Re: [PATCH 2/2] Arm32: tidy the memset() macro

[Jan Beulich <jbeulich@xxxxxxxx>]

On 19.08.2022 11:41, Wei Chen wrote:
> Hi Jan,
> 
>> -----Original Message-----
>> From: Jan Beulich <jbeulich@xxxxxxxx>
>> Sent: 2022年8月19日 16:04
>> To: Wei Chen <Wei.Chen@xxxxxxx>
>> Cc: Julien Grall <julien@xxxxxxx>; Stefano Stabellini
>> <sstabellini@xxxxxxxxxx>; Volodymyr Babchuk <volodymyr_babchuk@xxxxxxxx>;
>> Bertrand Marquis <Bertrand.Marquis@xxxxxxx>; xen-
>> devel@xxxxxxxxxxxxxxxxxxxx
>> Subject: Re: [PATCH 2/2] Arm32: tidy the memset() macro
>>
>> On 19.08.2022 09:59, Wei Chen wrote:
>>>> -----Original Message-----
>>>> From: Xen-devel <xen-devel-bounces@xxxxxxxxxxxxxxxxxxxx> On Behalf Of
>> Jan
>>>> Beulich
>>>> Sent: 2022年8月19日 15:50
>>>>
>>>> - add parentheses where they were missing (MISRA)
>>>> - make sure to evaluate also v exactly once (MISRA)
>>>> - remove excess parentheses
>>>> - rename local variables to not have leading underscores
>>>> - apply Xen coding style
>>>>
>>>> Signed-off-by: Jan Beulich <jbeulich@xxxxxxxx>
>>>> ---
>>>> I wonder whether "if ( n_ )" is really helpful: It's extra code in all
>>>> callers passing a non-constant size, just to cover a pretty rare case
>>>> which memset() is required to deal with correctly anyway, and which
>>>
>>> What rare case we need to use n_ that can make memset happy?
>>
>> I'm afraid I don't understand the question.
>>
> 
> Sorry I didn't describe the problem clearly in the last email. You mentioned
> whether if (n_) is useful in your patch comments. I looked at the 
> implementation
> of the current memset macro, and I didn't feel it was too useful.
> 
> Then in the comments you mentioned that if (n_) is just to cover a very rare 
> case.
> Does the rare case is memset(p, v, 0)?

Yes, albeit not in the form you've written it, but with the last argument
being a variable which happens to be zero. With literal zero, the compiler
would dead-code eliminate the construct anyway.

Jan

> If this is the case, I agree with you,
> memset itself should be able to handle with size=0.
> 
> Sorry again for confusing you!
> 
> Thanks,
> Wei Chen