parisc: Add <asm/hash.h>

PA-RISC is interesting; integer multiplies are implemented in the
FPU, so are painful in the kernel.  But it tries to be friendly to
shift-and-add sequences for constant multiplies.

__hash_32 is implemented using the same shift-and-add sequence as
Microblaze, just scheduled for the PA7100.  (It's 2-way superscalar
but in-order, like the Pentium.)

hash_64 was tricky, but a suggestion from Jason Thong allowed a
good solution by breaking up the multiplier.  After a lot of manual
optimization, I found a 19-instruction sequence for the multiply that
can be executed in 10 cycles using only 4 temporaries.

(The PA8xxx can issue 4 instructions per cycle, but 2 must be ALU ops
and 2 must be loads/stores.  And the final add can't be paired.)

An alternative considered, but ultimately not used, was Thomas Wang's
64-to-32-bit integer hash.  At 12 instructions, it's smaller, but they're
all sequentially dependent, so it has longer latency.

https://web.archive.org/web/2011/http://www.concentric.net/~Ttwang/tech/inthash.htm
http://burtleburtle.net/bob/hash/integer.html

Signed-off-by: George Spelvin <linux@sciencehorizons.net>
Cc: Helge Deller <deller@gmx.de>
Cc: linux-parisc@vger.kernel.org
Signed-off-by: Helge Deller <deller@gmx.de>

