Re: [Qemu-devel] [PATCH v6 01/13] bitops: Add MAKE_64BIT_MASK macro

[Peter Maydell]

On 12 May 2016 at 23:45, Alistair Francis <address@hidden> wrote:
> Add a macro that creates a 64bit value which has length number of ones
> shifted acrros by the value of shift.

"across"

>
> Signed-off-by: Alistair Francis <address@hidden>
> Reviewed-by: Alex Bennée <address@hidden>
> ---
> V5:
>  - Re-write to a 64-bit mask instead of ONES()
>  - Re-order this patch in the series
>
>  include/qemu/bitops.h | 3 +++
>  1 file changed, 3 insertions(+)
>
> diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
> index 755fdd1..3c45791 100644
> --- a/include/qemu/bitops.h
> +++ b/include/qemu/bitops.h
> @@ -24,6 +24,9 @@
>  #define BIT_WORD(nr)            ((nr) / BITS_PER_LONG)
>  #define BITS_TO_LONGS(nr)       DIV_ROUND_UP(nr, BITS_PER_BYTE * 
> sizeof(long))
>
> +#define MAKE_64BIT_MASK(shift, length) \
> +    (((1ull << (length)) - 1) << shift)
> +

This is undefined behaviour for a 64-bit length. The expression
we use in deposit64() to create a mask is
   ((~0ULL >> (64 - length)) << start)

thanks
-- PMM