Re: [XEN PATCH][for-4.19 v3 1/8] xen/include: add macro LOWEST_BIT

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 23/10/2023 11:19, Nicola Vetrini wrote:
> On 23/10/2023 09:48, Jan Beulich wrote:
> > On 20.10.2023 17:28, Nicola Vetrini wrote:
> > > --- a/automation/eclair_analysis/ECLAIR/deviations.ecl
> > > +++ b/automation/eclair_analysis/ECLAIR/deviations.ecl
> > > @@ -246,6 +246,12 @@ constant expressions are required.\""
> > >    "any()"}
> > >  -doc_end
> > >
> > > +-doc_begin="The macro LOWEST_BIT encapsulates a well-known pattern 
> > > to obtain the value
> > > +2^ffs(x) for unsigned integers on two's complement architectures
> > > +(all the architectures supported by Xen satisfy this requirement)."
> > > +-config=MC3R1.R10.1,reports+={safe, 
> > > "any_area(any_loc(any_exp(macro(^LOWEST_BIT$))))"}
> > > +-doc_end
> > This being deviated this way (rather than by SAF-* comments) wants
> > justifying in the description. You did reply to my respective
> > comment on v2, but such then (imo) needs propagating into the actual
> > patch as well.
> Sure, will do.
>
> > > --- a/docs/misra/deviations.rst
> > > +++ b/docs/misra/deviations.rst
> > > @@ -192,6 +192,13 @@ Deviations related to MISRA C:2012 Rules:
> > >         See automation/eclair_analysis/deviations.ecl for the full 
> > > explanation.
> > >       - Tagged as `safe` for ECLAIR.
> > >
> > > +   * - R10.1
> > > +     - The well-known pattern (x & -x) applied to unsigned integer 
> > > values on 2's
> > > +       complement architectures (i.e., all architectures supported 
> > > by Xen), used
> > > +       to obtain the value 2^ffs(x), where ffs(x) is the position of 
> > > the first
> > > +       bit set. If no bits are set, zero is returned.
> > > +     - Tagged as `safe` for ECLAIR.
> > In such an explanation there shall not be any ambiguity. Here I see
> > an issue with ffs() returning 1-based bit position numbers, which
> > isn't in line with the use in 2^ffs(x). (Arguably use of ^ itself is
> > also problematic, as that's XOR in C, not POW. I'd suggest 2^^ffs(x)
> > or 2**ffs(x).)
> Makes sense, I think I'll use an plain english explanation to avoid
> any confusion
> about notation.
>
> > > --- a/xen/include/xen/macros.h
> > > +++ b/xen/include/xen/macros.h
> > > @@ -8,8 +8,11 @@
> > >  #define DIV_ROUND(n, d) (((n) + (d) / 2) / (d))
> > >  #define DIV_ROUND_UP(n, d) (((n) + (d) - 1) / (d))
> > >
> > > -#define MASK_EXTR(v, m) (((v) & (m)) / ((m) & -(m)))
> > > -#define MASK_INSR(v, m) (((v) * ((m) & -(m))) & (m))
> > > +/* Returns the 2^ffs(x) or 0, where ffs(x) is the index of the 
> > > lowest set bit */
> > > +#define LOWEST_BIT(x) ((x) & -(x))
> > I'm afraid my concern regarding this new macro's name (voiced on v2) 
> > hasn't
> > been addressed (neither verbally nor by finding a more suitable name).
> >
> > Jan
> I didn't come up with much better names, so I left it as is. Here's a 
> few ideas:
> - LOWEST_SET
> - MASK_LOWEST_SET
> - MASK_LOWEST_BIT
> - MASK_FFS_0
> - LOWEST_ONE
>
> and also yours, included here for convenience, even though you felt it
> was too long:
>
> - ISOLATE_LOW_BIT()
>
> All maintainers from THE REST are CC-ed, so we can see if anyone has
> any suggestion.
There's also LOWEST_BIT_MASK as another possible name.

--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)