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).
> > 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.
--
i.
