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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)


  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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