Try integer-length. https://docs.racket-lang.org/reference/generic-numbers.html?q=integer-length#%28def._%28%28quote._~23~25kernel%29._integer-length%29%29
I don't know how it is implemented. /Jens Axel Den tor. 25. feb. 2021 kl. 09.52 skrev Dominik Pantůček < dominik.pantu...@trustica.cz>: > Hello Racketeers, > > I'm slightly stuck with speeding up some calculations and the reason is > that I need to compute the index of highest set bit in a number. So for > 1 it is 0, for 2 it is 1, for 8 3, 1023 9 and 1024 10 ... > > The fastest Racket code I can come up with is as follows: > > (begin-encourage-inline > (define (highest-bit num) > (let loop ((num num) > (bit 0)) > (if (unsafe-fx> num 0) > (loop (unsafe-fxrshift num 1) > (unsafe-fx+ bit 1)) > bit)))) > > (Yes, it returns incorrect result for 0, but that must be checked > elsewhere as the result cannot be defined for 0 anyway). > > However this is a single instruction operation on x86 (bsr) or two > instruction operation (rbit and clz if I am not mistaken) on ARM. Dunno > about PPC yet. > > I looked at CS internals a bit and although there is a "name collision" > as the bsr mnemonics is used for ret (branch subroutine return?), it > should be fairly easy to add something like fxbsr (-> fixnum? fixnum?) > to Racket core. > > I also experimented with x64asm package but the results were even worse > than the aforementioned tight loop (there is a lot of overhead in > define-λ! generated functions). > > As the routine is typically called 40k-60k times per frame in my code > (real-time rendering) it could really make a difference. > > Should I try to add it? How should it be named? What about BC? > > And more generic question - aren't there more similar operations that > can be implemented efficiently on all supported CPUs that might be > useful in general? Yes, I am aiming at SIMD as many of you know :) > Especially with the expanded number of available FP registers to 8 on > 64-bit x86 CS there surely is quite some potential to it. > > > Cheers, > Dominik > > -- > You received this message because you are subscribed to the Google Groups > "Racket Users" group. > To unsubscribe from this group and stop receiving emails from it, send an > email to racket-users+unsubscr...@googlegroups.com. > To view this discussion on the web visit > https://groups.google.com/d/msgid/racket-users/a2a4fe83-877d-d7ed-4812-bd628c128659%40trustica.cz > . > -- -- Jens Axel Søgaard -- You received this message because you are subscribed to the Google Groups "Racket Users" group. To unsubscribe from this group and stop receiving emails from it, send an email to racket-users+unsubscr...@googlegroups.com. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CABefVgwvxcsy1EN_W04fYUfp%2BGm2nbBdoeTEnD3UbKfCHcHyKw%40mail.gmail.com.
