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

[malc]

On Mon, 25 Jun 2012, 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.
> 
> Suggested-by: Jia Liu <address@hidden>
> Signed-off-by: Peter Maydell <address@hidden>
> ---

[..snip..]

> +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));
> +}
> +
> +/**
> + * field32 - return a specified bit field from a uint32_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 uint32_t field32(uint32_t value, int start, int length)
> +{
> +    assert(start >= 0 && start <= 31 && length > 0 && start + length <= 32);
> +    return (value >> start) & ~0U >> (32 - length);
> +}
> +
>  #endif
> 

The field32 and field64 are inconsistent w.r.t. grouping, while both are
correct, one has to go through precedence list mentally in the latter
case.

-- 
mailto:address@hidden