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

[Blue Swirl]

On Mon, Jun 25, 2012 at 7:38 PM, Peter Maydell <address@hidden> 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>
> ---
> Jia Liu had a function like this in the OpenRISC support patchset;
> this implementation borrows the API but has a different implementation,
> because I wanted to handle the length == wordsize case.
> I've also provided a 64 bit version as well as a 32 bit one (alas, gcc
> is not smart enough to notice that it only needs to do 32 bit arithmetic
> if you pass in a uint32_t to the 64 bit function).
> Based on previous experience with a different codebase I think that this
> will result in much more comprehensible code than manual shift-and-mask
> which is what we tend to do today.
>
> No users yet, but I wanted to throw this out for review anyway. If
> people really don't want it until it gets a first user I can throw it
> into my next random patchset that does bit ops...

I like this.

>
>  bitops.h |   28 ++++++++++++++++++++++++++++
>  1 files changed, 28 insertions(+), 0 deletions(-)
>
> diff --git a/bitops.h b/bitops.h
> index 07d1a06..36e4c78 100644
> --- a/bitops.h
> +++ b/bitops.h
> @@ -269,4 +269,32 @@ static inline unsigned long hweight_long(unsigned long w)
>     return count;
>  }
>
> +/**
> + * 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)

start and length could be unsigned.

> +{
> +    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
> --
> 1.7.1
>