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m68k: Add <asm/hash.h>

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This provides a multiply by constant GOLDEN_RATIO_32 = 0x61C88647
for the original mc68000, which lacks a 32x32-bit multiply instruction.

Yes, the amount of optimization effort put in is excessive. :-)

Shift-add chain found by Yevgen Voronenko's Hcub algorithm at
http://spiral.ece.cmu.edu/mcm/gen.html

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Philippe De Muyter <phdm@macq.eu>
Cc: linux-m68k@lists.linux-m68k.org
This commit is contained in:
George Spelvin 2016-05-26 11:36:19 -04:00
parent 468a942852
commit 14c44b95b3
2 changed files with 60 additions and 0 deletions
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1
arch/m68k/Kconfig.cpu
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@ -40,6 +40,7 @@ config M68000
select CPU_HAS_NO_MULDIV64
select CPU_HAS_NO_UNALIGNED
select GENERIC_CSUM
select HAVE_ARCH_HASH
help
The Freescale (was Motorola) 68000 CPU is the first generation of
the well known M68K family of processors. The CPU core as well as

59
arch/m68k/include/asm/hash.h Normal file
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@ -0,0 +1,59 @@
#ifndef _ASM_HASH_H
#define _ASM_HASH_H
/*
* If CONFIG_M68000=y (original mc68000/010), this file is #included
* to work around the lack of a MULU.L instruction.
*/
#define HAVE_ARCH__HASH_32 1
/*
* While it would be legal to substitute a different hash operation
* entirely, let's keep it simple and just use an optimized multiply
* by GOLDEN_RATIO_32 = 0x61C88647.
*
* The best way to do that appears to be to multiply by 0x8647 with
* shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
*
* Because the 68000 has multi-cycle shifts, this addition chain is
* chosen to minimise the shift distances.
*
* Despite every attempt to spoon-feed it simple operations, GCC
* 6.1.1 doggedly insists on doing annoying things like converting
* "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
*
* It also likes to notice two shifts in a row, like "a = x << 2" and
* "a <<= 7", and convert that to "a = x << 9". But shifts longer
* than 8 bits are extra-slow on m68k, so that's a lose.
*
* Since the 68000 is a very simple in-order processor with no
* instruction scheduling effects on execution time, we can safely
* take it out of GCC's hands and write one big asm() block.
*
* Without calling overhead, this operation is 30 bytes (14 instructions
* plus one immediate constant) and 166 cycles.
*
* (Because %2 is fetched twice, it can't be postincrement, and thus it
* can't be a fully general "g" or "m". Register is preferred, but
* offsettable memory or immediate will work.)
*/
static inline u32 __attribute_const__ __hash_32(u32 x)
{
u32 a, b;
asm( "move.l %2,%0" /* a = x * 0x0001 */
"\n lsl.l #2,%0" /* a = x * 0x0004 */
"\n move.l %0,%1"
"\n lsl.l #7,%0" /* a = x * 0x0200 */
"\n add.l %2,%0" /* a = x * 0x0201 */
"\n add.l %0,%1" /* b = x * 0x0205 */
"\n add.l %0,%0" /* a = x * 0x0402 */
"\n add.l %0,%1" /* b = x * 0x0607 */
"\n lsl.l #5,%0" /* a = x * 0x8040 */
: "=&d,d" (a), "=&r,r" (b)
: "r,roi?" (x)); /* a+b = x*0x8647 */
return ((u16)(x*0x61c8) << 16) + a + b;
}
#endif /* _ASM_HASH_H */