From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from mail-io1-xd42.google.com (mail-io1-xd42.google.com [IPv6:2607:f8b0:4864:20::d42]) by sourceware.org (Postfix) with ESMTPS id 28CB53857C69 for ; Thu, 1 Oct 2020 11:27:57 +0000 (GMT) DMARC-Filter: OpenDMARC Filter v1.3.2 sourceware.org 28CB53857C69 Received: by mail-io1-xd42.google.com with SMTP id m17so6208278ioo.1 for ; Thu, 01 Oct 2020 04:27:57 -0700 (PDT) X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=CrEDQQm/x9Vgdpk82a1ymug92oKtBW0/ts4fw2xU1fI=; b=h6M4iFTNlC/Uys0Fy16BMSRPLc+TRxdF0Cv4/3Jp0HjjIsn7kGrwY8AjJYA8Mk12AI sdtV2nzOjrTEgpf7ziPovgZB6PX8ulz3wFLJUYVqy8u5e7wVCpCE68ru3oS8SgBlbKrp C6pfEt9/YXxfDNwl2eTAmH/9MryYea9dqbjm43GlGa2tdVZ4uv0xxfdS26M9OgYUdpm5 f1X6L16D7jGVsDssClXAlRIDYKlagO2euUMih95Ko/M46hM6w2m4q765lCzhbG/xBmT6 TYdB5skG/7yJJOMErg2TU7V9lljGfqgIb4Nc2CduVLfJq8WEWISOhrKJ54ycP/mJ5YW1 /U0g== X-Gm-Message-State: AOAM532M54FShcb4TOXDVc/D5Hz3w5Tzza52uJ302JsYm/gpSQGXJvOK EEwmg9aJpmkhisKf9HlqGga5uQH/hr0wJj9CvQY= X-Google-Smtp-Source: ABdhPJyKRNyzPuCyW1c+OXlotwr8Jl2ocfi/fDrZ4Ims8PYPnN/rtlc7XDmapooIyDGdqesNcjEulorVTAVLX4mUSng= X-Received: by 2002:a02:e47:: with SMTP id 68mr5502918jae.78.1601551676687; Thu, 01 Oct 2020 04:27:56 -0700 (PDT) MIME-Version: 1.0 References: <20201001101559.77163-1-colomar.6.4.3@gmail.com> <20201001101559.77163-6-colomar.6.4.3@gmail.com> <8dc9e60d-f838-87d5-2fc6-c34a84118916@gmail.com> In-Reply-To: <8dc9e60d-f838-87d5-2fc6-c34a84118916@gmail.com> From: Jonathan Wakely Date: Thu, 1 Oct 2020 12:27:45 +0100 Message-ID: Subject: Re: [PATCH 05/16] system_data_types.7: Add int_fastN_t family of types To: Alejandro Colomar Cc: mtk.manpages@gmail.com, linux-man@vger.kernel.org, GNU C Library , "gcc@gcc.gnu.org" Content-Type: text/plain; charset="UTF-8" X-Spam-Status: No, score=-1.4 required=5.0 tests=BAYES_00, DKIM_SIGNED, DKIM_VALID, DKIM_VALID_AU, DKIM_VALID_EF, FREEMAIL_FROM, RCVD_IN_DNSWL_NONE, SPF_HELO_NONE, SPF_PASS, TXREP autolearn=ham autolearn_force=no version=3.4.2 X-Spam-Checker-Version: SpamAssassin 3.4.2 (2018-09-13) on server2.sourceware.org X-BeenThere: libc-alpha@sourceware.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Libc-alpha mailing list List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , X-List-Received-Date: Thu, 01 Oct 2020 11:27:58 -0000 On Thu, 1 Oct 2020 at 12:15, Alejandro Colomar wrote: > > > > On 2020-10-01 13:07, Jonathan Wakely wrote: > [...] > >> +Notes: > >> +Some of these types may be optimized for size > >> +instead of raw performance. > > > > I'm not sure what this tells me as a programmer. What does "raw > > performance" means exactly? The text above says it's "the fastest", > > but then it says "may be optimized for size". I don't know how to > > interpret this. Is it fast or is it small, or something else? Is it > > optimized for small size? Natural word size? Cacheline size? > > > > I prefer the phrasing of the caveats in the C and POSIX standards > > which just say it might not be fastest for all purposes. > > > > How about "Where there is no single type that is fastest for all > > purposes, the implementation may choose any type with the required > > signedness and at least the minimum width." > > > > I don't see anything in this man page saying that the types > > are all typedefs, rather than new types that are distinct from the > > standard integer types. That seems like useful information. > > > > Hi Jonathan, > > I wasn't sure about how to word it. > > In theory, they should be the fastest types; just that. > But then, for some reason, GCC decided that > int_fast8_t should be int8_t instead of int64_t, > because when using arrays of int_fast8_t, > it will create smaller arrays, which will be faster (less cache, etc.). > > (I remember having read that a long time ago, but I don't remember the > source, or if it's the actual reason). So then that's still optimized for "raw performance", isn't it? The "raw performance" of copying an array of bytes is better than the "raw performance" of copying an array of 64-bit types. The meaning of "raw performance" depends on what you're doing, so I don't think it's a useful term without context. > How would you word that? I gave a suggestion above. Don't use terms like "raw performance" that are meaningless without context. Using "no single type that is fastest for all purposes" makes it clearer that "fastest" isn't something universally true, it might be fastest for some purposes and not others.