--- Log opened Thu Nov 17 00:00:19 2022
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06:37 < roconnor> sipa: ^^
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07:52 < roconnor> Although I don't think I have much influance, I've personally decided that (mod n) means (Z/nZ) and in particular (mod 0) means (Z/0Z) = Z, and thus x mod 0 = x for all x:Z.
07:52 < sipa> roconnor: Hmm, are you suggesting something like `return (int64_t)((uint64_t)a * (uint64_t)b)` ?
07:52 < sipa> Just `a * b` will cause a signed integer overflow.
07:52 < sipa> Which cppreference says is UB. I'm surprised ubsan doesn't complain.
07:52 < sipa> Oh, but ubsan isn't enabled on any of the platforms where that code path is tested.
07:52 < sipa> Added a commit to address that in #1156
07:52 < sipa> Ok, by forcing that codepath to be used (using the inline asm replacement for _mulh and _umulh) and enabling ubsan, it triggers.
07:52 < sipa> And by changing to `(uint64_t)a * (uint64_t)b` it doesn't.
07:53 < sipa> That makes sense. It means that that "the integers mod n" have characteristic n.
07:58 < roconnor> it also makes the equation x = (x `div` 0)*0 + (x `mod` 0) work out, which is promising. ... for some definition of work out.
07:58 < sipa> Even for n=0.
07:59 < roconnor> I was about to say my IRC client is acting weird, however https://gnusha.org/secp256k1/2022-11-17.log suggests that sipa, it is your IRC client that is buffering things oddly.
07:59 < sipa> Also gcd(a,0) = a.
08:01 < roconnor> -
08:01 < sipa> Not my client, but the matrix-IRC bridge seems to have some latency...
08:01 < sipa> Also gcd(a,b) = gcd(a,b mod a) = gcd(a mod b, b)
08:01 < sipa> Keeps working when a=0 or b=0.
08:05 < roconnor> I brought this up because of your mod infinity comment in mulmod256.  I don't actually think you should change your comment.  I just wish we could.
08:10 < andytoshi> this is a really cool observation roconnor 
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10:18 < roconnor> Do we need new configurations to run UBSAN through these codepaths.  That sounds irritating to do.  We'd have to do something like link against an implementation of __umulh and __mulh?
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--- Log closed Fri Nov 18 00:00:20 2022