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Date:   Mon, 18 Mar 2019 10:14:28 +0100
From:   Peter Zijlstra <peterz@infradead.org>
To:     Nick Desaulniers <ndesaulniers@google.com>
Cc:     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,
        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,
        Miguel Ojeda <miguel.ojeda.sandonis@gmail.com>
Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc
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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".

So where "-Os" is "Optimize for Sadness" this "-Oz" thing is like a
downright depression. Please just take it out back. Don't enable crap
like this.