[PATCH v2 08/10] m68k: Add <asm/hash.h>
From: George Spelvin
Date: Fri May 27 2016 - 11:06:15 EST
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
Signed-off-by: George Spelvin <linux@xxxxxxxxxxxxxxxxxxx>
Cc: Geert Uytterhoeven <geert@xxxxxxxxxxxxxx>
Cc: Greg Ungerer <gerg@xxxxxxxxxxxxxx>
Cc: Andreas Schwab <schwab@xxxxxxxxxxxxxx>
Cc: Philippe De Muyter <phdm@xxxxxxx>
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
@@ -40,6 +40,7 @@ config M68000
+ select HAVE_ARCH_HASH
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
@@ -0,0 +1,59 @@
+ * 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 */