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

Nick Desaulniers
2019-03-15T22:06:52Z

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?
--
Thanks,
~Nick Desaulniers