Re: [PATCH v4 4/8] bits: introduce fixed-type BIT

Andy Shevchenko
2025-03-05T19:56:30Z

On Thu, Mar 06, 2025 at 02:17:18AM +0900, Vincent Mailhol wrote:
> On 06/03/2025 at 00:48, Andy Shevchenko wrote:
> > On Wed, Mar 05, 2025 at 11:48:10PM +0900, Vincent Mailhol wrote:
> >> On 05/03/2025 at 23:33, Andy Shevchenko wrote:
> >>> On Wed, Mar 05, 2025 at 10:00:16PM +0900, Vincent Mailhol via B4 Relay wrote:

...

> >>>> +#define BIT_U8(b) (BIT_INPUT_CHECK(u8, b) + (unsigned int)BIT(b))
> >>>> +#define BIT_U16(b) (BIT_INPUT_CHECK(u16, b) + (unsigned int)BIT(b))
> >>>
> >>> Why not u8 and u16? This inconsistency needs to be well justified.
> >>
> >> Because of the C integer promotion rules, if casted to u8 or u16, the
> >> expression will immediately become a signed integer as soon as it is get
> >> used. For example, if casted to u8
> >>
> >> BIT_U8(0) + BIT_U8(1)
> >>
> >> would be a signed integer. And that may surprise people.
> >
> > Yes, but wouldn't be better to put it more explicitly like
> >
> > #define BIT_U8(b) (BIT_INPUT_CHECK(u8, b) + (u8)BIT(b) + 0 + UL(0)) // + ULL(0) ?
>
> OK, the final result would be unsigned. But, I do not follow how this is
> more explicit.
>
> Also, why doing:
>
> (u8)BIT(b) + 0 + UL(0)
>
> and not just:
>
> (u8)BIT(b) + UL(0)
>
> ?
>
> What is that intermediary '+ 0' for?
>
> I am sorry, but I am having a hard time understanding how casting to u8
> and then doing an addition with an unsigned long is more explicit than
> directly doing a cast to the desired type.

Reading this again, I think we don't need it at all. u8, aka unsigned char,
will be promoted to int, but it will be int with a value < 256, can't be signed
as far as I understand this correctly.

> As I mentioned in my answer to Yuri, I have a slight preference for the
> unsigned int cast, but I am OK to go back to the u8/u16 cast as it was
> in v3.

Which means that the simples uXX castings should suffice. In any case we need
test cases for that.

> However, I really do not see how that '+ 0 + UL(0)' would be an improvement.
>
> > Also, BIT_Uxx() gives different type at the end, shouldn't they all be promoted
> > to unsigned long long at the end? Probably it won't work in real assembly.
> > Can you add test cases which are written in assembly? (Yes, I understand that it will
> > be architecture dependent, but still.)
>
> No. I purposely guarded the definition of the BIT_Uxx() by a
>
> #if !defined(__ASSEMBLY__)
>
> so that these are never visible in assembly. I actually put a comment to
> explain why the GENMASK_U*() are not available in assembly. I can copy
> paste the same comment to explain why why BIT_U*() are not made
> available either:
>
> /*
> * Missing asm support
> *
> * BIT_U*() depends on BITS_PER_TYPE() which would not work in the asm
> * code as BITS_PER_TYPE() relies on sizeof(), something not available
> * in asm. Nethertheless, the concept of fixed width integers is a C
> * thing which does not apply to assembly code.
> */
>
> I really believe that it would be a mistake to make the GENMASK_U*() or
> the BIT_U*() available to assembly.

Ah, okay then!

> >> David also pointed this in the v3:
> >>
> >> https://lore.kernel.org/intel-xe/d42dc197a15649e69d459362849a37f2@AcuMS.aculab.com/
> >>
> >> and I agree with his comment.

Why unsigned char won't work?

> >> I explained this in the changelog below the --- cutter, but it is
> >> probably better to make the explanation more visible. I will add a
> >> comment in the code to explain this.
> >>
> >>>> +#define BIT_U32(b) (BIT_INPUT_CHECK(u32, b) + (u32)BIT(b))
> >>>> +#define BIT_U64(b) (BIT_INPUT_CHECK(u64, b) + (u64)BIT_ULL(b))

--
With Best Regards,
Andy Shevchenko