On March 18, 2019 3:16:39 PM PDT, Matthias Kaehlcke <m...@chromium.org> wrote: >On Mon, Mar 18, 2019 at 02:50:44PM -0700, h...@zytor.com wrote: >> On March 18, 2019 2:31:13 PM PDT, Matthias Kaehlcke ><m...@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 <m...@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?! -- Sent from my Android device with K-9 Mail. Please excuse my brevity.
