On 07/03/2025 12:01 pm, Jan Beulich wrote:
> On 07.03.2025 12:50, Oleksii Kurochko wrote:
>> On 3/6/25 9:19 PM, Andrew Cooper wrote:
>>> On 05/03/2025 7:34 am, Jan Beulich wrote:
>>>> I was actually hoping to eliminate BITS_PER_LONG at some point, in favor
>>>> of using sizeof(long) * BITS_PER_BYTE. (Surely in common code we could
>>>> retain a shorthand of that name, if so desired, but I see no reason why
>>>> each arch would need to provide all three BITS_PER_{BYTE,INT,LONG}.)
>>> The concern is legibility and clarity.
>>>
>>> This:
>>>
>>> ((x) ? 32 - __builtin_clz(x) : 0)
>>>
>>> is a clear expression in a way that this:
>>>
>>> ((x) ? (sizeof(int) * BITS_PER_BYTE) - __builtin_clz(x) : 0)
>>>
>>> is not. The problem is the extra binary expression, and this:
>>>
>>> ((x) ? BITS_PER_INT - __builtin_clz(x) : 0)
>>>
>>> is still clear, because the reader doesn't have to perform a multiply to
>>> just to figure out what's going on.
>>>
>>>
>>> It is definitely stupid to have each architecture provide their own
>>> BITS_PER_*. The compiler is in a superior position to provide those
>>> details, and it should be in a common location.
>>>
>>> I don't particularly mind how those constants are derived, but one key
>>> thing that BITS_PER_* can do which sizeof() can't is be used in #ifdef/etc.
>> What about moving them to xen/config.h? (if it isn't the best one place, any
>> suggestion which is better?)
>>
>> #define BYTES_PER_INT (1 << INT_BYTEORDER)
>> #define BITS_PER_INT (BYTES_PER_INT << 3)
>>
>> #define BYTES_PER_LONG (1 << LONG_BYTEORDER)
>> #define BITS_PER_LONG (BYTES_PER_LONG << 3)
>> #define BITS_PER_BYTE 8
The *_BYTEORDER's are useless indirection. BITS_PER_* should be defined
straight up, and this will simplify quite a lot of preprocessing.
>>
>> Also, it seems like the follwoing could be moved there too:
>>
>> #define POINTER_ALIGN BYTES_PER_LONG
> This one is likely fine to move.
>
>> #define BITS_PER_LLONG 64
> This one is only fine to move imo when converted to
>
> #define BITS_PER_LONG (BYTES_PER_LLONG << 3)
Erm? That's mixing long and long long types. Presuming that's an
errant L, then sure.
>
>> #define BITS_PER_BYTE 8
> Personally I'd rather leave this per-arch. The others can truly be derived;
> this one can't be. If we centralize, imo we should also convert the " << 3"
> to " * BITS_PER_BYTE".
It is highly unlikely that Xen will ever run on a system where CHAR_BIT
isn't 8.
So I suggest it stays central to reduce complexity. If an arch ever
needs to change it, the complexity can be added then.
~Andrew
