220 37391 article Path: news.gmane.org!.POSTED!not-for-mail From: Mathias Gaunard Newsgroups: gmane.comp.lang.c++.isocpp.proposals Subject: Re: Would SIMD consider signed overflow undefined? 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[46.105.43.105]) by mx.google.com with ESMTPS id k190si992279qkc.189.2018.03.19.14.22.30 for (version=TLS1_2 cipher=ECDHE-RSA-AES128-GCM-SHA256 bits=128/128); Mon, 19 Mar 2018 14:22:30 -0700 (PDT) Received-SPF: pass (google.com: domain of mathias@gaunard.com designates 46.105.43.105 as permitted sender) client-ip=46.105.43.105; Original-Received: from player770.ha.ovh.net (unknown [10.109.120.149]) by mo69.mail-out.ovh.net (Postfix) with ESMTP id 83ED88637 for ; Mon, 19 Mar 2018 22:22:28 +0100 (CET) Original-Received: from mail-ot0-f173.google.com (mail-ot0-f173.google.com [74.125.82.173]) (Authenticated sender: mathias@gaunard.com) by player770.ha.ovh.net (Postfix) with ESMTPSA id CDC7E3C0091 for ; Mon, 19 Mar 2018 22:22:27 +0100 (CET) Original-Received: by mail-ot0-f173.google.com with SMTP id v23-v6so3670550oth.9 for ; Mon, 19 Mar 2018 14:22:27 -0700 (PDT) X-Received: by 2002:a9d:1f2a:: with SMTP id x39-v6mr9192594otd.125.1521494546859; Mon, 19 Mar 2018 14:22:26 -0700 (PDT) Original-Received: by 10.201.21.34 with HTTP; Mon, 19 Mar 2018 14:22:26 -0700 (PDT) In-Reply-To: X-Gmail-Original-Message-ID: X-Ovh-Tracer-Id: 3032048453107562111 X-VR-SPAMSTATE: OK X-VR-SPAMSCORE: 0 X-VR-SPAMCAUSE: gggruggvucftvghtrhhoucdtuddrgedtgedrudefgddugeelucetufdoteggodetrfdotffvucfrrhhofhhilhgvmecuqfggjfdpvefjgfevmfevgfenuceurghilhhouhhtmecufedttdenuc X-Original-Sender: mathias@gaunard.com X-Original-Authentication-Results: mx.google.com; spf=pass (google.com: domain of mathias@gaunard.com designates 46.105.43.105 as permitted sender) smtp.mailfrom=mathias@gaunard.com Precedence: list Mailing-list: list std-proposals@isocpp.org; contact std-proposals+owners@isocpp.org List-ID: X-Spam-Checked-In-Group: std-proposals@isocpp.org X-Google-Group-Id: 399137483710 List-Post: , List-Help: , List-Archive: List-Subscribe: , List-Unsubscribe: , Xref: news.gmane.org gmane.comp.lang.c++.isocpp.proposals:37391 Archived-At: --000000000000fe4d670567ca8d06 Content-Type: text/plain; charset="UTF-8" On 19 March 2018 at 21:04, Myriachan wrote: > Would the SIMD proposals consider signed integer overflow to be undefined? > > SIMD programming uses a lot of tricks for performance, including things > like reinterpreting values as the "wrong" type, and abusing > two's-complement overflow mechanics. This is often done irrespective of > whether the result is ultimately considered signed or unsigned. > > The standard library should expose any functionality that any trick can stay in the implementation without the user having to worry about it. One example of a "trick" is that the value for true is usually the same as -1. So you can implement `a.where(condition)++' as `a - condition'. -- You received this message because you are subscribed to the Google Groups "ISO C++ Standard - Future Proposals" group. To unsubscribe from this group and stop receiving emails from it, send an email to std-proposals+unsubscribe@isocpp.org. To post to this group, send email to std-proposals@isocpp.org. To view this discussion on the web visit https://groups.google.com/a/isocpp.org/d/msgid/std-proposals/CALnjya-MHDV%2BYOKY2x%3DxGjLRaAztaABz4T0cR%3DY5%2B1MC_Pf_dg%40mail.gmail.com. --000000000000fe4d670567ca8d06 Content-Type: text/html; charset="UTF-8" Content-Transfer-Encoding: quoted-printable
On 1= 9 March 2018 at 21:04, Myriachan <myriachan@gmail.com> wro= te:
Would the SIMD propo= sals consider signed integer overflow to be undefined?

SIMD programm= ing uses a lot of tricks for performance, including things like reinterpret= ing values as the "wrong" type, and abusing two's-complement = overflow mechanics.=C2=A0 This is often done irrespective of whether the re= sult is ultimately considered signed or unsigned.


The standard library should = expose any functionality that any trick can stay in the implementation with= out the user having to worry about it.
One example of a "trick" is that the value for true is = usually the same as -1. So you can implement `a.where(condition)++' as = `a - condition'.
=C2=A0

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