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

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Date: Fri, 3 May 2024 12:12:47 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Reinette Chatre <reinette.chatre@...el.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 5/3/24 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.
> 
> 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.

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

I assumed that this code did not expect to handle negative numbers,
because it is using unsigned math throughout.

If you do expect it to handle cases where, for example, this happens:

    avg_bw_imc > avg_bw_resc

..then a proper solution is easy, and looks like this:

diff --git a/tools/testing/selftests/resctrl/mbm_test.c b/tools/testing/selftests/resctrl/mbm_test.c
index c873793d016d..b87f91a41494 100644
--- a/tools/testing/selftests/resctrl/mbm_test.c
+++ b/tools/testing/selftests/resctrl/mbm_test.c
@@ -17,8 +17,8 @@
  static int
  show_bw_info(unsigned long *bw_imc, unsigned long *bw_resc, size_t span)
  {
-       unsigned long avg_bw_imc = 0, avg_bw_resc = 0;
-       unsigned long sum_bw_imc = 0, sum_bw_resc = 0;
+       long avg_bw_imc = 0, avg_bw_resc = 0;
+       long sum_bw_imc = 0, sum_bw_resc = 0;
         int runs, ret, avg_diff_per;
         float avg_diff = 0;

Should I resend the patch with that approach?

thanks,
-- 
John Hubbard
NVIDIA