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

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Message-ID: <d93c08c2-6fa8-4c3c-8887-f5ccabd7777a@nvidia.com>
Date: Mon, 6 May 2024 10:21:01 -0700
From: John Hubbard <jhubbard@...dia.com>
To: Ilpo Järvinen <ilpo.jarvinen@...ux.intel.com>
Cc: Reinette Chatre <reinette.chatre@...el.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 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