linux-kernel - Re: [PATCH] lib: Add shared copy of _

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Message-ID: <20190318091428.GI6058@hirez.programming.kicks-ass.net>
Date:   Mon, 18 Mar 2019 10:14:28 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     Nick Desaulniers <ndesaulniers@...gle.com>
Cc:     Matthias Kaehlcke <mka@...omium.org>,
        Andy Lutomirski <luto@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H . Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        LKML <linux-kernel@...r.kernel.org>,
        Manoj Gupta <manojgupta@...omium.org>,
        Tiancong Wang <tcwang@...omium.org>,
        Stephen Hines <srhines@...gle.com>,
        clang-built-linux@...glegroups.com,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
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 <mka@...omium.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.