linux-kernel - Re: [PATCH 08/10] m68k: Add <asm/archhash.h>

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Message-ID: <20160525085653.GA9917@frolo.macqel>
Date:	Wed, 25 May 2016 10:56:53 +0200
From:	Philippe De Muyter <phdm@...q.eu>
To:	George Spelvin <linux@...encehorizons.net>
Cc:	geert@...ux-m68k.org, gerg@...ux-m68k.org,
	linux-kernel@...r.kernel.org, linux-m68k@...ts.linux-m68k.org,
	torvalds@...ux-foundation.org, tglx@...utronix.de
Subject: Re: [PATCH 08/10] m68k: Add <asm/archhash.h>

On Wed, May 25, 2016 at 03:34:55AM -0400, George Spelvin wrote:
> +static inline u32 __attribute_const__ __hash_32(u32 x)
> +{
> +	u32 a, b;
> +
> +	asm(   "move.l %2,%0"	/* 0x0001 */
> +	"\n	lsl.l #2,%0"	/* 0x0004 */
> +	"\n	move.l %0,%1"
> +	"\n	lsl.l #7,%0"	/* 0x0200 */
> +	"\n	add.l %2,%0"	/* 0x0201 */
> +	"\n	add.l %0,%1"	/* 0x0205 */
> +	"\n	add.l %0,%0"	/* 0x0402 */
> +	"\n	add.l %0,%1"	/* 0x0607 */
> +	"\n	lsl.l #5,%0"	/* 0x8040 */
> +				/* 0x8647 */

There is no standard way to write asm in the kernel, but I prefer
a simple semicolon after each insn

	asm("move.l	%2,%0;"	/* 0x0001 */
	    "lsl.l	#2,%0;"	/* 0x0004 */
	    "move.l	%0,%1;"
	    "lsl.l	#7,%0;"	/* 0x0200 */
	    "add.l	%2,%0;"	/* 0x0201 */
	    "add.l	%0,%1;"	/* 0x0205 */
	    "add.l	%0,%0;"	/* 0x0402 */
	    "add.l	%0,%1;"	/* 0x0607 */
	    "lsl.l	#5,%0"	/* 0x8040 */
				/* 0x8647 */

Also, it took me some time to understand the hexadecimal constants
in the comments (and the last one predicts a future event :)).

> +	: "=&d" (a), "=&r" (b)
> +	: "g" (x));
> +
> +	return ((u16)(x*0x61c8) << 16) + a + b;
> +}

Just my two cents

Philippe