took me only 30 minutes of CPU time but now i know if there's a more efficient* way to rotate a multibyte value by 2 bits in 8051 than just rotating by one bit twice (the anwer is no, there isn't**)pic.twitter.com/MbfNCFT1R2
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Vulnerability researcher at Google. This is a personal stream, opinions expressed are mine.
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took me only 30 minutes of CPU time but now i know if there's a more efficient* way to rotate a multibyte value by 2 bits in 8051 than just rotating by one bit twice (the anwer is no, there isn't**)pic.twitter.com/MbfNCFT1R2
* by instruction count ** within a model that considers only these instructions, which is to say, without indirect addressing and without assuming a specific register bankpic.twitter.com/WbmtuLl1iF
on the second half-hour of waiting, my SMT solver sent to me: this way to rotate a 16-bit value in R1:R0 by 3 bits. i don't quite understand how it works and i'm not sure i want to.pic.twitter.com/Qq66AJKGp7
4 bits but we've seen that before, it's just a bunch of xors and nibble swapspic.twitter.com/602zctvRSu
spent around 3 hours of CPU time to get this rotate by 5pic.twitter.com/eYqWmjq9BM
enjoying how it rotates by 6 bits with 5 RLC instructionspic.twitter.com/RC1Iy1jyl8
i -really- love this one. it made a rotate by 10 by doing two rotates by 2 on -individual bytes- and then doing some sort of XOR swapping of the 2 remaining bits. this is a brilliant solution i wouldn't even consider on my own!pic.twitter.com/SJylCYeDjB
You should post this on code golf SE, someone might just come up with something better
as in, using instructions outside the model I'm using?
There could be side effects you weren't modeling, I've seen a superoptimizer use totally unexpected status flags as part of a calculation, for example (I don't know 8051 though, maybe it doesn't apply!).
there are definitely effects I'm not modelling (I'm using only 1/3 of the useful instructions here, omitting any use of direct- or indirect-memory access to registers), but relying on those would result in a different ABI
there are *also* flags I'm not modelling, but none of those seem any more useful for rotates. if I was doing arithmetics though I would definitely expand the model considerably.
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