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 <m...@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 - Registered Address Lakeside, Bramley Road, Mount Farm, Milton Keynes, MK1 1PT, UK Registration No: 1397386 (Wales)