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 1BC79C433DB for ; Thu, 11 Feb 2021 12:08:12 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id DCC6D64E9A for ; Thu, 11 Feb 2021 12:08:11 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S230019AbhBKMHa (ORCPT ); Thu, 11 Feb 2021 07:07:30 -0500 Received: from gate.crashing.org ([63.228.1.57]:49270 "EHLO gate.crashing.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S231272AbhBKL4j (ORCPT ); Thu, 11 Feb 2021 06:56:39 -0500 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 Cc: Benjamin Herrenschmidt , Paul Mackerras , Michael Ellerman , npiggin@gmail.com, linuxppc-dev@lists.ozlabs.org, linux-kernel@vger.kernel.org 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 Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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