From: Dave Thaler <dthaler@microsoft.com>
To: Alexei Starovoitov <alexei.starovoitov@gmail.com>,
"dthaler1968@googlemail.com" <dthaler1968@googlemail.com>
Cc: "bpf@vger.kernel.org" <bpf@vger.kernel.org>
Subject: RE: [PATCH 07/15] ebpf-docs: Fix modulo zero, division by zero, overflow, and underflow
Date: Fri, 30 Sep 2022 21:54:17 +0000 [thread overview]
Message-ID: <DM4PR21MB3440CDB9D8E325CBEA20FFA7A3569@DM4PR21MB3440.namprd21.prod.outlook.com> (raw)
In-Reply-To: <20220930205211.tb26v4rzhqrgog2h@macbook-pro-4.dhcp.thefacebook.com>
[...]
> > +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.
Dave
next prev parent reply other threads:[~2022-09-30 21:54 UTC|newest]
Thread overview: 50+ messages / expand[flat|nested] mbox.gz Atom feed top
2022-09-27 18:59 [PATCH 01/15] ebpf-docs: Move legacy packet instructions to a separate file dthaler1968
2022-09-27 18:59 ` [PATCH 02/15] ebpf-docs: Linux byteswap note dthaler1968
2022-09-27 18:59 ` [PATCH 03/15] ebpf-docs: Move Clang notes to a separate file dthaler1968
2022-09-27 18:59 ` [PATCH 04/15] ebpf-docs: Add Clang note about BPF_ALU dthaler1968
2022-09-27 18:59 ` [PATCH 05/15] ebpf-docs: Add TOC and fix formatting dthaler1968
2022-09-27 18:59 ` [PATCH 06/15] ebpf-docs: Use standard type convention in standard doc dthaler1968
2022-09-30 20:49 ` Alexei Starovoitov
2022-09-27 18:59 ` [PATCH 07/15] ebpf-docs: Fix modulo zero, division by zero, overflow, and underflow dthaler1968
2022-09-30 20:52 ` Alexei Starovoitov
2022-09-30 21:54 ` Dave Thaler [this message]
2022-09-30 21:59 ` Alexei Starovoitov
2022-09-30 22:41 ` Dave Thaler
2022-09-30 23:41 ` Alexei Starovoitov
2022-10-04 16:36 ` Dave Thaler
2022-10-04 17:24 ` div_k. Was: " Alexei Starovoitov
2022-10-04 18:23 ` Dave Thaler
2022-10-04 18:34 ` Alexei Starovoitov
2023-09-29 21:03 ` Signed modulo operations Dave Thaler
2023-09-29 21:03 ` [Bpf] " Dave Thaler
2023-09-30 6:07 ` Carsten Bormann
2023-09-30 6:07 ` Carsten Bormann
2023-09-30 14:48 ` Alexei Starovoitov
2023-09-30 14:48 ` Alexei Starovoitov
2023-10-02 13:19 ` Eduard Zingerman
2022-09-27 18:59 ` [PATCH 08/15] ebpf-docs: Use consistent names for the same field dthaler1968
2022-09-30 20:57 ` Alexei Starovoitov
2022-10-04 14:44 ` Dave Thaler
2022-09-27 18:59 ` [PATCH 09/15] ebpf-docs: Explain helper functions dthaler1968
2022-09-30 22:01 ` Alexei Starovoitov
2022-09-30 23:01 ` Dave Thaler
2022-09-27 18:59 ` [PATCH 10/15] ebpf-docs: Add appendix of all opcodes in order dthaler1968
2022-09-30 22:02 ` Alexei Starovoitov
2022-09-30 22:43 ` Dave Thaler
2022-09-27 18:59 ` [PATCH 11/15] ebpf-docs: Improve English readability dthaler1968
2022-09-30 22:16 ` Alexei Starovoitov
2022-10-04 14:32 ` Dave Thaler
2022-10-04 15:38 ` Alexei Starovoitov
2022-10-04 15:55 ` Dave Thaler
2022-10-04 15:56 ` Dave Thaler
2022-10-04 16:19 ` Alexei Starovoitov
2022-10-04 16:41 ` Dave Thaler
2022-10-04 16:54 ` Dave Thaler
2022-10-06 20:44 ` Jim Harris
2022-09-27 18:59 ` [PATCH 12/15] ebpf-docs: Add Linux note about register calling convention dthaler1968
2022-09-30 22:17 ` Alexei Starovoitov
2022-09-27 18:59 ` [PATCH 13/15] ebpf-docs: Add extended 64-bit immediate instructions dthaler1968
2022-09-27 18:59 ` [PATCH 14/15] ebpf-docs: Add extended call instructions dthaler1968
2022-09-27 18:59 ` [PATCH 15/15] ebpf-docs: Add note about invalid instruction dthaler1968
2022-09-30 22:21 ` Alexei Starovoitov
2022-09-30 20:50 ` [PATCH 01/15] ebpf-docs: Move legacy packet instructions to a separate file patchwork-bot+netdevbpf
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