Re: [Qemu-devel] [PATCH v2] bitops.h: Add field32() and field64() functions to extract bitfields

[Avi Kivity]

On 06/27/2012 01:29 PM, Peter Maydell wrote:
> Add field32() and field64() functions which extract a particular
> bit field from a word and return it. Based on an idea by Jia Liu.
> 
>  
> +/**
> + * field64 - return a specified bit field from a uint64_t value
> + * @value: The value to extract the bit field from
> + * @start: The lowest bit in the bit field (numbered from 0)
> + * @length: The length of the bit field
> + *
> + * Returns the value of the bit field extracted from the input value.
> + */
> +static inline uint64_t field64(uint64_t value, int start, int length)
> +{
> +    assert(start >= 0 && start <= 63 && length > 0 && start + length <= 64);
> +    return (value >> start) & (~0ULL >> (64 - length));
> +}
> +

Undefined for length == 64, so better add that to the assert.

I suggest adding the analogous functions for writing.  I believe the
common naming is extract/deposit.

static inline uint64_t deposit64(uint64_t *pvalue, unsigned start,
                                 unsigned length, uint64_t fieldval)
{
    uint64_t mask = (((uint64_t)1 << length) - 1) << start;
    *pvalue = (*pvalue & ~mask) | ((fieldval << start) & mask);
}

Useful for setting a bit to a specific value.

-- 
error compiling committee.c: too many arguments to function