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

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Message-ID: <d46b81df-e018-a9bd-1550-31c825c487cf@linux.intel.com>
Date: Mon, 6 May 2024 12:07:57 +0300 (EEST)
From: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
To: John Hubbard <jhubbard@...dia.com>
cc: Reinette Chatre <reinette.chatre@...el.com>, 
    Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>, 
    Shuah Khan <shuah@...nel.org>, Nathan Chancellor <nathan@...nel.org>, 
    Nick Desaulniers <ndesaulniers@...gle.com>, 
    Bill Wendling <morbo@...gle.com>, Justin Stitt <justinstitt@...gle.com>, 
    Fenghua Yu <fenghua.yu@...el.com>, Valentin Obst <kernel@...entinobst.de>, 
    linux-kselftest@...r.kernel.org, LKML <linux-kernel@...r.kernel.org>, 
    llvm@...ts.linux.dev
Subject: Re: [PATCH] selftests/resctrl: fix clang build warnings related to
 abs(), labs() calls

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.