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Date:   Mon, 18 Mar 2019 10:14:28 +0100
From:   Peter Zijlstra <>
To:     Nick Desaulniers <>
Cc:     Matthias Kaehlcke <>,
        Andy Lutomirski <>,
        Thomas Gleixner <>,
        Ingo Molnar <>, Borislav Petkov <>,
        "H . Peter Anvin" <>,,
        LKML <>,
        Manoj Gupta <>,
        Tiancong Wang <>,
        Stephen Hines <>,,
        Miguel Ojeda <>
Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc

On Fri, Mar 15, 2019 at 03:06:37PM -0700, Nick Desaulniers wrote:
> On Fri, Mar 15, 2019 at 1:54 PM Matthias Kaehlcke <> 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):

*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.

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