From: David Laight <David.Laight@ACULAB.COM>
To: 'Peter Zijlstra' <peterz@infradead.org>,
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" <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
Date: Mon, 18 Mar 2019 14:43:41 +0000 [thread overview]
Message-ID: <cccfcd81efcf47a9ad2ddec7219a797d@AcuMS.aculab.com> (raw)
In-Reply-To: <20190318091428.GI6058@hirez.programming.kicks-ass.net>
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.
David
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Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK
Registration No: 1397386 (Wales)
next prev parent reply other threads:[~2019-03-18 14:42 UTC|newest]
Thread overview: 21+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-03-15 20:54 [PATCH] lib: Add shared copy of __lshrti3 from libgcc Matthias Kaehlcke
2019-03-15 22:06 ` Nick Desaulniers
2019-03-15 22:08 ` hpa
2019-03-15 23:34 ` Matthias Kaehlcke
2019-03-15 23:51 ` hpa
2019-03-15 22:12 ` hpa
2019-03-15 23:47 ` Matthias Kaehlcke
2019-03-15 23:53 ` hpa
2019-03-18 16:38 ` Matthias Kaehlcke
2019-03-15 23:29 ` Matthias Kaehlcke
2019-03-18 9:14 ` Peter Zijlstra
2019-03-18 14:43 ` David Laight [this message]
2019-03-18 14:54 ` Peter Zijlstra
2019-03-18 21:31 ` Matthias Kaehlcke
2019-03-18 21:50 ` hpa
2019-03-18 22:16 ` Matthias Kaehlcke
2019-03-18 23:44 ` hpa
2019-03-18 23:52 ` Matthias Kaehlcke
2019-03-18 23:54 ` hpa
2019-03-18 23:55 ` hpa
2019-03-19 17:10 ` Matthias Kaehlcke
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