Re: [XEN PATCH][for-4.19 1/9] xen/include: add macro LOWEST_POW2

[Julien Grall <julien@xxxxxxx>]

On 06/10/2023 11:02, Nicola Vetrini wrote:
> On 06/10/2023 11:29, Julien Grall wrote:
> > Hi,
> >
> > On 06/10/2023 09:26, Nicola Vetrini wrote:
> > > The purpose of this macro is to encapsulate the well-known expression
> > > 'x & -x', that in 2's complement architectures on unsigned integers will
> > > give 2^ffs(x), where ffs(x) is the position of the lowest set bit in x.
> > >
> > > A deviation for ECLAIR is also introduced.
> > Can you explain why this is a deviation in ECLAIR rather than one with
> > /* SAF-* */ (or whichever name we decide to rename to)? Is this
> > because the code is correct from MISRA perspective but the tool is
> > confused?
> The code does violate Rule 10.1 (a unary minus applied to an unsigned 
> value is
> deemed inappropriate by MISRA), but rather than changing a whole lot of 
> places where this
> construct is used (mainly in x86 code), the reasoning is that it makes 
> more sense to isolate
> it and justify its presence by the fact that on 2's complement 
> architectures the result is
> indeed correct.
This is explaining to me why you are adding LOWEST_POW2(). But this 
doesn't explain why you are not using /* SAF-* */ on top of LOWEST_POW2().
To me, we should only use ECLAIR specific deviation when this is a 
shortcoming with the tool.
Cheers,

--
Julien Grall