Re: [XEN PATCH][for-4.19 v4 1/8] xen/include: add macro ISOLATE_LOW_BIT

[Nicola Vetrini <nicola.vetrini@xxxxxxxxxxx>]

On 2023-10-31 11:20, Jan Beulich wrote:
> On 31.10.2023 11:03, Nicola Vetrini wrote:
> > On 2023-10-31 09:28, Nicola Vetrini wrote:
> > > On 2023-10-31 08:43, Jan Beulich wrote:
> > > > On 30.10.2023 23:44, Stefano Stabellini wrote:
> > > > > On Mon, 30 Oct 2023, Jan Beulich wrote:
> > > > > > On 27.10.2023 15:34, Nicola Vetrini wrote:
> > > > > > > --- a/xen/include/xen/macros.h
> > > > > > > +++ b/xen/include/xen/macros.h
> > > > > > > @@ -8,8 +8,14 @@
> > > > > > >  #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))
> > > > > > > +/*
> > > > > > > + * Given an unsigned integer argument, expands to a mask where
> > > > > > > just the least
> > > > > > > + * significant nonzero bit of the argument is set, or 0 if no 
> > > > > > > bits
> > > > > > > are set.
> > > > > > > + */
> > > > > > > +#define ISOLATE_LOW_BIT(x) ((x) & -(x))
> > > > > > Not even considering future Misra changes (which aiui may require
> > > > > > that
> > > > > > anyway), this generalization of the macro imo demands that its
> > > > > > argument
> > > > > > now be evaluated only once.
> > > > > Fur sure that would be an improvement, but I don't see a trivial 
> > > > > way
> > > > > to
> > > > > do it and this issue is also present today before the patch.
> > > > This was an issue here for MASK_EXTR() and MASK_INSR(), yes, but the
> > > > new
> > > > macro has wider use, and there was no issue elsewhere so far.
> > > >
> > > > > I think it
> > > > > would be better to avoid scope-creeping this patch as we are 
> > > > > already
> > > > > at
> > > > > v4 for something that was expected to be a trivial mechanical 
> > > > > change.
> > > > > I
> > > > > would rather review the fix as a separate patch, maybe sent by you 
> > > > > as
> > > > > you probably have a specific implementation in mind?
> > > > #define ISOLATE_LOW_BIT(x) ({ \
> > > >     typeof(x) x_ = (x); \
> > > >     x_ & -x_; \
> > > > })
> > > >
> > > > Hard to see the scope creep here. What I would consider scope creep 
> > > > I
> > > > specifically didn't even ask for: I'd like this macro to be
> > > > overridable
> > > > by an arch. Specifically (see my earlier naming hint) I'd like to 
> > > > use
> > > > x86's BMI insn BLSI in the context of "x86: allow Kconfig control 
> > > > over
> > > > psABI level", when ABI v2 or higher is in use.
> > > I appreciate you suggesting an implementation; I'll send a v5
> > > incorporating it.
> > There's an issue with this approach, though: since the macro is used
> > indirectly
> > in expressions that are e.g. case labels or array sizes, the build 
> > fails
> > (see [1] for instance).
> > Perhaps it's best to leave it as is?
> Hmm. I'm afraid it's not an option to "leave as is", not the least 
> because
> - as said - I'm under the impression that another Misra rule requires
> macro arguments to be evaluated exactly once. Best I can think of right
> away is to have a macro for limited use (to address such build issues)
> plus an inline function (for general use). But yes, maybe that then 
> indeed
> needs to be a 2nd step.
>
> Jan
>
> > [1] 
> > https://gitlab.com/xen-project/people/bugseng/xen/-/jobs/5423693947
There is no such MISRA Rule afaik: R23.7 is similar, but only for C11 
generic selections.
--
Nicola Vetrini, BSc
Software Engineer, BUGSENG srl (https://bugseng.com)