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=-4.2 required=3.0 tests=BAYES_00, HEADER_FROM_DIFFERENT_DOMAINS,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS, TVD_SUBJ_WIPE_DEBT,USER_AGENT_SANE_1 autolearn=no 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 3D06CC433DB for ; Thu, 11 Feb 2021 11:57:51 +0000 (UTC) Received: from lists.ozlabs.org (lists.ozlabs.org [203.11.71.2]) (using TLSv1.2 with cipher ECDHE-RSA-AES256-GCM-SHA384 (256/256 bits)) (No client certificate requested) by mail.kernel.org (Postfix) with ESMTPS id D69DB64E13 for ; Thu, 11 Feb 2021 11:57:47 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org D69DB64E13 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=kernel.crashing.org Authentication-Results: mail.kernel.org; spf=pass smtp.mailfrom=linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Received: from bilbo.ozlabs.org (lists.ozlabs.org [IPv6:2401:3900:2:1::3]) by lists.ozlabs.org (Postfix) with ESMTP id 4Dbw9h5JtKzDwh5 for ; Thu, 11 Feb 2021 22:57:44 +1100 (AEDT) Authentication-Results: lists.ozlabs.org; spf=permerror (SPF Permanent Error: Unknown mechanism found: ip:192.40.192.88/32) smtp.mailfrom=kernel.crashing.org (client-ip=63.228.1.57; helo=gate.crashing.org; envelope-from=segher@kernel.crashing.org; receiver=) Received: from gate.crashing.org (gate.crashing.org [63.228.1.57]) by lists.ozlabs.org (Postfix) with ESMTP id 4Dbw3m2bhvzDvX9 for ; Thu, 11 Feb 2021 22:52:35 +1100 (AEDT) Received: from gate.crashing.org (localhost.localdomain [127.0.0.1]) by gate.crashing.org (8.14.1/8.14.1) with ESMTP id 11BBnBgY023491; Thu, 11 Feb 2021 05:49:11 -0600 Received: (from segher@localhost) by gate.crashing.org (8.14.1/8.14.1/Submit) id 11BBnA8Q023484; Thu, 11 Feb 2021 05:49:10 -0600 X-Authentication-Warning: gate.crashing.org: segher set sender to segher@kernel.crashing.org using -f Date: Thu, 11 Feb 2021 05:49:10 -0600 From: Segher Boessenkool To: Christophe Leroy Subject: Re: [PATCH] powerpc/bug: Remove specific powerpc BUG_ON() Message-ID: <20210211114910.GA28121@gate.crashing.org> References: <694c7195c81d1bcc781b3c14f452886683d6c524.1613029237.git.christophe.leroy@csgroup.eu> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <694c7195c81d1bcc781b3c14f452886683d6c524.1613029237.git.christophe.leroy@csgroup.eu> User-Agent: Mutt/1.4.2.3i X-BeenThere: linuxppc-dev@lists.ozlabs.org X-Mailman-Version: 2.1.29 Precedence: list List-Id: Linux on PowerPC Developers Mail List List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , Cc: linux-kernel@vger.kernel.org, npiggin@gmail.com, Paul Mackerras , linuxppc-dev@lists.ozlabs.org Errors-To: linuxppc-dev-bounces+linuxppc-dev=archiver.kernel.org@lists.ozlabs.org Sender: "Linuxppc-dev" On Thu, Feb 11, 2021 at 07:41:52AM +0000, Christophe Leroy wrote: > powerpc BUG_ON() is based on using twnei or tdnei instruction, > which obliges gcc to format the condition into a 0 or 1 value > in a register. Huh? Why is that? Will it work better if this used __builtin_trap? Or does the kernel only detect very specific forms of trap instructions? > By using a generic implementation, gcc will generate a branch > to the unconditional trap generated by BUG(). That is many more instructions than ideal. > As modern powerpc implement branch folding, that's even more efficient. What PowerPC cpus implement branch folding? I know none. Some example code generated via __builtin_trap: void trap(void) { __builtin_trap(); } void trap_if_0(int x) { if (x == 0) __builtin_trap(); } void trap_if_not_0(int x) { if (x != 0) __builtin_trap(); } -m64: trap: trap trap_if_0: tdeqi 3,0 blr trap_if_not_0: tdnei 3,0 blr -m32: trap: trap trap_if_0: tweqi 3,0 blr trap_if_not_0: twnei 3,0 blr Segher