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[209.132.180.67]) by mx.google.com with ESMTP id l128si4224706pgl.248.2018.04.04.13.59.57; Wed, 04 Apr 2018 14:00:13 -0700 (PDT) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; dkim=pass header.i=@chromium.org header.s=google header.b=nlO3PW4E; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org; dmarc=pass (p=NONE sp=NONE dis=NONE) header.from=chromium.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1752262AbeDDU6v (ORCPT + 99 others); Wed, 4 Apr 2018 16:58:51 -0400 Received: from mail-pl0-f65.google.com ([209.85.160.65]:43273 "EHLO mail-pl0-f65.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752051AbeDDU6u (ORCPT ); Wed, 4 Apr 2018 16:58:50 -0400 Received: by mail-pl0-f65.google.com with SMTP id c21-v6so12237076plz.10 for ; Wed, 04 Apr 2018 13:58:49 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=chromium.org; s=google; h=date:from:to:cc:subject:message-id:references:mime-version :content-disposition:in-reply-to:user-agent; bh=bqaqfZ9MJW/hawIwANrTTj52GCfKVJd8SzqCBjQZ98Q=; b=nlO3PW4ErhJd18N8AONK8GCrklkxUHIlDOHbsPy9OTQNa3T2e8RFph0uiqpcIuxAaM JrMHEv9r0pw4a0Y9GcR5qMESRXQx4HrlbNzu+AYEgr5qJ/aSn/T9uODK4ZLGS81iTITD rDtmCrgYfTTtsTelWsuzCTGlreLTED2RH1K10= X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:date:from:to:cc:subject:message-id:references :mime-version:content-disposition:in-reply-to:user-agent; bh=bqaqfZ9MJW/hawIwANrTTj52GCfKVJd8SzqCBjQZ98Q=; b=V8nK1/QnqKXw8jrS3IVXvv0uf7UxWt3Fbslpg+Y85JprAzBoolvmLFsdPlOus9hhlP PdR0/NgVaRRwkDqApQNa+SnzuaqQWepB9vVa9hkpPvvz9eGozdYZosKuYHHoKHIuqZmI WOPV3N2BqTFzCYw1+mvFm4RDyFGY9sRUhg6K0fpZZNvwpwv/6RAFjyWflhVoLeeBYDyh vKkNjQljyU1b2VrSaJdgJiQwfH8zydTt3V5HxNsCp5W+3/aR84mDgtl8trE0Za87IzXB oK8VA1VtaBRaI45q9lrkjgRpohUDdaSYQZp6YPn1Fn/iAox2jk6Oh3zjYCDMBkjTTFho 97xw== X-Gm-Message-State: AElRT7GUj4ifu9QI9chyB4b7l2m6TWWFe2swXiViV3mF1xVcDQ94B2UB s84ZNyD7ybvSqXXpTSuG2Cwv5Q== X-Received: by 2002:a17:902:7288:: with SMTP id d8-v6mr20115620pll.79.1522875529628; Wed, 04 Apr 2018 13:58:49 -0700 (PDT) Received: from localhost ([2620:0:1000:1501:8e2d:4727:1211:622]) by smtp.gmail.com with ESMTPSA id z14sm9796636pgv.26.2018.04.04.13.58.48 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Wed, 04 Apr 2018 13:58:49 -0700 (PDT) Date: Wed, 4 Apr 2018 13:58:48 -0700 From: Matthias Kaehlcke To: Arnd Bergmann Cc: Peter Zijlstra , Ingo Molnar , Linus Torvalds , Linux Kernel Mailing List , Thomas Gleixner , Andrew Morton , James Y Knight , Chandler Carruth , Stephen Hines , Nick Desaulniers , Kees Cook , Guenter Roeck , Greg Hackmann , Greg Kroah-Hartman Subject: Re: [GIT PULL] x86/build changes for v4.17 Message-ID: <20180404205848.GG87376@google.com> References: <20180402095033.nfzcrmxvpm46dhbl@gmail.com> <20180403085904.GY4082@hirez.programming.kicks-ass.net> <20180403095118.rpf7tj577dppvx7d@gmail.com> <20180403180658.GE87376@google.com> <20180404093007.GI4082@hirez.programming.kicks-ass.net> <20180404191724.GF87376@google.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.9.2 (2017-12-15) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org El Wed, Apr 04, 2018 at 10:33:19PM +0200 Arnd Bergmann ha dit: > On Wed, Apr 4, 2018 at 9:17 PM, Matthias Kaehlcke wrote: > > El Wed, Apr 04, 2018 at 11:30:07AM +0200 Peter Zijlstra ha dit: > > > >> On Tue, Apr 03, 2018 at 11:06:58AM -0700, Matthias Kaehlcke wrote: > >> > >> > Yes, Chrome OS R67 (currently dev, soon beta) will ship a kernel built > >> > with Clang for multiple x86 Chromebooks. > >> > >> But there are still _known_ miscompilations.... > > > > Our compiler team is looking into this (missing option > > -fno-delete-null-pointer-checks) > > Do you know if anyone is looking into __builtin_constant_p() > optimization as well? We have a lot of uses of this gcc feature > in the kernel, and if I remember correctly, clang implements > this by basically always returning false for the cases we > are interested in. > > In most cases, this is used to implement a fast-path for a helper > function, so not doing it the same way as gcc just results in > slower execution, but I assume we also have code that behaves > differently on clang compared to gcc because of this. I think I didn't come (knowingly) across that one yet. Could you point me to an instance that could be used as an example in a bug report?