Re: [PATCH] RISCV/bitops: Use Zbb to provide arch-optimised bitops

[Jan Beulich <jbeulich@xxxxxxxx>]

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
> 
> 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)

> #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".

Jan