Re: [PATCH] selftests/resctrl: fix clang build warnings related to abs(), labs() calls

[John Hubbard]

On 5/6/24 2:07 AM, Ilpo Järvinen wrote:
> On Fri, 3 May 2024, John Hubbard wrote:
>
> > On 5/3/24 11:37 AM, Reinette Chatre wrote:
> > > On 5/3/2024 9:52 AM, John Hubbard wrote:
> > > > On 5/3/24 1:00 AM, Ilpo Järvinen wrote:
> > > > > On Thu, 2 May 2024, John Hubbard wrote:
> > > > ...
> > > > > > diff --git a/tools/testing/selftests/resctrl/mbm_test.c
> > > > > > b/tools/testing/selftests/resctrl/mbm_test.c
> > > > > > index d67ffa3ec63a..c873793d016d 100644
> > > > > > --- a/tools/testing/selftests/resctrl/mbm_test.c
> > > > > > +++ b/tools/testing/selftests/resctrl/mbm_test.c
> > > > > > @@ -33,7 +33,7 @@ show_bw_info(unsigned long *bw_imc, unsigned long
> > > > > > *bw_resc, size_t span)
> > > > > >           avg_bw_imc = sum_bw_imc / 4;
> > > > > >         avg_bw_resc = sum_bw_resc / 4;
> > > > > > -    avg_diff = (float)labs(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
> > > > > > +    avg_diff = (float)(avg_bw_resc - avg_bw_imc) / avg_bw_imc;
> > > > > >         avg_diff_per = (int)(avg_diff * 100);
> > > > > >           ret = avg_diff_per > MAX_DIFF_PERCENT;
> > > > >
> > > > > But how are these two cases same after your change when you ended up
> > > > > removing taking the absolute value entirely?
> > > >
> > > > All of the arguments are unsigned integers, so all arithmetic results
> > > > are interpreted as unsigned, so taking the absolute value of that is
> > > > always a no-op.
>
> (I see there's a better patch posted already but since there are a few
> incorrect claims in this discussion, I'll do for the record type of
> reply.)
>
> This discussion now went to a tangent about the warning. My main point is
> that logic is not correct after removing labs().
>
> I also disagree with the claim that using labs() on unsigned value is
> no-op because labs() takes long so unsigned is just forced into signed
> when calling which is why the warning triggers but it's very misleading
> warning (see below).

Yes you are correct.

> > > It does not seem as though clang can see when values have been casted.
> > > I tried to do so explicitly with a:
> > >    	avg_diff = labs((long)avg_bw_resc - avg_bw_imc) / (float)avg_bw_imc;
> >
> > The subtraction result will get promoted to an unsigned long, before being
> > passed into labs(3).
> >
> > > But that still triggers:
> > > warning: taking the absolute value of unsigned type 'unsigned long' has no
> > > effect [-Wabsolute-value]
> >
> > As expected, yes.
>
> That error message isn't factually correct:
>
>          unsigned long a = LONG_MAX;
>          long b;
>
>          a += 2;
>          b = (long)a;
>          printf("%llu %lli %lli\n", a, b, labs(a));
>
> Prints (at least when built with gcc):
>
> 9223372036854775809 -9223372036854775807 9223372036854775807
>
> labs(LONG_MAX + 1) won't work though since it's not positively presentable
> with long and the value is left untouched.

Thanks for setting the detailed record straight! :)


thanks,
--
John Hubbard
NVIDIA