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>,
Geert Uytterhoeven <geert@linux-m68k.org>,
Greg Ungerer <gerg@linux-m68k.org>,
Andreas Schwab <schwab@linux-m68k.org>,
Philippe De Muyter <phdm@macq.eu>,
linux-m68k@vger.kernel.org
Subject: [PATCH v3 08/10] m68k: Add <asm/hash.h>
Date: Sat, 28 May 2016 15:57:21 -0400 [thread overview]
Message-ID: <1464465443-25305-9-git-send-email-linux@sciencehorizons.net> (raw)
In-Reply-To: <1464465443-25305-1-git-send-email-linux@sciencehorizons.net>
This provides a multiply by constant GOLDEN_RATIO_32 = 0x61C88647
for the original mc68000, which lacks a 32x32-bit multiply instruction.
Yes, the amount of optimization effort put in is excessive. :-)
Shift-add chain found by Yevgen Voronenko's Hcub algorithm at
http://spiral.ece.cmu.edu/mcm/gen.html
Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Cc: Geert Uytterhoeven <geert@linux-m68k.org>
Cc: Greg Ungerer <gerg@linux-m68k.org>
Cc: Andreas Schwab <schwab@linux-m68k.org>
Cc: Philippe De Muyter <phdm@macq.eu>
Cc: linux-m68k@lists.linux-m68k.org
---
arch/m68k/Kconfig.cpu | 1 +
arch/m68k/include/asm/hash.h | 59 ++++++++++++++++++++++++++++++++++++++++++++
2 files changed, 60 insertions(+)
create mode 100644 arch/m68k/include/asm/hash.h
diff --git a/arch/m68k/Kconfig.cpu b/arch/m68k/Kconfig.cpu
index 0dfcf128..bf3de464 100644
--- a/arch/m68k/Kconfig.cpu
+++ b/arch/m68k/Kconfig.cpu
@@ -40,6 +40,7 @@ config M68000
select CPU_HAS_NO_MULDIV64
select CPU_HAS_NO_UNALIGNED
select GENERIC_CSUM
+ select HAVE_ARCH_HASH
help
The Freescale (was Motorola) 68000 CPU is the first generation of
the well known M68K family of processors. The CPU core as well as
diff --git a/arch/m68k/include/asm/hash.h b/arch/m68k/include/asm/hash.h
new file mode 100644
index 00000000..6407af84
--- /dev/null
+++ b/arch/m68k/include/asm/hash.h
@@ -0,0 +1,59 @@
+#ifndef _ASM_HASH_H
+#define _ASM_HASH_H
+
+/*
+ * If CONFIG_M68000=y (original mc68000/010), this file is #included
+ * to work around the lack of a MULU.L instruction.
+ */
+
+#define HAVE_ARCH__HASH_32 1
+/*
+ * While it would be legal to substitute a different hash operation
+ * entirely, let's keep it simple and just use an optimized multiply
+ * by GOLDEN_RATIO_32 = 0x61C88647.
+ *
+ * The best way to do that appears to be to multiply by 0x8647 with
+ * shifts and adds, and use mulu.w to multiply the high half by 0x61C8.
+ *
+ * Because the 68000 has multi-cycle shifts, this addition chain is
+ * chosen to minimise the shift distances.
+ *
+ * Despite every attempt to spoon-feed it simple operations, GCC
+ * 6.1.1 doggedly insists on doing annoying things like converting
+ * "lsl.l #2,<reg>" (12 cycles) to two adds (8+8 cycles).
+ *
+ * It also likes to notice two shifts in a row, like "a = x << 2" and
+ * "a <<= 7", and convert that to "a = x << 9". But shifts longer
+ * than 8 bits are extra-slow on m68k, so that's a lose.
+ *
+ * Since the 68000 is a very simple in-order processor with no
+ * instruction scheduling effects on execution time, we can safely
+ * take it out of GCC's hands and write one big asm() block.
+ *
+ * Without calling overhead, this operation is 30 bytes (14 instructions
+ * plus one immediate constant) and 166 cycles.
+ *
+ * (Because %2 is fetched twice, it can't be postincrement, and thus it
+ * can't be a fully general "g" or "m". Register is preferred, but
+ * offsettable memory or immediate will work.)
+ */
+static inline u32 __attribute_const__ __hash_32(u32 x)
+{
+ u32 a, b;
+
+ asm( "move.l %2,%0" /* a = x * 0x0001 */
+ "\n lsl.l #2,%0" /* a = x * 0x0004 */
+ "\n move.l %0,%1"
+ "\n lsl.l #7,%0" /* a = x * 0x0200 */
+ "\n add.l %2,%0" /* a = x * 0x0201 */
+ "\n add.l %0,%1" /* b = x * 0x0205 */
+ "\n add.l %0,%0" /* a = x * 0x0402 */
+ "\n add.l %0,%1" /* b = x * 0x0607 */
+ "\n lsl.l #5,%0" /* a = x * 0x8040 */
+ : "=&d,d" (a), "=&r,r" (b)
+ : "r,roi?" (x)); /* a+b = x*0x8647 */
+
+ return ((u16)(x*0x61c8) << 16) + a + b;
+}
+
+#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 ` George Spelvin [this message]
2016-05-28 19:57 ` [PATCH v3 09/10] microblaze: Add <asm/hash.h> George Spelvin
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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