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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2025 17:35, Oleksii Kurochko wrote:
> 
> On 3/7/25 1:12 PM, Andrew Cooper wrote:
>> 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.
> 
> Could we really drop *_BYTEORDER?
> 
> For example, LONG_BYTEORDER for Arm could be either 2 or 3 depends on Arm32 
> or Arm64 is used.

The can still #define BITS_PER_LONG to 32 or 64 respectively, can't they?

>>>> 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.
> 
> Does it make sense to ifdef that? Or, at least, before defintion of 
> BITS_PER_BYTE something like:
> #if CHAR_BIT != 8
> #error "CHAR_BIT isn't 8"
> #endif

Where would CHAR_BIT come from? Oh, perhaps you mean __CHAR_BIT__? If all
compilers we can build with supply that value, we could indeed centrally
use either

#define BITS_PER_BYTE __CHAR_BIT__

or

#define BITS_PER_BYTE 8
#if BITS_PER_BYTE != __CHAR_BIT__
# error "..."
#endif

Jan