Re: [PATCH 1/2] include: bitops: Add mask extract64/deposit64

[Richard Henderson <richard.henderson@linaro.org>]

On 8/5/24 14:33, Alistair Francis wrote:
> Based on the RISC-V get_field() and set_field() macros add
> mask_extract64() and mask_deposit64() bitop functions. These can extrac
> and deposit values into fields using a bit field mask directly instead
> of a length and shift.
> 
> Signed-off-by: Alistair Francis <alistair.francis@wdc.com>
> ---
>   include/qemu/bitops.h | 35 +++++++++++++++++++++++++++++++++++
>   1 file changed, 35 insertions(+)
> 
> diff --git a/include/qemu/bitops.h b/include/qemu/bitops.h
> index 2c0a2fe751..dd26f4a6b5 100644
> --- a/include/qemu/bitops.h
> +++ b/include/qemu/bitops.h
> @@ -409,6 +409,22 @@ static inline uint64_t extract64(uint64_t value, int start, int length)
>       return (value >> start) & (~0ULL >> (64 - length));
>   }
>   
> +/**
> + * mask_extract64:
> + * @value: the value to extract the bit field from
> + * @mask: the mask bit field to extract
> + *
> + * Extract from the 64 bit input @value the bit mask specified by the
> + * @mask parameter, and return it. The value returned is shifted
> + * so that only the bit field is returned.
> + *
> + * Returns: the value of the bit field extracted from the input value.
> + */
> +static inline uint64_t mask_extract64(uint64_t value, uint64_t mask)
> +{
> +    return (value & mask) / (mask & ~(mask << 1));
> +}

Adding these miss the point of using "standard" qemu operations.

But if we were going to add this, avoid the division.

  (value & mask) >> ctz64(mask)

I presume the original formulation is so that the macro can be used in the context of a 
compile-time constant.


r~