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From: Peter Maydell <peter.maydell@linaro.org>
Date: Thu, 9 Jun 2016 19:46:17 +0100
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Subject: Re: [Qemu-devel] [PATCH v6 01/13] bitops: Add MAKE_64BIT_MASK macro
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To: Alistair Francis <alistair.francis@xilinx.com>
Cc: QEMU Developers <qemu-devel@nongnu.org>, Peter Crosthwaite <crosthwaitepeter@gmail.com>, Edgar Iglesias <edgar.iglesias@xilinx.com>, "Edgar E. Iglesias" <edgar.iglesias@gmail.com>, =?UTF-8?Q?Andreas_F=C3=A4rber?= <afaerber@suse.de>, =?UTF-8?B?S09OUkFEIEZyw6lkw6lyaWM=?= <fred.konrad@greensocs.com>, =?UTF-8?B?QWxleCBCZW5uw6ll?= <alex.bennee@linaro.org>

On 12 May 2016 at 23:45, Alistair Francis <alistair.francis@xilinx.com> wro=
te:
> 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 <alistair.francis@xilinx.com>
> Reviewed-by: Alex Benn=C3=A9e <alex.bennee@linaro.org>
> ---
> 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