Messages in this thread | | | Date | Mon, 18 Mar 2019 15:54:54 +0100 | From | Peter Zijlstra <> | Subject | Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc |
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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.
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