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

[Blue Swirl]

On Tue, Jun 26, 2012 at 6:41 PM, Peter Maydell <address@hidden> wrote:
> On 26 June 2012 19:25, Blue Swirl <address@hidden> wrote:
>> On Tue, Jun 26, 2012 at 6:11 PM, Peter Maydell <address@hidden> wrote:
>>> On 26 June 2012 18:58, Blue Swirl <address@hidden> wrote:
>>>> On Mon, Jun 25, 2012 at 7:38 PM, Peter Maydell <address@hidden> wrote:
>>>>> +static inline uint64_t field64(uint64_t value, int start, int length)
>>>>
>>>> start and length could be unsigned.
>>>
>>> They could be, but is there any reason why they should be?
>>> set_bit(), clear_bit() etc use 'int' for bit numbers, so this
>>> is consistent with that.
>>
>> Negative shifts don't work, the line with assert() would get shorter
>> and simpler and I like unsigned values.
>
> I don't like using unsigned for numbers that merely happen to always
> be positive (as opposed to actually requiring unsigned arithmetic)[*],
> so I think I'll stick with being consistent with the existing bitops
> functions, thanks :-)

Using unsigned types also produces better code in some cases. There
are also operations which do not work well with signed integers (%,
>>).

>
> [*] the classic example of where that kind of thing can trip you up
> is the way it complicates the termination condition on a "for (i = N;
> i >= 0; i--)" decreasing loop.
>
> -- PMM