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

[Nicola Vetrini <nicola.vetrini@bugseng.com>]

On 06/10/2023 12:22, Julien Grall wrote:
> 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,

Because of the way ECLAIR deviation comments work implies that in most 
cases the real
place where to put the deviation is the usage site
(the so-called top expansion location of the macro). Now, for 
widely-used macros this is
cumbersome and would clutter the code unnecessarily. It's way cleaner 
imo to have a single
line in the configuration with a clear justification that is present in 
the textual output
of the tool.
But then there are tool interoperability considerations, that would call 
for standardized
deviation mechanisms, if they do detect this as a violation (which I 
don't know).

In the end, it could be done with a textual deviation, if that's 
preferred, but keep in mind
that those are more fragile w.r.t. code movement.

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