Messages in this thread | | | Date | Fri, 15 Mar 2019 16:29:35 -0700 | From | Matthias Kaehlcke <> | Subject | Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc |
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On Fri, Mar 15, 2019 at 03:06:37PM -0700, 'Nick Desaulniers' via Clang Built Linux 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?
Our compiler folks who maintain this gcc version dug this up for me. In the gcc sources there is no direct implementation of __lshrti3, I was told it is somehow derived from __lshrdi3.
> > 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);
ok
> > #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)
This signature matches with the gcc documentation (https://gcc.gnu.org/onlinedocs/gccint/Integer-library-routines.html), however the gcc implementation of the function has 128-bit values as input/output, so I guess to meet gcc's expectations the 'long long's need to be changed to TItype.
> > +{ > > + 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?
I put it preemptively, since I know from https://lkml.org/lkml/2019/3/15/669 that the vDSO build is unhappy about EXPORT_SYMBOL. Obviously we can also wait until a vDSO needs to include this file.
Thanks
Matthias
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