From mboxrd@z Thu Jan  1 00:00:00 1970
Return-Path: <linux-kernel-owner@vger.kernel.org>
Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand
	id S1752847AbbINHMn (ORCPT <rfc822;w@1wt.eu>);
	Mon, 14 Sep 2015 03:12:43 -0400
Received: from bombadil.infradead.org ([198.137.202.9]:34140 "EHLO
	bombadil.infradead.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org
	with ESMTP id S1750801AbbINHMm (ORCPT
	<rfc822;linux-kernel@vger.kernel.org>);
	Mon, 14 Sep 2015 03:12:42 -0400
Date: Mon, 14 Sep 2015 09:12:31 +0200
From: Peter Zijlstra <peterz@infradead.org>
To: Sedat Dilek <sedat.dilek@gmail.com>
Cc: Baoquan He <bhe@redhat.com>, Denys Vlasenko <dvlasenk@redhat.com>,
        Tejun Heo <tj@kernel.org>, Christoph Lameter <cl@linux.com>,
        LKML <linux-kernel@vger.kernel.org>,
        Andrew Morton <akpm@linux-foundation.org>,
        David Rientjes <rientjes@google.com>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Thomas Gleixner <tglx@linutronix.de>, Thomas Graf <tgraf@suug.ch>,
        Ingo Molnar <mingo@kernel.org>,
        the arch/x86 maintainers <x86@kernel.org>
Subject: Re: [llvmlinux] percpu | bitmap issue? (Cannot boot on bare metal
 due to a kernel NULL pointer dereference)
Message-ID: <20150914071231.GM18489@twins.programming.kicks-ass.net>
References: <CA+icZUUgVEaYmToXd6j4sHT2PW3CH++B7DcO-Mj48W2rBD6yAQ@mail.gmail.com>
 <20150909032541.GC1998@dhcp-17-102.nay.redhat.com>
 <CA+icZUWOcDAeu+e7aP9J5ZwpKOgjuMKtJyeM8McHZe33EM65Og@mail.gmail.com>
 <CA+icZUXNWWHrx4mgzudafMwq1HOZd5FZY2QTicY5a-eYF=D8Kg@mail.gmail.com>
 <20150909071410.GD1998@dhcp-17-102.nay.redhat.com>
 <CA+icZUVTHJGkg9mTs3TSy3U4VX_cTyNpwNeX9R+73Y9=ZD_E0A@mail.gmail.com>
 <CA+icZUWo1-7Kqo36Dp-pjjq1hUdiqvwhZhvaXM1RwA8mqter_A@mail.gmail.com>
 <20150909125424.GP3644@twins.programming.kicks-ass.net>
 <CA+icZUUJOsoon2Vk09yu6Z=p2N8mpHkVnp=TZyfSxpKKw_x2iQ@mail.gmail.com>
 <CA+icZUU=qAJ7xqYs_k=NNG-Sj3Xv770MUNToA4JqiO3rAzUxNA@mail.gmail.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <CA+icZUU=qAJ7xqYs_k=NNG-Sj3Xv770MUNToA4JqiO3rAzUxNA@mail.gmail.com>
User-Agent: Mutt/1.5.21 (2012-12-30)
Sender: linux-kernel-owner@vger.kernel.org
List-ID: <linux-kernel.vger.kernel.org>
X-Mailing-List: linux-kernel@vger.kernel.org

On Sun, Sep 13, 2015 at 04:33:39AM +0200, Sedat Dilek wrote:
> > It looks like an inline-optimization bug in CLANG when the compiler's
> > optimization-level is higher than -O2.

> > [1] http://lists.linuxfoundation.org/pipermail/llvmlinux/2015-September/001355.html
> 
> After some discussion on #llvm it turned out to be a known issue in LLVMLinux!
> 
> Unfortunately, an existing patch [1] got archived which is still
> required to build x86_64 correctly.

> [1] http://git.linuxfoundation.org/?p=llvmlinux.git;a=blob_plain;f=arch/x86_64/patches/ARCHIVE/0029-Fix-ARCH_HWEIGHT-for-compilation-with-clang.patch;hb=HEAD

As long as LLVM cannot do things like that and requires full function
calls I cannot see it being a sensible compiler to use from a
performance POV.

There's a fairly large difference between an inline POPCNT instruction
and a full out-of-line function call.

/me goes back to ignoring LLVM for the time being.