On Mon, Jun 25, 2012 at 7:38 PM, Peter Maydell <peter.mayd...@linaro.org> 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 <pro...@gmail.com> > Signed-off-by: Peter Maydell <peter.mayd...@linaro.org> > --- > 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 >