From: George Spelvin <linux@sciencehorizons.net>
To: Linus Torvalds <torvalds@linux-foundation.org>,
lkml <linux-kernel@vger.kernel.org>
Cc: "J . Bruce Fields" <bfields@redhat.com>,
George Spelvin <linux@sciencehorizons.net>,
Alistair Francis <alistair.francis@xilinx.com>,
Michal Simek <michal.simek@xilinx.com>
Subject: [PATCH v3 09/10] microblaze: Add <asm/hash.h>
Date: Sat, 28 May 2016 15:57:22 -0400 [thread overview]
Message-ID: <1464465443-25305-10-git-send-email-linux@sciencehorizons.net> (raw)
In-Reply-To: <1464465443-25305-1-git-send-email-linux@sciencehorizons.net>
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>
---
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
--- 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 00000000..753513ae
--- /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 */
--
2.8.1
next prev parent reply other threads:[~2016-05-28 19:58 UTC|newest]
Thread overview: 55+ messages / expand[flat|nested] mbox.gz Atom feed top
[not found] <CA+55aFxPSW+84KfQ1N_WmND-wtvgj2zQm8nFPkRcc+gyU=uing@mail.gmail.com>
2016-05-25 7:20 ` [PATCH 00/10] String hash improvements George Spelvin
2016-05-25 8:00 ` Geert Uytterhoeven
2016-05-25 8:11 ` George Spelvin
2016-05-25 8:50 ` Geert Uytterhoeven
2016-05-25 9:07 ` George Spelvin
2016-05-25 16:08 ` Linus Torvalds
2016-05-28 19:57 ` [PATCH v3 " George Spelvin
2016-05-28 19:57 ` [PATCH v3 01/10] Pull out string hash to <linux/stringhash.h> George Spelvin
2016-05-28 19:57 ` [PATCH v3 02/10] fs/namei.c: Add hashlen_string() function George Spelvin
2016-05-28 19:57 ` [PATCH v3 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hashlen_string() George Spelvin
2016-05-28 19:57 ` [PATCH v3 04/10] Change hash_64() return value to 32 bits George Spelvin
2016-05-28 19:57 ` [PATCH v3 05/10] Eliminate bad hash multipliers from hash_32() and hash_64() George Spelvin
2016-05-28 19:57 ` [PATCH v3 06/10] fs/namei.c: Improve dcache hash function George Spelvin
2016-05-30 15:11 ` Peter Zijlstra
2016-05-30 16:06 ` George Spelvin
2016-05-30 16:27 ` Peter Zijlstra
2016-05-30 18:10 ` George Spelvin
2016-06-02 1:18 ` Linus Torvalds
2016-06-02 2:31 ` George Spelvin
2016-06-02 16:35 ` Linus Torvalds
2016-06-02 18:23 ` George Spelvin
2016-05-28 19:57 ` [PATCH v3 07/10] <linux/hash.h>: Add support for architecture-specific functions George Spelvin
2016-05-29 7:57 ` Geert Uytterhoeven
2016-05-28 19:57 ` [PATCH v3 08/10] m68k: Add <asm/hash.h> George Spelvin
2016-05-28 19:57 ` George Spelvin [this message]
2016-05-28 19:57 ` [PATCH v3 10/10] h8300: " George Spelvin
2016-05-28 20:47 ` [PATCH v3 00/10] String hash improvements Linus Torvalds
2016-05-28 20:54 ` George Spelvin
2016-06-02 22:59 ` [PATCH " Fubo Chen
2016-05-26 17:09 ` [PATCH v2 " George Spelvin
2016-05-25 7:21 ` [PATCH 01/10] Pull out string hash to <linux/stringhash.h> George Spelvin
2016-05-25 7:22 ` [PATCH 02/10] fs/namei.c: Add hash_string() function George Spelvin
2016-05-25 7:26 ` [PATCH 03/10] <linux/sunrpc/svcauth.h>: Define hash_str() in terms of hash_string() George Spelvin
2016-05-25 7:28 ` [PATCH 04/10] Change hash_64() return value to 32 bits George Spelvin
2016-05-25 7:29 ` [PATCH 05/10] Eliminate bad hash multipliers from hash_32() and hash_64() George Spelvin
2016-05-25 7:31 ` [PATCH 06/10] fs/namei.c: Improve dcache hash function George Spelvin
2016-05-25 7:33 ` [PATCH 07/10] <linux/hash.h>: Add support for architecture-specific functions George Spelvin
2016-05-26 17:16 ` [PATCH v2 " George Spelvin
2016-05-25 7:34 ` [PATCH 08/10] m68k: Add <asm/archhash.h> George Spelvin
2016-05-25 7:34 ` George Spelvin
2016-05-25 8:07 ` Geert Uytterhoeven
2016-05-25 8:19 ` George Spelvin
2016-05-25 8:24 ` [PATCH 08v2/10] " George Spelvin
2016-05-25 8:48 ` Geert Uytterhoeven
2016-05-25 8:56 ` [PATCH 08/10] " Philippe De Muyter
2016-05-25 9:14 ` George Spelvin
2016-05-25 9:31 ` Andreas Schwab
2016-05-25 9:51 ` Philippe De Muyter
2016-05-25 13:24 ` Philippe De Muyter
2016-05-25 13:42 ` George Spelvin
2016-05-26 17:19 ` [PATCH v2 08/10] m68k: Add <asm/hash.h> George Spelvin
2016-05-25 7:37 ` [PATCH 09/10] microblaze: Add <asm/archhash.h> George Spelvin
2016-05-26 17:21 ` [PATCH v2 09/10] microblaze: Add <asm/hash.h> George Spelvin
2016-05-25 7:38 ` [PATCH 10/10] h8300: Add <asm/archhash.h> George Spelvin
2016-05-26 17:23 ` [PATCH v2 10/10] h8300: Add <asm/hash.h> George Spelvin
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