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

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Message-ID: <20210412220821.GN26583@gate.crashing.org>
Date:   Mon, 12 Apr 2021 17:08:22 -0500
From:   Segher Boessenkool <segher@...nel.crashing.org>
To:     Christophe Leroy <christophe.leroy@...roup.eu>
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

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.

Nice patches!

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


Segher