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 < [email protected]>: > 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 [email protected]. > 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 [email protected]. To view this discussion on the web visit https://groups.google.com/d/msgid/racket-users/CABefVgwvxcsy1EN_W04fYUfp%2BGm2nbBdoeTEnD3UbKfCHcHyKw%40mail.gmail.com.
