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

[Philippe De Muyter]

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