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   ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 16:08:38 JST ✧✦Catherine✦✧

   gcc now has builtin functions for CRCs https://gcc.gnu.org/onlinedocs/gcc/CRC-Builtins.html

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 17:48:01 JST ✧✦Catherine✦✧

     in reply to

     • F4GRX Sébastien

     @f4grx beats me

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     F4GRX Sébastien (f4grx@chaos.social)'s status on Saturday, 30-Aug-2025 17:48:03 JST F4GRX Sébastien

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     @whitequark whats the goal of these? Performance?

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     8051 enthusiast (8051enthusiast@mastodon.social)'s status on Saturday, 30-Aug-2025 19:29:10 JST 8051 enthusiast

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     @whitequark i can't believe we're getting crc builtins and a llvm loopidiom pr that recognizes crcs but there is still no clmul builtin

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:32:28 JST ✧✦Catherine✦✧

     in reply to

     • 8051 enthusiast

     @8051enthusiast eli5: why is clmul so useful?

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:36:04 JST ✧✦Catherine✦✧

     in reply to

     • 8051 enthusiast

     @8051enthusiast also, loopidiom, seriously? who added that?

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     8051 enthusiast (8051enthusiast@mastodon.social)'s status on Saturday, 30-Aug-2025 19:36:50 JST 8051 enthusiast

     in reply to

     @whitequark it's not merged yet, just a pr: https://github.com/llvm/llvm-project/pull/143208

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:40:07 JST ✧✦Catherine✦✧

     in reply to

     • 8051 enthusiast

     @8051enthusiast oh, the code is less bad than I thought

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:47:24 JST ✧✦Catherine✦✧

     in reply to

     • F4GRX Sébastien
     • Simon Brenner

     @olsner @f4grx hm, can you use CLMUL to implement rotation?

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     Simon Brenner (olsner@social.vivaldi.net)'s status on Saturday, 30-Aug-2025 19:47:25 JST Simon Brenner

     in reply to

     • F4GRX Sébastien

     @f4grx @whitequark I would say the point is convenient arch-independent access to crc hardware - you should get the special instruction when the polynomial matches whatever your cpu has and hopefully a clmul based fallback before resorting to a table. Much nicer than a tangle of ifdefs for every architecture and the research needed to know every crc instruction and which crc variant it produces…

     I guess if you were properly enthusiastic about crcs you’d already be maintaining specialized assembly for 14 architectures and every commonly used polynomial and this is not so interesting… but now the rest of us can get gcc (or gcc’s architecture maintainers) to do it for us?

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     8051 enthusiast (8051enthusiast@mastodon.social)'s status on Saturday, 30-Aug-2025 19:48:01 JST 8051 enthusiast

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     @whitequark for CRCs specifically, since CRC is basically a polynomial remainder, you can optimize it to clmul the same way you can optimize "integer modulo a constant" to integer multiplications. it's also useful in cryptographic contexts whenever there are fields over GF(2). i also would have found it useful a bunch of times i wanted to rearrange some bits (which can be done in a way similar to normal integer multiplication to spread bits, but without having to worry about carry bits)

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:48:31 JST ✧✦Catherine✦✧

     in reply to

     • 8051 enthusiast

     @8051enthusiast aha so this is on point: https://mastodon.social/@whitequark/115117315959503306

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     ✧✦Catherine✦✧ (whitequark@mastodon.social)'s status on Saturday, 30-Aug-2025 19:48:31 JST ✧✦Catherine✦✧

     in reply to

     • 8051 enthusiast

     @8051enthusiast I do not at all understand CRCs or how one can be a "polynomial remainder"

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     8051 enthusiast (8051enthusiast@mastodon.social)'s status on Saturday, 30-Aug-2025 19:59:55 JST 8051 enthusiast

     in reply to

     • F4GRX Sébastien
     • Simon Brenner

     @whitequark @olsner @f4grx hmm i think that's wrong. but if you had a 32-bit integer, you could do clmul(x, 1 ^ (1 << 32)) >> r (with some extra integer casting) which is the same as (x | (x << 32)) >> r

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     8051 enthusiast (8051enthusiast@mastodon.social)'s status on Saturday, 30-Aug-2025 21:38:37 JST 8051 enthusiast

     in reply to

     @whitequark ok so i wanted to attempt to explain this, but it got too long and still skipped too many steps, sorry! anyways, here's the draft in case you still want to read it: https://gist.github.com/8051Enthusiast/9d737c081a356bf47b4ea1f434ffe417