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

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Message-ID: <cccfcd81efcf47a9ad2ddec7219a797d@AcuMS.aculab.com>
Date:   Mon, 18 Mar 2019 14:43:41 +0000
From:   David Laight <David.Laight@...LAB.COM>
To:     'Peter Zijlstra' <peterz@...radead.org>,
        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" <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" 
        <clang-built-linux@...glegroups.com>,
        Miguel Ojeda <miguel.ojeda.sandonis@...il.com>
Subject: RE: [PATCH] lib: Add shared copy of __lshrti3 from libgcc

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@...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".

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.

	David

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