linux-kernel - Re: [PATCH] drivers/staging/wilc1000: fix sparse warning: right shift by bigger than source value

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Date:   Wed, 12 Jul 2017 10:23:02 +0800
From:   Rui Teng <rui.teng@...ux.vnet.ibm.com>
To:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>
Cc:     Aditya Shankar <aditya.shankar@...rochip.com>,
        Ganesh Krishna <ganesh.krishna@...rochip.com>,
        devel@...verdev.osuosl.org, linux-wireless@...r.kernel.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] drivers/staging/wilc1000: fix sparse warning: right shift
 by bigger than source value

On 12/07/2017 1:04 AM, Greg Kroah-Hartman wrote:
> On Mon, Jul 10, 2017 at 04:57:31PM +0800, Rui Teng wrote:
>> This patch sets memory to zero directly to avoid unnecessary shift and
>> bitwise operations on bool type, which can fix a sparse warning and also
>> improve performance.
>
> It does?  How did you measure the performance impact?  What was now
> faster?

It can avoid 3 times right shift and 3 times bitwise operations.
And once memory set should also faster than 4 times copy operations.
And add number 4 once should also faster than 4 times plus plus.

>
> thanks,
>
> greg k-h
>
>
>>
>> Signed-off-by: Rui Teng <rui.teng@...ux.vnet.ibm.com>
>> ---
>>  drivers/staging/wilc1000/host_interface.c | 7 +++----
>>  1 file changed, 3 insertions(+), 4 deletions(-)
>>
>> diff --git a/drivers/staging/wilc1000/host_interface.c b/drivers/staging/wilc1000/host_interface.c
>> index 2568dfc15181..036c5c19a016 100644
>> --- a/drivers/staging/wilc1000/host_interface.c
>> +++ b/drivers/staging/wilc1000/host_interface.c
>> @@ -2416,10 +2416,9 @@ static void Handle_SetMulticastFilter(struct wilc_vif *vif,
>>  		goto ERRORHANDLER;
>>
>>  	pu8CurrByte = wid.val;
>> -	*pu8CurrByte++ = (strHostIfSetMulti->enabled & 0xFF);
>> -	*pu8CurrByte++ = ((strHostIfSetMulti->enabled >> 8) & 0xFF);
>> -	*pu8CurrByte++ = ((strHostIfSetMulti->enabled >> 16) & 0xFF);
>> -	*pu8CurrByte++ = ((strHostIfSetMulti->enabled >> 24) & 0xFF);
>> +	memset(pu8CurrByte, 0, 4);
>> +	*pu8CurrByte = (strHostIfSetMulti->enabled & 0xFF);
>> +	pu8CurrByte += 4;
>
> Are you sure enabled isn't larger than 8 bits?

The size of bool may larger than 8 bits. But when we assign any value
to bool type, the value of bool type will only be 1 or 0.
For example, the following output will be 1 other than 0x100.
	bool b = 0x100;
	printf("b: %d\n", b);

Or I think it should change 'enabled' type from bool to u32, to make 
sure that the right shift operation can take effect.

>
> thanks,
>
> greg k-h
>