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

[Jan Beulich <jbeulich@xxxxxxxx>]

On 06.03.2025 21:19, Andrew Cooper wrote:
> On 05/03/2025 7:34 am, Jan Beulich wrote:
>> On 28.02.2025 17:24, Andrew Cooper wrote:
>>> On 27/02/2025 8:11 am, Jan Beulich wrote:
>>>> On 26.02.2025 18:20, Andrew Cooper wrote:
>>>>> --- a/xen/arch/riscv/include/asm/bitops.h
>>>>> +++ b/xen/arch/riscv/include/asm/bitops.h
>>>>> @@ -125,6 +125,13 @@ static inline void clear_bit(int nr, volatile void 
>>>>> *p)
>>>>>  #undef NOT
>>>>>  #undef __AMO
>>>>>  
>>>>> +#define arch_ffs(x)     ((x) ? 1 + __builtin_ctz(x) : 0)
>>>>> +#define arch_ffsl(x)    ((x) ? 1 + __builtin_ctzl(x) : 0)
>>>>> +#define arch_fls(x)     ((x) ? 32 - __builtin_clz(x) : 0)
>>>> I fear you won't like me to say this, but can't we avoid baking in yet
>>>> another assumption on sizeof(int) == 4, by using at least sizeof(int) * 8
>>>> here (yet better might be sizeof(int) * BITS_PER_BYTE)?
>>> Yes and no.
>>>
>>> No, because 32 here is consistent with ARM and PPC when it comes to
>>> arch_fls().  Given the effort it took to get these consistent, I'm not
>>> interested in letting them diverge.
>>>
>>> But, if someone wants to introduce BITS_PER_INT to mirror BITS_PER_LONG
>>> and use it consistently, then that would be ok too.
> 
> Oleksii: I see your patch is committed, but when I said "use it
> consistently", I meant "patch ARM and PPC too".
>> 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.

This is a fair point indeed.

Jan