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

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