013 8:17 AM To: IBM-MAIN@LISTSERV.UA.EDU Subject: Re: Is there a "reverse bits" hardware instruction? How fast does this code need to be? David's ffs64 looked pretty good to my inexpert eye, I think you would have to be running it very frequently for something to be measurably faster. There are some similar discussions here, including some branchless techniques that probably would be faster (not necessarily detectably): http://stackoverflow.com/questions/757059/position-of-least-significant-bit- that-is-set One answer also talks about clearing the lowest set bit. ---------------------------------------------------------------------- For IBM-MAIN subscribe / signoff / archive access instructions, send email to lists...@listserv.ua.edu with the message: INFO IBM-MAIN
