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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 25.03.2025 18:04, Oleksii Kurochko wrote:
> 
> On 3/25/25 5:46 PM, Jan Beulich wrote:
>> 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?
> 
> Yes, but then if we want to move it to xen/config.h then it will be needed to:
> in Arm's asm/config.h to have something like:
>    #ifdef CONFIG_ARM_32
>        #define BITS_PER_LONG 32
>    #endif
> 
> and then in xen/config.h
>    #ifndef BITS_PER_LONG
>        #define BITS_PER_LONG 64
>    #endif
> 
> But I wanted to not have #ifndef BITS_PER_LONG in xen/config.h. (or using 
> CONFIG_ARM_32 in xen/config.h)

Whatever the fundamental definitions that can vary per arch - those should
imo live in asm/config.h. For the case here, if we deem *_BYTEORDER as
fundamental, they go there (and BITS_PER_* go into xen/config.h). If we deem
BITS_PER_* fundamental, they go into asm/config.h.

Stuff that we expect to remain uniform across ports (BITS_PER_BYTE, possibly
also BITS_PER_INT BITS_PER_LLONG) may also go into xen/config.h, as long as
we're okay with the possible future churn if a port appeared that has
different needs.

I agree that there better wouldn't be #ifndef for such in xen/config.h.

Jan