Commit 7b13277b authored by George Spelvin's avatar George Spelvin

microblaze: Add <asm/hash.h>

Microblaze is an FPGA soft core that can be configured various ways.

If it is configured without a multiplier, the standard __hash_32()
will require a call to __mulsi3, which is a slow software loop.

Instead, use a shift-and-add sequence for the constant multiply.
GCC knows how to do this, but it's not as clever as some.
Signed-off-by: default avatarGeorge Spelvin <linux@sciencehorizons.net>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>
parent 14c44b95
......@@ -16,6 +16,7 @@ config MICROBLAZE
select GENERIC_IRQ_SHOW
select GENERIC_PCI_IOMAP
select GENERIC_SCHED_CLOCK
select HAVE_ARCH_HASH
select HAVE_ARCH_KGDB
select HAVE_DEBUG_KMEMLEAK
select HAVE_DMA_API_DEBUG
......
#ifndef _ASM_HASH_H
#define _ASM_HASH_H
/*
* Fortunately, most people who want to run Linux on Microblaze enable
* both multiplier and barrel shifter, but omitting them is technically
* a supported configuration.
*
* With just a barrel shifter, we can implement an efficient constant
* multiply using shifts and adds. GCC can find a 9-step solution, but
* this 6-step solution was found by Yevgen Voronenko's implementation
* of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html.
*
* That software is really not designed for a single multiplier this large,
* but if you run it enough times with different seeds, it'll find several
* 6-shift, 6-add sequences for computing x * 0x61C88647. They are all
* c = (x << 19) + x;
* a = (x << 9) + c;
* b = (x << 23) + a;
* return (a<<11) + (b<<6) + (c<<3) - b;
* with variations on the order of the final add.
*
* Without even a shifter, it's hopless; any hash function will suck.
*/
#if CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL == 0
#define HAVE_ARCH__HASH_32 1
/* Multiply by GOLDEN_RATIO_32 = 0x61C88647 */
static inline u32 __attribute_const__ __hash_32(u32 a)
{
#if CONFIG_XILINX_MICROBLAZE0_USE_BARREL
unsigned int b, c;
/* Phase 1: Compute three intermediate values */
b = a << 23;
c = (a << 19) + a;
a = (a << 9) + c;
b += a;
/* Phase 2: Compute (a << 11) + (b << 6) + (c << 3) - b */
a <<= 5;
a += b; /* (a << 5) + b */
a <<= 3;
a += c; /* (a << 8) + (b << 3) + c */
a <<= 3;
return a - b; /* (a << 11) + (b << 6) + (c << 3) - b */
#else
/*
* "This is really going to hurt."
*
* Without a barrel shifter, left shifts are implemented as
* repeated additions, and the best we can do is an optimal
* addition-subtraction chain. This one is not known to be
* optimal, but at 37 steps, it's decent for a 31-bit multiplier.
*
* Question: given its size (37*4 = 148 bytes per instance),
* and slowness, is this worth having inline?
*/
unsigned int b, c, d;
b = a << 4; /* 4 */
c = b << 1; /* 1 5 */
b += a; /* 1 6 */
c += b; /* 1 7 */
c <<= 3; /* 3 10 */
c -= a; /* 1 11 */
d = c << 7; /* 7 18 */
d += b; /* 1 19 */
d <<= 8; /* 8 27 */
d += a; /* 1 28 */
d <<= 1; /* 1 29 */
d += b; /* 1 30 */
d <<= 6; /* 6 36 */
return d + c; /* 1 37 total instructions*/
#endif
}
#endif /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */
#endif /* _ASM_HASH_H */
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