On Fri, Sep 30, 2022 at 3:41 PM Dave Thaler <dthaler@xxxxxxxxxxxxx> wrote: > > > -----Original Message----- > > From: Alexei Starovoitov <alexei.starovoitov@xxxxxxxxx> > > Sent: Friday, September 30, 2022 2:59 PM > > To: Dave Thaler <dthaler@xxxxxxxxxxxxx> > > Cc: dthaler1968@xxxxxxxxxxxxxx; bpf@xxxxxxxxxxxxxxx > > Subject: Re: [PATCH 07/15] ebpf-docs: Fix modulo zero, division by zero, > > overflow, and underflow > > > > On Fri, Sep 30, 2022 at 09:54:17PM +0000, Dave Thaler wrote: > > > [...] > > > > > +Also note that the modulo operation often varies by language when > > > > > +the dividend or divisor are negative, where Python, Ruby, etc. > > > > > +differ from C, Go, Java, etc. This specification requires that > > > > > +modulo use truncated division (where -13 % 3 == -1) as > > > > > +implemented in C, Go, > > > > > +etc.: > > > > > + > > > > > + a % n = a - n * trunc(a / n) > > > > > + > > > > > > > > Interesting bit of info, but I'm not sure how it relates to the ISA doc. > > > > > > It's because there's multiple definitions of modulo out there as the > > > paragraph notes, which differ in what they do with negative numbers. > > > The ISA defines the modulo operation as being the specific version above. > > > If you tried to implement the ISA in say Python and didn't know that, > > > you'd have a non-compliant implementation. > > > > Is it because the languages have weird rules to pick between signed vs > > unsigned mod? > > At least from llvm pov the smod and umod have fixed behavior. > > It's because there's different mathematical definitions and different languages have chosen different definitions. E.g., languages/libraries that follow Knuth use a > different mathematical definition than C uses. For details see: > > https://en.wikipedia.org/wiki/Modulo_operation#Variants_of_the_definition > > https://torstencurdt.com/tech/posts/modulo-of-negative-numbers/ Those differences are in signed div/mod only, right? Unsigned div/mod doesn't have it, right? bpf has only unsigned div/mod.
