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    Subject[PATCH 4.2.y-ckt 33/59] powerpc: Fix bad inline asm constraint in create_zero_mask()
    Date
    4.2.8-ckt10 -stable review patch.  If anyone has any objections, please let me know.

    ---8<------------------------------------------------------------

    From: Anton Blanchard <anton@samba.org>

    commit b4c112114aab9aff5ed4568ca5e662bb02cdfe74 upstream.

    In create_zero_mask() we have:

    addi %1,%2,-1
    andc %1,%1,%2
    popcntd %0,%1

    using the "r" constraint for %2. r0 is a valid register in the "r" set,
    but addi X,r0,X turns it into an li:

    li r7,-1
    andc r7,r7,r0
    popcntd r4,r7

    Fix this by using the "b" constraint, for which r0 is not a valid
    register.

    This was found with a kernel build using gcc trunk, narrowed down to
    when -frename-registers was enabled at -O2. It is just luck however
    that we aren't seeing this on older toolchains.

    Thanks to Segher for working with me to find this issue.

    Fixes: d0cebfa650a0 ("powerpc: word-at-a-time optimization for 64-bit Little Endian")
    Signed-off-by: Anton Blanchard <anton@samba.org>
    Signed-off-by: Michael Ellerman <mpe@ellerman.id.au>
    Signed-off-by: Kamal Mostafa <kamal@canonical.com>
    ---
    arch/powerpc/include/asm/word-at-a-time.h | 2 +-
    1 file changed, 1 insertion(+), 1 deletion(-)

    diff --git a/arch/powerpc/include/asm/word-at-a-time.h b/arch/powerpc/include/asm/word-at-a-time.h
    index 5b3a903..7043539 100644
    --- a/arch/powerpc/include/asm/word-at-a-time.h
    +++ b/arch/powerpc/include/asm/word-at-a-time.h
    @@ -77,7 +77,7 @@ static inline unsigned long create_zero_mask(unsigned long bits)
    "andc %1,%1,%2\n\t"
    "popcntd %0,%1"
    : "=r" (leading_zero_bits), "=&r" (trailing_zero_bit_mask)
    - : "r" (bits));
    + : "b" (bits));

    return leading_zero_bits;
    }
    --
    2.7.4
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