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    SubjectRe: [PATCH] x86: bitops: fix build regression
    On Thu, May 7, 2020 at 3:02 AM <hpa@zytor.com> wrote:
    >
    > On May 6, 2020 11:18:09 PM PDT, Brian Gerst <brgerst@gmail.com> wrote:
    > >On Tue, May 5, 2020 at 1:47 PM Nick Desaulniers
    > ><ndesaulniers@google.com> wrote:
    > >>
    > >> From: Sedat Dilek <sedat.dilek@gmail.com>
    > >>
    > >> It turns out that if your config tickles __builtin_constant_p via
    > >> differences in choices to inline or not, this now produces invalid
    > >> assembly:
    > >>
    > >> $ cat foo.c
    > >> long a(long b, long c) {
    > >> asm("orb\t%1, %0" : "+q"(c): "r"(b));
    > >> return c;
    > >> }
    > >> $ gcc foo.c
    > >> foo.c: Assembler messages:
    > >> foo.c:2: Error: `%rax' not allowed with `orb'
    > >>
    > >> The "q" constraint only has meanting on -m32 otherwise is treated as
    > >> "r".
    > >>
    > >> This is easily reproducible via
    > >Clang+CONFIG_STAGING=y+CONFIG_VT6656=m,
    > >> or Clang+allyesconfig.
    > >>
    > >> Keep the masking operation to appease sparse (`make C=1`), add back
    > >the
    > >> cast in order to properly select the proper 8b register alias.
    > >>
    > >> [Nick: reworded]
    > >>
    > >> Cc: stable@vger.kernel.org
    > >> Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>
    > >> Link: https://github.com/ClangBuiltLinux/linux/issues/961
    > >> Link:
    > >https://lore.kernel.org/lkml/20200504193524.GA221287@google.com/
    > >> Fixes: 1651e700664b4 ("x86: Fix bitops.h warning with a moved cast")
    > >> Reported-by: Sedat Dilek <sedat.dilek@gmail.com>
    > >> Reported-by: kernelci.org bot <bot@kernelci.org>
    > >> Suggested-by: Andy Shevchenko <andriy.shevchenko@intel.com>
    > >> Suggested-by: Ilie Halip <ilie.halip@gmail.com>
    > >> Tested-by: Sedat Dilek <sedat.dilek@gmail.com>
    > >> Signed-off-by: Sedat Dilek <sedat.dilek@gmail.com>
    > >> Signed-off-by: Nick Desaulniers <ndesaulniers@google.com>
    > >> ---
    > >> arch/x86/include/asm/bitops.h | 4 ++--
    > >> 1 file changed, 2 insertions(+), 2 deletions(-)
    > >>
    > >> diff --git a/arch/x86/include/asm/bitops.h
    > >b/arch/x86/include/asm/bitops.h
    > >> index b392571c1f1d..139122e5b25b 100644
    > >> --- a/arch/x86/include/asm/bitops.h
    > >> +++ b/arch/x86/include/asm/bitops.h
    > >> @@ -54,7 +54,7 @@ arch_set_bit(long nr, volatile unsigned long *addr)
    > >> if (__builtin_constant_p(nr)) {
    > >> asm volatile(LOCK_PREFIX "orb %1,%0"
    > >> : CONST_MASK_ADDR(nr, addr)
    > >> - : "iq" (CONST_MASK(nr) & 0xff)
    > >> + : "iq" ((u8)(CONST_MASK(nr) & 0xff))
    > >
    > >I think a better fix would be to make CONST_MASK() return a u8 value
    > >rather than have to cast on every use.
    > >
    > >Also I question the need for the "q" constraint. It was added in
    > >commit 437a0a54 as a workaround for GCC 3.4.4. Now that the minimum
    > >GCC version is 4.6, is this still necessary?
    > >
    > >--
    > >Brian Gerst
    >
    > Yes, "q" is needed on i386.

    I think the bug this worked around was that the compiler didn't detect
    that CONST_MASK(nr) was also constant and doesn't need to be put into
    a register. The question is does that bug still exist on compiler
    versions we care about?

    --
    Brian Gerst

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