[PATCH v3 09/10] microblaze: Add <asm/hash.h>
From: George Spelvin
Date: Sat May 28 2016 - 15:58:33 EST
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@xxxxxxxxxxxxxxxxxxx>
Cc: Alistair Francis <alistair.francis@xxxxxxxxxx>
Cc: Michal Simek <michal.simek@xxxxxxxxxx>
arch/microblaze/Kconfig | 1 +
arch/microblaze/include/asm/hash.h | 81 ++++++++++++++++++++++++++++++++++++++
2 files changed, 82 insertions(+)
create mode 100644 arch/microblaze/include/asm/hash.h
diff --git a/arch/microblaze/Kconfig b/arch/microblaze/Kconfig
index 3d793b55..ce3e5125 100644
@@ -16,6 +16,7 @@ config MICROBLAZE
+ select HAVE_ARCH_HASH
diff --git a/arch/microblaze/include/asm/hash.h b/arch/microblaze/include/asm/hash.h
new file mode 100644
@@ -0,0 +1,81 @@
+ * 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)
+ 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 */
+ * "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 /* !CONFIG_XILINX_MICROBLAZE0_USE_HW_MUL */
+#endif /* _ASM_HASH_H */