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:

quoted

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!