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

David Laight
2019-03-18T14:42:48Z

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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