linux-kernel - Re: [PATCH 08v2/10] m68k: Add <asm/archhash.h>

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Message-ID: <20160525082401.7249.qmail@ns.sciencehorizons.net>
Date:	25 May 2016 04:24:01 -0400
From:	"George Spelvin" <linux@...encehorizons.net>
To:	geert@...ux-m68k.org, linux@...encehorizons.net
Cc:	gerg@...ux-m68k.org, linux-kernel@...r.kernel.org,
	linux-m68k@...ts.linux-m68k.org, tglx@...utronix.de,
	torvalds@...ux-foundation.org
Subject: Re: [PATCH 08v2/10] m68k: Add <asm/archhash.h>

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. :-)

Addition chains found by Yevgen Voronenko's Hcub algorithm at
http://spiral.ece.cmu.edu/mcm/gen.html

Signed-off-by: George Spelvin <linux@...encehorizons.net>
Cc: Geert Uytterhoeven <geert@...ux-m68k.org>
Cc: Greg Ungerer <gerg@...ux-m68k.org>
Cc: linux-m68k@...ts.linux-m68k.org
---
 arch/m68k/Kconfig.cpu            |  1 +
 arch/m68k/include/asm/archhash.h | 58 ++++++++++++++++++++++++++++++++++++++++
 2 files changed, 59 insertions(+)
 create mode 100644 arch/m68k/include/asm/archhash.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/archhash.h b/arch/m68k/include/asm/archhash.h
new file mode 100644
index 00000000..2532cf92
--- /dev/null
+++ b/arch/m68k/include/asm/archhash.h
@@ -0,0 +1,58 @@
+#ifndef _ASM_ARCHHASH_H
+#define _ASM_ARCHHASH_H
+
+/*
+ * The only 68k processors that lack MULU.L and so need this workaround
+ * are the original 68000 and 68010.
+ */
+#if defined(CONFIG_M68000) || defined(CONFIG_M68010)
+
+#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 GCC 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.
+ */
+static inline u32 __attribute_const__ __hash_32(u32 x)
+{
+	u32 a, b;
+
+	asm(   "move.l %2,%0"	/* 0x0001 */
+	"\n	lsl.l #2,%0"	/* 0x0004 */
+	"\n	move.l %0,%1"
+	"\n	lsl.l #7,%0"	/* 0x0200 */
+	"\n	add.l %2,%0"	/* 0x0201 */
+	"\n	add.l %0,%1"	/* 0x0205 */
+	"\n	add.l %0,%0"	/* 0x0402 */
+	"\n	add.l %0,%1"	/* 0x0607 */
+	"\n	lsl.l #5,%0"	/* 0x8040 */
+				/* 0x8647 */
+	: "=&d" (a), "=&r" (b)
+	: "g" (x));
+
+	return ((u16)(x*0x61c8) << 16) + a + b;
+}
+#endif	/* HAVE_ARCH__HASH_32 */
+
+#endif	/* _ASM_ARCHHASH_H */
-- 
2.8.1