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: George Spelvin <linux@sciencehorizons.net>
Cc: Alistair Francis <alistair.francis@xilinx.com>
Cc: Michal Simek <michal.simek@xilinx.com>

diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index 3d793b5..ce3e512 100644 (file)
--- a/arch/microblaze/Kconfig
+++ b/arch/microblaze/Kconfig
@@ -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

diff --git a/arch/microblaze/include/asm/hash.h b/arch/microblaze/include/asm/hash.h
new file mode 100644 (file)
index 0000000..753513a
--- /dev/null
+++ b/arch/microblaze/include/asm/hash.h
@@ -0,0 +1,81 @@
+#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 */