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Mon, 18 Mar 2019 16:52:20 -0700 (PDT) Received: from localhost ([2620:15c:202:1:75a:3f6e:21d:9374]) by smtp.gmail.com with ESMTPSA id e8sm15134594pfn.103.2019.03.18.16.52.20 (version=TLS1_2 cipher=ECDHE-RSA-CHACHA20-POLY1305 bits=256/256); Mon, 18 Mar 2019 16:52:20 -0700 (PDT) Date: Mon, 18 Mar 2019 16:52:19 -0700 From: Matthias Kaehlcke To: hpa@zytor.com Cc: Andy Lutomirski , Thomas Gleixner , Ingo Molnar , Borislav Petkov , x86@kernel.org, linux-kernel@vger.kernel.org, Nick Desaulniers , Manoj Gupta , Tiancong Wang , Stephen Hines , clang-built-linux@googlegroups.com Subject: Re: [PATCH] lib: Add shared copy of __lshrti3 from libgcc Message-ID: <20190318235219.GL112750@google.com> References: <20190318213113.GI112750@google.com> <25133BBA-9121-4B3A-865B-085BFC5F154C@zytor.com> <20190318221639.GK112750@google.com> <278CBEF0-9D09-4091-A630-73D912587383@zytor.com> MIME-Version: 1.0 Content-Type: text/plain; charset=utf-8 Content-Disposition: inline In-Reply-To: <278CBEF0-9D09-4091-A630-73D912587383@zytor.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 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 > > 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 > >> > > >> >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 > >> >> > >> >> 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 > >> >> +#include > >> >> + > >> >> +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 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.