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

Matthias Kaehlcke
2019-03-18T23:52:24Z

On Mon, Mar 18, 2019 at 04:44:03PM -0700, hpa@zytor.com wrote:
> On March 18, 2019 3:16:39 PM PDT, Matthias Kaehlcke <mka@chromium.org> wrote:
> >On Mon, Mar 18, 2019 at 02:50:44PM -0700, hpa@zytor.com wrote:
> >> On March 18, 2019 2:31:13 PM PDT, Matthias Kaehlcke
> ><mka@chromium.org> wrote:
> >> >On Fri, Mar 15, 2019 at 01:54:50PM -0700, Matthias Kaehlcke 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'
> >> >>
> >> >> Add a copy of the __lshrti3 libgcc routine (from gcc v4.9.2).
> >> >>
> >> >> Include the function for x86 builds with clang, which is the
> >> >> environment where the above error was observed.
> >> >>
> >> >> Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
> >> >
> >> >With "Revert "kbuild: use -Oz instead of -Os when using clang"
> >> >(https://lore.kernel.org/patchwork/patch/1051932/) the above
> >> >error is fixed, a few comments inline for if the patch is
> >> >resurrected in the future because __lshrti3 is emitted in a
> >> >different context.
> >> >
> >> >> diff --git a/include/linux/libgcc.h b/include/linux/libgcc.h
> >> >> index 32e1e0f4b2d0..a71036471838 100644
> >> >> --- a/include/linux/libgcc.h
> >> >> +++ b/include/linux/libgcc.h
> >> >> @@ -22,15 +22,26 @@
> >> >> #include <asm/byteorder.h>
> >> >>
> >> >> typedef int word_type __attribute__ ((mode (__word__)));
> >> >> +typedef int TItype __attribute__ ((mode (TI)));
> >> >
> >> >Consider using __int128 instead. Definition and use need a
> >> >'defined(__SIZEOF_INT128__)' guard (similar for mode (TI)), since
> >> >these 128 bit types aren't supported on all platforms.
> >> >
> >> >> #ifdef __BIG_ENDIAN
> >> >> struct DWstruct {
> >> >> int high, low;
> >> >> };
> >> >> +
> >> >> +struct DWstruct128 {
> >> >> + long long high, low;
> >> >> +};
> >> >
> >> >This struct isn't needed, struct DWstruct can be used.
> >> >
> >> >> diff --git a/lib/lshrti3.c b/lib/lshrti3.c
> >> >> new file mode 100644
> >> >> index 000000000000..2d2123bb3030
> >> >> --- /dev/null
> >> >> +++ b/lib/lshrti3.c
> >> >> @@ -0,0 +1,31 @@
> >> >> +// SPDX-License-Identifier: GPL-2.0
> >> >> +
> >> >> +#include <linux/export.h>
> >> >> +#include <linux/libgcc.h>
> >> >> +
> >> >> +long long __lshrti3(long long u, word_type b)
> >> >
> >> >use TItype for input/output, which is what gcc does, though the
> >above
> >> >matches the interface in the documentation.
> >> >
> >> >> +{
> >> >> + DWunion128 uu, w;
> >> >> + word_type bm;
> >> >> +
> >> >> + if (b == 0)
> >> >> + return u;
> >> >> +
> >> >> + uu.ll = u;
> >> >> + bm = 64 - b;
> >> >> +
> >> >> + if (bm <= 0) {
> >> >> + w.s.high = 0;
> >> >> + w.s.low = (unsigned long long) uu.s.high >> -bm;
> >> >
> >> >include <linux/types.h> and use u64 instead of unsigned long long.
> >>
> >> Ok, now I'm really puzzled.
> >>
> >> How could we need a 128-bit shift when the prototype only has 64 bits
> >of input?!
> >
> >Good question, this is the code from libgcc:
> >
> >TItype
> >__lshrti3 (TItype u, shift_count_type b)
> >{
> > if (b == 0)
> > return u;
> >
> > const DWunion uu = {.ll = u};
> > const shift_count_type bm = (8 * (8)) - b;
> > DWunion w;
> >
> > if (bm <= 0)
> > {
> > w.s.high = 0;
> > w.s.low = (UDItype) uu.s.high >> -bm;
> > }
> > else
> > {
> > const UDItype carries = (UDItype) uu.s.high << bm;
> >
> > w.s.high = (UDItype) uu.s.high >> b;
> > w.s.low = ((UDItype) uu.s.low >> b) | carries;
> > }
> >
> > return w.ll;
> >}
> >
> >
> >My compiler knowledge is limited, my guess is that the function is a
> >generic implementation, and while a long long is 64-bit wide under
> >Linux it could be 128-bit on other platforms.
>
> Yes, long long is just plain wrong.
>
> How could we end up calling this function on 32 bits?!

We didn't, in this case the function is called in 64-bit code
(arch/x86/kvm/x86.o: In function `mul_u64_u64_shr'), for the 32-bit
vDSO it was __lshrdi3.