From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_PASS,USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 29ED0C43381 for ; Sat, 30 Mar 2019 23:14:50 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id E137320989 for ; Sat, 30 Mar 2019 23:14:49 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1731003AbfC3XOt (ORCPT ); Sat, 30 Mar 2019 19:14:49 -0400 Received: from gate.crashing.org ([63.228.1.57]:35933 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1730968AbfC3XOt (ORCPT ); Sat, 30 Mar 2019 19:14:49 -0400 Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id x2UNE8OW009809; Sat, 30 Mar 2019 18:14:09 -0500 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id x2UNE5ku009804; Sat, 30 Mar 2019 18:14:05 -0500 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Sat, 30 Mar 2019 18:14:03 -0500 From: Segher Boessenkool To: Michael Cree , George Spelvin , linux-alpha@vger.kernel.org, linux-mips@vger.kernel.org, linux-s390@vger.kernel.org, linuxppc-dev@lists.ozlabs.org Subject: Re: CONFIG_ARCH_SUPPORTS_INT128: Why not mips, s390, powerpc, and alpha? Message-ID: <20190330231402.GM3969@gate.crashing.org> References: <201903291307.x2TD772v013534@sdf.org> <20190329200015.ujmjrvn6ta67h74j@tower> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20190329200015.ujmjrvn6ta67h74j@tower> User-Agent: Mutt/1.4.2.3i Sender: linux-mips-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-mips@vger.kernel.org On Sat, Mar 30, 2019 at 09:00:15AM +1300, Michael Cree wrote: > It does move the umulh inside the loop but that seems sensible since > the use of unlikely() implies that the loop is unlikely to be taken > so on average it would be a good bet to start the calculation of > umulh earlier since it has a few cycles latency to get the result, > and it is pipelined so it can be calculated in the shadow of the > mulq instruction on the same execution unit. That may make sense, but it is not what happens, sorry. It _starts off_ as part of the loop, and it is never moved outside. The only difference between a likely loop and an unlikely loop here I've seen (on all targets I tried) is that with a likely loop the loop target is aligned, while with an unlikely loop it isn't. > On the older CPUs > (before EV6 which are not out-of-order execution) having the umulh > inside the loop may be a net gain. Yes. Similarly, on Power you can often calculate the high mul at the same time as the low mul, for no extra cost. This may be true on many archs. Segher