Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc

Peter Zijlstra
2019-03-18T14:55:17Z

On Mon, Mar 18, 2019 at 02:43:41PM +0000, David Laight wrote:
> 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.

This is clang, no GCC on that list comes even close to generating
anything as 'brilliant' as that.