linux-kernel - Re: [PATCH v1 2/2] powerpc/atomics: Use immediate operand when possible

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Date:   Tue, 13 Apr 2021 18:36:04 +0200
From:   Christophe Leroy <christophe.leroy@...roup.eu>
To:     Segher Boessenkool <segher@...nel.crashing.org>
Cc:     Benjamin Herrenschmidt <benh@...nel.crashing.org>,
        Paul Mackerras <paulus@...ba.org>,
        Michael Ellerman <mpe@...erman.id.au>,
        linuxppc-dev@...ts.ozlabs.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v1 2/2] powerpc/atomics: Use immediate operand when
 possible



Le 13/04/2021 à 00:08, Segher Boessenkool a écrit :
> Hi!
> 
> On Thu, Apr 08, 2021 at 03:33:45PM +0000, Christophe Leroy wrote:
>> +#define ATOMIC_OP(op, asm_op, dot, sign)				\
>>   static __inline__ void atomic_##op(int a, atomic_t *v)			\
>>   {									\
>>   	int t;								\
>>   									\
>>   	__asm__ __volatile__(						\
>>   "1:	lwarx	%0,0,%3		# atomic_" #op "\n"			\
>> -	#asm_op " %0,%2,%0\n"						\
>> +	#asm_op "%I2" dot " %0,%0,%2\n"					\
>>   "	stwcx.	%0,0,%3 \n"						\
>>   "	bne-	1b\n"							\
>> -	: "=&r" (t), "+m" (v->counter)					\
>> -	: "r" (a), "r" (&v->counter)					\
>> +	: "=&b" (t), "+m" (v->counter)					\
>> +	: "r"#sign (a), "r" (&v->counter)				\
>>   	: "cc");							\
>>   }									\
> 
> You need "b" (instead of "r") only for "addi".  You can use "addic"
> instead, which clobbers XER[CA], but *all* inline asm does, so that is
> not a downside here (it is also not slower on any CPU that matters).
> 
>> @@ -238,14 +238,14 @@ static __inline__ int atomic_fetch_add_unless(atomic_t *v, int a, int u)
>>   "1:	lwarx	%0,0,%1		# atomic_fetch_add_unless\n\
>>   	cmpw	0,%0,%3 \n\
>>   	beq	2f \n\
>> -	add	%0,%2,%0 \n"
>> +	add%I2	%0,%0,%2 \n"
>>   "	stwcx.	%0,0,%1 \n\
>>   	bne-	1b \n"
>>   	PPC_ATOMIC_EXIT_BARRIER
>> -"	subf	%0,%2,%0 \n\
>> +"	sub%I2	%0,%0,%2 \n\
>>   2:"
>> -	: "=&r" (t)
>> -	: "r" (&v->counter), "r" (a), "r" (u)
>> +	: "=&b" (t)
>> +	: "r" (&v->counter), "rI" (a), "r" (u)
>>   	: "cc", "memory");
> 
> Same here.

Yes, I thought about addic, I didn't find an early solution because I forgot the matching 'addc'.

Now with the couple addc/addic it works well.

Thanks

> 
> Nice patches!
> 
> Acked-by: Segher Boessenkool <segher@...nel.crashing.org>
> 

Christophe