Re: [PATCH v2 06/13] xen/bitops: Implement ffs() in common logic

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 31/05/2024 7:56 am, Nicola Vetrini wrote:
> On 2024-05-31 03:14, Stefano Stabellini wrote:
>> On Fri, 24 May 2024, Andrew Cooper wrote:
>>> Perform constant-folding unconditionally, rather than having it
>>> implemented
>>> inconsistency between architectures.
>>>
>>> Confirm the expected behaviour with compile time and boot time tests.
>>>
>>> For non-constant inputs, use arch_ffs() if provided but fall back to
>>> generic_ffsl() if not.  In particular, RISC-V doesn't have a builtin
>>> that
>>> works in all configurations.
>>>
>>> For x86, rename ffs() to arch_ffs() and adjust the prototype.
>>>
>>> For PPC, __builtin_ctz() is 1/3 of the size of size of the transform to
>>> generic_fls().  Drop the definition entirely.  ARM too benefits in
>>> the general
>>> case by using __builtin_ctz(), but less dramatically because it using
>>> optimised asm().
>>>
>>> Signed-off-by: Andrew Cooper <andrew.cooper3@xxxxxxxxxx>
>>
>> This patch made me realize that we should add __builtin_ctz,
>> __builtin_constant_p and always_inline to
>> docs/misra/C-language-toolchain.rst as they don't seem to be currently
>> documented and they are not part of the C standard
>>
>> Patch welcome :-)
>>
>
> I can send a patch for the builtins.

That's very kind of you.

In total by the end of this series, we've got __builtin_constant_p() 
(definitely used elsewhere already), and __builtin_{ffs,ctz,clz}{,l}() 
(3x primitives, 2x input types).

If we're going for a list of the primitive operations, lets add
__builtin_popcnt{,l}() too right away, because if it weren't for 4.19
code freeze, I'd have cleaned up the hweight() helpers too.

~Andrew