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

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

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.

The following files use BITS_PER_LONG preprocessing:

arch/arm/include/asm/div64.h
arch/x86/cpu/mcheck/mce.c
arch/x86/smpboot.c
common/bitops.c
common/coverage/gcov.h
common/coverage/llvm.c
common/cpu.c
common/event_channel.c
common/time.c
common/ubsan/ubsan.c
include/asm-generic/div64.h
include/xen/cpumask.h
include/xen/inttypes.h
include/xen/nodemask.h
include/xen/sched.h
include/xen/xxhash.h
lib/find-next-bit.c
lib/generic-ffsl.c
lib/generic-flsl.c
lib/generic-hweightl.c

And I really don't think they can be replaced with a sizeof().

~Andrew