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

[Matthias Kaehlcke <mka@chromium.org>]

Hi Peter,

On Fri, Mar 15, 2019 at 03:08:57PM -0700, hpa@zytor.com wrote:
> On March 15, 2019 3:06:37 PM PDT, Nick Desaulniers <ndesaulniers@google.com> 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
> >
> >>
> >> Add a copy of the __lshrti3 libgcc routine (from gcc v4.9.2).
> >
> >Has it changed since? If so why not a newer version of libgcc_s?
> >
> >>
> >> 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>
> >> ---
> >>  arch/x86/Kconfig       |  1 +
> >>  include/linux/libgcc.h | 16 ++++++++++++++++
> >>  lib/Kconfig            |  3 +++
> >>  lib/Makefile           |  1 +
> >>  lib/lshrti3.c          | 31 +++++++++++++++++++++++++++++++
> >>  5 files changed, 52 insertions(+)
> >>  create mode 100644 lib/lshrti3.c
> >>
> >> diff --git a/arch/x86/Kconfig b/arch/x86/Kconfig
> >> index c1f9b3cf437c..a5e0d923845d 100644
> >> --- a/arch/x86/Kconfig
> >> +++ b/arch/x86/Kconfig
> >> @@ -105,6 +105,7 @@ config X86
> >>         select GENERIC_IRQ_PROBE
> >>         select GENERIC_IRQ_RESERVATION_MODE
> >>         select GENERIC_IRQ_SHOW
> >> +       select GENERIC_LIB_LSHRTI3              if CC_IS_CLANG
> >>         select GENERIC_PENDING_IRQ              if SMP
> >>         select GENERIC_SMP_IDLE_THREAD
> >>         select GENERIC_STRNCPY_FROM_USER
> >> 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)));
> >
> >Well that's an interesting new compiler attribute.
> >https://stackoverflow.com/questions/4559025/what-does-gcc-attribute-modexx-actually-do
> >Please use `__mode(TI)` from include/linux/compiler_attributes.h ex.
> >typedef int TItype __mode(TI);
> >
> >>
> >>  #ifdef __BIG_ENDIAN
> >>  struct DWstruct {
> >>         int high, low;
> >>  };
> >> +
> >> +struct DWstruct128 {
> >> +       long long high, low;
> >> +};
> >> +
> >>  #elif defined(__LITTLE_ENDIAN)
> >>  struct DWstruct {
> >>         int low, high;
> >>  };
> >> +
> >> +struct DWstruct128 {
> >> +       long long low, high;
> >> +};
> >> +
> >>  #else
> >>  #error I feel sick.
> >>  #endif
> >> @@ -40,4 +51,9 @@ typedef union {
> >>         long long ll;
> >>  } DWunion;
> >>
> >> +typedef union {
> >> +       struct DWstruct128 s;
> >> +       TItype ll;
> >> +} DWunion128;
> >> +
> >>  #endif /* __ASM_LIBGCC_H */
> >> diff --git a/lib/Kconfig b/lib/Kconfig
> >> index a9e56539bd11..369e10259ea6 100644
> >> --- a/lib/Kconfig
> >> +++ b/lib/Kconfig
> >> @@ -624,6 +624,9 @@ config GENERIC_LIB_ASHRDI3
> >>  config GENERIC_LIB_LSHRDI3
> >>         bool
> >>
> >> +config GENERIC_LIB_LSHRTI3
> >> +       bool
> >> +
> >>  config GENERIC_LIB_MULDI3
> >>         bool
> >>
> >> diff --git a/lib/Makefile b/lib/Makefile
> >> index 4e066120a0d6..42648411f451 100644
> >> --- a/lib/Makefile
> >> +++ b/lib/Makefile
> >> @@ -277,6 +277,7 @@ obj-$(CONFIG_PARMAN) += parman.o
> >>  obj-$(CONFIG_GENERIC_LIB_ASHLDI3) += ashldi3.o
> >>  obj-$(CONFIG_GENERIC_LIB_ASHRDI3) += ashrdi3.o
> >>  obj-$(CONFIG_GENERIC_LIB_LSHRDI3) += lshrdi3.o
> >> +obj-$(CONFIG_GENERIC_LIB_LSHRTI3) += lshrti3.o
> >>  obj-$(CONFIG_GENERIC_LIB_MULDI3) += muldi3.o
> >>  obj-$(CONFIG_GENERIC_LIB_CMPDI2) += cmpdi2.o
> >>  obj-$(CONFIG_GENERIC_LIB_UCMPDI2) += ucmpdi2.o
> >> 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)
> >> +{
> >> +       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;
> >> +       } else {
> >> +               const unsigned long long carries =
> >> +                       (unsigned long long) uu.s.high << bm;
> >> +               w.s.high = (unsigned long long) uu.s.high >> b;
> >> +               w.s.low = ((unsigned long long) uu.s.low >> b) |
> >carries;
> >> +       }
> >> +
> >> +       return w.ll;
> >> +}
> >> +#ifndef BUILD_VDSO
> >> +EXPORT_SYMBOL(__lshrti3);
> >> +#endif
> >
> >I don't think you want this.  Maybe that was carried over from
> >https://lkml.org/lkml/2019/3/15/669
> >by accident?  The above linkage error mentions arch/x86/kvm/x86.o
> >which I wouldn't expect to be linked into the VDSO image?
> 
> If we compile with the default ABI we can simply link with libgcc;
> with -mregparm=3 all versions of gcc are broken.

I suppose you refer to linking the VDSO against libgcc? Would this be
acceptable (as opposed to linking the kernel against it)?