Messages in this thread | | | Date | Mon, 18 Mar 2019 16:55:38 -0700 | Subject | Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc | From | hpa@zytor ... |
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On March 18, 2019 4:52:19 PM PDT, Matthias Kaehlcke <mka@chromium.org> wrote: >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.
Again, for 64 bits we can include libgcc in the link (with --no-whole-archive). -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
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