2 files changed, 82 insertions, 0 deletions

diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index 3d793b55f60c..ce3e512517ce 100644
--- 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
index 000000000000..753513ae8cb0
--- /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 */