Re:Re: [PATCH] arm/mach-hisi: Use BUG_ON instead of if condition followed by BUG

[周传高 <zhouchuangao@vivo.com>]

>On 2021-04-23 09:14, zhouchuangao wrote:
>> BUG_ON uses unlikely in if(). Through disassembly, we can see that
>> brk #0x800 is compiled to the end of the function.
>> As you can see below:
>>      ......
>>      ffffff8008660bec:   d65f03c0    ret
>>      ffffff8008660bf0:   d4210000    brk #0x800
>> 
>> Usually, the condition in if () is not satisfied. For the
>> multi-stage pipeline, we do not need to perform fetch decode
>> and excute operation on brk instruction.
>
>32-bit Arm does not have "ret" and "brk" instructions, and either way 
>the relevant BUG() instruction(s) aren't executed unless the condition 
>is met, so this really makes very little sense.
>

Sorry, this is just an analysis based on ARM64.

>> In my opinion, this can improve the efficiency of the
>> multi-stage pipeline.
>
>It has very little to do with the pipeline - modern cores are 
>considerably more sophisticated than the 3-stage Acorn RISC Machine of 
>1985, and are not usually limited by frontend throughput. The point of 
>unlikely() is to avoid having a normally-taken forward branch to skip 
>over in-line code, and instead make sure the only thing in the normal 
>execution path is a normally-not-taken branch to handle the condition 
>out-of-line. Yes, the impact of branches - and thus why it can be 
>desirable to avoid them - is indeed *related* to pipelining, but that's 
>rather tangential.
>
>Even then, it's only worth considering things at this level in 
>frequently-executed and/or performance-critical code. Saving a couple of 
>CPU cycles in something that is effectively a one-time operation is 
>utterly immaterial.
>
>The realistic justification for these patches is that that BUG_ON() 
>exists for implementing conditional BUG()s, so we may as well use it if 
>it makes the source code more readable.
>

Thank you for your excellent analysis, Indeed, only in the case of
Frequently Executed and/or Performance-Critical Code, the patch is of
great value.

Hmm...Perhaps the best reason is to make the code more readable.

BR,
zhouchuangao

>> Signed-off-by: zhouchuangao <zhouchuangao@vivo.com>
>> ---
>>   arch/arm/mach-hisi/hotplug.c  | 3 +--
>>   arch/arm/mach-hisi/platmcpm.c | 4 ++--
>>   2 files changed, 3 insertions(+), 4 deletions(-)
>> 
>> diff --git a/arch/arm/mach-hisi/hotplug.c b/arch/arm/mach-hisi/hotplug.c
>> index c517941..b9ced60 100644
>> --- a/arch/arm/mach-hisi/hotplug.c
>> +++ b/arch/arm/mach-hisi/hotplug.c
>> @@ -193,8 +193,7 @@ void hix5hd2_set_cpu(int cpu, bool enable)
>>   	u32 val = 0;
>>   
>>   	if (!ctrl_base)
>> -		if (!hix5hd2_hotplug_init())
>> -			BUG();
>> +		BUG_ON(!hix5hd2_hotplug_init());
>
>Whatever tool you're using to detect these patterns, consider improving 
>it, or at least giving a bit more thought to the results beyond blindly 
>applying one single rule - "if(x) BUG_ON(y);" arguably makes even less 
>sense since it's now neither one thing nor the other.
>
>Robin.
>
>>   	if (enable) {
>>   		/* power on cpu1 */
>> diff --git a/arch/arm/mach-hisi/platmcpm.c b/arch/arm/mach-hisi/platmcpm.c
>> index 96a4840..6c90039 100644
>> --- a/arch/arm/mach-hisi/platmcpm.c
>> +++ b/arch/arm/mach-hisi/platmcpm.c
>> @@ -82,8 +82,8 @@ static void hip04_set_snoop_filter(unsigned int cluster, unsigned int on)
>>   {
>>   	unsigned long data;
>>   
>> -	if (!fabric)
>> -		BUG();
>> +	BUG_ON(!fabric);
>> +
>>   	data = readl_relaxed(fabric + FAB_SF_MODE);
>>   	if (on)
>>   		data |= 1 << cluster;
>>