1 files changed, 146 insertions, 0 deletions

diff --git a/arch/parisc/include/asm/hash.h b/arch/parisc/include/asm/hash.h
new file mode 100644
index 000000000000..dbe93311aa26
--- /dev/null
+++ b/arch/parisc/include/asm/hash.h
@@ -0,0 +1,146 @@
+#ifndef _ASM_HASH_H
+#define _ASM_HASH_H
+
+/*
+ * HP-PA only implements integer multiply in the FPU.  However, for
+ * integer multiplies by constant, it has a number of shift-and-add
+ * (but no shift-and-subtract, sigh!) instructions that a compiler
+ * can synthesize a code sequence with.
+ *
+ * Unfortunately, GCC isn't very efficient at using them.  For example
+ * it uses three instructions for "x *= 21" when only two are needed.
+ * But we can find a sequence manually.
+ */
+
+#define HAVE_ARCH__HASH_32 1
+
+/*
+ * This is a multiply by GOLDEN_RATIO_32 = 0x61C88647 optimized for the
+ * PA7100 pairing rules.  This is an in-order 2-way superscalar processor.
+ * Only one instruction in a pair may be a shift (by more than 3 bits),
+ * but other than that, simple ALU ops (including shift-and-add by up
+ * to 3 bits) may be paired arbitrarily.
+ *
+ * PA8xxx processors also dual-issue ALU instructions, although with
+ * fewer constraints, so this schedule is good for them, too.
+ *
+ * This 6-step sequence was found by Yevgen Voronenko's implementation
+ * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html.
+ */
+static inline u32 __attribute_const__ __hash_32(u32 x)
+{
+	u32 a, b, c;
+
+	/*
+	 * Phase 1: Compute  a = (x << 19) + x,
+	 * b = (x << 9) + a, c = (x << 23) + b.
+	 */
+	a = x << 19;		/* Two shifts can't be paired */
+	b = x << 9;	a += x;
+	c = x << 23;	b += a;
+			c += b;
+	/* Phase 2: Return (b<<11) + (c<<6) + (a<<3) - c */
+	b <<= 11;
+	a += c << 3;	b -= c;
+	return (a << 3) + b;
+}
+
+#if BITS_PER_LONG == 64
+
+#define HAVE_ARCH_HASH_64 1
+
+/*
+ * Finding a good shift-and-add chain for GOLDEN_RATIO_64 is tricky,
+ * because available software for the purpose chokes on constants this
+ * large.  (It's mostly designed for compiling FIR filter coefficients
+ * into FPGAs.)
+ *
+ * However, Jason Thong pointed out a work-around.  The Hcub software
+ * (http://spiral.ece.cmu.edu/mcm/gen.html) is designed for *multiple*
+ * constant multiplication, and is good at finding shift-and-add chains
+ * which share common terms.
+ *
+ * Looking at 0x0x61C8864680B583EB in binary:
+ * 0110000111001000100001100100011010000000101101011000001111101011
+ *  \______________/    \__________/       \_______/     \________/
+ *   \____________________________/         \____________________/
+ * you can see the non-zero bits are divided into several well-separated
+ * blocks.  Hcub can find algorithms for those terms separately, which
+ * can then be shifted and added together.
+ *
+ * Dividing the input into 2, 3 or 4 blocks, Hcub can find solutions
+ * with 10, 9 or 8 adds, respectively, making a total of 11 for the
+ * whole number.
+ *
+ * Using just two large blocks, 0xC3910C8D << 31 in the high bits,
+ * and 0xB583EB in the low bits, produces as good an algorithm as any,
+ * and with one more small shift than alternatives.
+ *
+ * The high bits are a larger number and more work to compute, as well
+ * as needing one extra cycle to shift left 31 bits before the final
+ * addition, so they are the critical path for scheduling.  The low bits
+ * can fit into the scheduling slots left over.
+ */
+
+
+/*
+ * This _ASSIGN(dst, src) macro performs "dst = src", but prevents GCC
+ * from inferring anything about the value assigned to "dest".
+ *
+ * This prevents it from mis-optimizing certain sequences.
+ * In particular, gcc is annoyingly eager to combine consecutive shifts.
+ * Given "x <<= 19; y += x; z += x << 1;", GCC will turn this into
+ * "y += x << 19; z += x << 20;" even though the latter sequence needs
+ * an additional instruction and temporary register.
+ *
+ * Because no actual assembly code is generated, this construct is
+ * usefully portable across all GCC platforms, and so can be test-compiled
+ * on non-PA systems.
+ *
+ * In two places, additional unused input dependencies are added.  This
+ * forces GCC's scheduling so it does not rearrange instructions too much.
+ * Because the PA-8xxx is out of order, I'm not sure how much this matters,
+ * but why make it more difficult for the processor than necessary?
+ */
+#define _ASSIGN(dst, src, ...) asm("" : "=r" (dst) : "0" (src), ##__VA_ARGS__)
+
+/*
+ * Multiply by GOLDEN_RATIO_64 = 0x0x61C8864680B583EB using a heavily
+ * optimized shift-and-add sequence.
+ *
+ * Without the final shift, the multiply proper is 19 instructions,
+ * 10 cycles and uses only 4 temporaries.  Whew!
+ *
+ * You are not expected to understand this.
+ */
+static __always_inline u32 __attribute_const__
+hash_64(u64 a, unsigned int bits)
+{
+	u64 b, c, d;
+
+	/*
+	 * Encourage GCC to move a dynamic shift to %sar early,
+	 * thereby freeing up an additional temporary register.
+	 */
+	if (!__builtin_constant_p(bits))
+		asm("" : "=q" (bits) : "0" (64 - bits));
+	else
+		bits = 64 - bits;
+
+	_ASSIGN(b, a*5);	c = a << 13;
+	b = (b << 2) + a;	_ASSIGN(d, a << 17);
+	a = b + (a << 1);	c += d;
+	d = a << 10;		_ASSIGN(a, a << 19);
+	d = a - d;		_ASSIGN(a, a << 4, "X" (d));
+	c += b;			a += b;
+	d -= c;			c += a << 1;
+	a += c << 3;		_ASSIGN(b, b << (7+31), "X" (c), "X" (d));
+	a <<= 31;		b += d;
+	a += b;
+	return a >> bits;
+}
+#undef _ASSIGN	/* We're a widely-used header file, so don't litter! */
+
+#endif /* BITS_PER_LONG == 64 */
+
+#endif /* _ASM_HASH_H */