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;
Date:   Mon, 18 Mar 2019 15:54:54 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     David Laight <David.Laight@ACULAB.COM>
Cc:     Nick Desaulniers <ndesaulniers@google.com>,
        Matthias Kaehlcke <mka@chromium.org>,
        Andy Lutomirski <luto@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
        "H . Peter Anvin" <hpa@zytor.com>,
        "x86@kernel.org" <x86@kernel.org>,
        LKML <linux-kernel@vger.kernel.org>,
        Manoj Gupta <manojgupta@chromium.org>,
        Tiancong Wang <tcwang@chromium.org>,
        Stephen Hines <srhines@google.com>,
        "clang-built-linux@googlegroups.com" 
        <clang-built-linux@googlegroups.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc
Message-ID: <20190318145454.GS6058@hirez.programming.kicks-ass.net>
References: <CAG5bF+S6OvBnsaR6UpMCqjDR9_hMo6qRMHGiW+iCaRYQW4C3YA@mail.gmail.com>
 <CAKwvOdkfb_DT2p2vvxK2ye6JpGYB4qqCc8yECU_JEkrc+6HXDA@mail.gmail.com>
 <20190318091428.GI6058@hirez.programming.kicks-ass.net>
 <cccfcd81efcf47a9ad2ddec7219a797d@AcuMS.aculab.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <cccfcd81efcf47a9ad2ddec7219a797d@AcuMS.aculab.com>
User-Agent: Mutt/1.10.1 (2018-07-13)
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Mon, Mar 18, 2019 at 02:43:41PM +0000, David Laight wrote:
> From: Peter Zijlstra
> > Sent: 18 March 2019 09:14
> > On Fri, Mar 15, 2019 at 03:06:37PM -0700, Nick Desaulniers wrote:
> > > On Fri, Mar 15, 2019 at 1:54 PM Matthias Kaehlcke <mka@chromium.org> wrote:
> > > >
> > > > The compiler may emit calls to __lshrti3 from the compiler runtime
> > > > library, which results in undefined references:
> > > >
> > > > arch/x86/kvm/x86.o: In function `mul_u64_u64_shr':
> > > >   include/linux/math64.h:186: undefined reference to `__lshrti3'
> > >
> > > Looks like Clang will emit this at -Oz (but not -O2):
> > > https://godbolt.org/z/w1_2YC
> > 
> > *OMG*, what is that compiler smoking and why do we want that?
> > 
> > It doesn't even do that for "-Os".
> 
> I like the way it moves %edx to %ecx, then %cl to %ecx and finally %ecx back to %edx.
> I'm guessing this is all made worse by the prototype containing 'char' not 'int'.
> 
> I'm sure the register tracking gets worse in every version of gcc.

This is clang, no GCC on that list comes even close to generating
anything as 'brilliant' as that.