Re: [PATCH 5/9] xen/bitops: Introduce generic_hweightl() and hweightl()

[Andrew Cooper <andrew.cooper3@xxxxxxxxxx>]

On 27/08/2024 12:41 pm, Jan Beulich wrote:
> On 27.08.2024 12:39, Andrew Cooper wrote:
>> On 26/08/2024 12:40 pm, Jan Beulich wrote:
>>> On 23.08.2024 01:06, Andrew Cooper wrote:
>>> --- a/xen/include/xen/bitops.h
>>> +++ b/xen/include/xen/bitops.h
>>> @@ -35,6 +35,12 @@ extern void __bitop_bad_size(void);
>>>  unsigned int __pure generic_ffsl(unsigned long x);
>>>  unsigned int __pure generic_flsl(unsigned long x);
>>>  
>>>> +/*
>>>> + * Hamming Weight, also called Population Count.  Returns the number of 
>>>> set
>>>> + * bits in @x.
>>>> + */
>>>> +unsigned int __pure generic_hweightl(unsigned long x);
>>> Aren't this and ...
>>>
>>>> @@ -284,6 +290,18 @@ static always_inline __pure unsigned int 
>>>> fls64(uint64_t x)
>>>>          (_v & (_v - 1)) != 0;                   \
>>>>      })
>>>>  
>>>> +static always_inline __pure unsigned int hweightl(unsigned long x)
>>> ... this even __attribute_const__?
>> Hmm.  This is following fls(), but on further consideration, they should
>> be const too.
>>
>> I'll do a prep patch fixing that, although I'm going to rename it to
>> __attr_const for brevity.
>>
>> Much as I'd prefer __const, I expect that is going too far, making it
>> too close to regular const.
> I was actually going to suggest using that name, if we want to shorten
> __attribute_const__. Yes, gcc treats __const (and __const__) as
> keywords, but do we care (much)? All it requires is that we don't start
> using __const as a (real) keyword.

Well also we'll get into more MISRA fun for overriding keywords.

But yes - the fact that GCC treats __const to mean const is precisely
why we shouldn't give it an unrelated meaning.

>
> Of course __const is a good example of why really we shouldn't use
> double-underscore prefixed names anywhere. Any of them can be assigned
> a meaning by the compiler, and here that's clearly the case. Therefore,
> taking your planned rename, maybe better make it attr_const then? And
> eventually switch stuff like __packed, __pure, and __weak to attr_* as
> well? Or even introduce something like
>
> #define attr(attr...) __attribute__((attr))
>
> and use attr(const) here?

Hmm - that's an interesting approach, and for other attributes which we
can use unconditionally.  It will end up shorter than multiple separate
__-prefixed names.

As a tangent, I've got some work from playing with -fanalyzer which
sprinkles some attr malloc/alloc_{size,align}()/free around.  It does
improve code generation (abeit marginally), but the function declaration
size suffers.

It won't work for attributes which are conditionally nothing (e.g.
cf_check), or ones that contain multiple aspects (e.g. __constructer
conataining cf_check).

In practice this means we're always going to end up with a mix, so maybe
attr_const is better for consistency.

>
>>>> +{
>>>> +    if ( __builtin_constant_p(x) )
>>>> +        return __builtin_popcountl(x);
>>> How certain are you that compilers (even old ones) will reliably fold
>>> constant expressions here, and never emit a libgcc call instead? The
>>> conditions look to be more tight than just __builtin_constant_p(); a
>>> pretty absurd example:
>>>
>>> unsigned ltest(void) {
>>>     return __builtin_constant_p("") ? __builtin_popcountl((unsigned 
>>> long)"") : ~0;
>>> }
>> How do you express that in terms of a call to hweightl()?
> hweightl((unsigned long)"");
>
> Yet as said - it's absurd. It merely serves to make the point that what
> __builtin_constant_p() returns true for doesn't necessarily constant-
> fold in expressions.

Yes, but as shown in the godbolt link, this form changes GCC's mind
about the __builtin_const-ness of the expression.

>
>> Again, this is following the layout started with fls() in order to avoid
>> each arch opencoding different versions of constant folding.
>>
>> https://godbolt.org/z/r544c49oY
>>
>> When it's forced through the hweightl() interface, even GCC 4.1 decides
>> that it's non-constant and falls back to generic_hweightl().
>>
>>
>> I did spend a *lot* of time with the fls() series checking that all
>> compilers we supported did what we wanted in this case, so I don't
>> expect it to be a problem.
> Right, and I guess I was pointlessly more concerned about popcount than
> I was for ffs() / fls(). The criteria upon which gcc decides whether to
> constant-fold the uses is exactly the same.
>
>>   But, if a library call is emitted, it will
>> be very obvious (link failure), and we can re-evaluate.
> Indeed, we certainly would notice, albeit the diagnostic may be cryptic
> to people.
>
> Bottom line - keep it as is.

Thanks.

~Andrew