On Thu, Nov 17, 2022 at 03:05:14AM +0100, Jason A. Donenfeld wrote: > On Thu, Nov 17, 2022 at 12:55:47AM +0100, Jason A. Donenfeld wrote: > > 1) How/whether to make f(0, UR2_MAX) safe, > > - without additional 64-bit arithmetic, > > - minimizing the number of branches. > > I have a few ideas I'll code golf for a bit. > > I think I can make progress with (1) alone by fiddling around with > > godbolt enough, like usual. > > The code gen is definitely worse. > > Original half-open interval: > > return floor + get_random_u32_below(ceil - floor); > > Suggested fully closed interval: > > ceil = ceil - floor + 1; > return likely(ceil) ? floor + get_random_u32_below(ceil) : get_random_u32(); How many of these uses are going to have ceil and floor as a variable? If they're constants (e.g. due to being in an inline function with constant arguments) then the compiler will optimise all of the above and the assembly code will just be either: 1. a call to get_random_u32() 2. a call to get_random_u32_below() and an addition. If one passes ceil or floor as a variable, then yes, the code gen is going to be as complicated as you suggest above. -- RMK's Patch system: https://www.armlinux.org.uk/developer/patches/ FTTP is here! 40Mbps down 10Mbps up. Decent connectivity at last!
