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

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Message-ID: <26f3effc-6ea1-4670-a301-76df3a710fa9@intel.com>
Date: Fri, 3 May 2024 11:37:04 -0700
From: Reinette Chatre <reinette.chatre@...el.com>
To: John Hubbard <jhubbard@...dia.com>, Ilpo Järvinen
	<ilpo.jarvinen@...ux.intel.com>
CC: 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 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.

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;

But that still triggers:
warning: taking the absolute value of unsigned type 'unsigned long' has no effect [-Wabsolute-value]

Looks like we may need to be more explicit types and not rely on casting so much
to make the compiler happy.

Reinette