@@ -30,6 +30,7 @@ config PARISC
select TTY # Needed for pdc_cons.c
select HAVE_DEBUG_STACKOVERFLOW
select HAVE_ARCH_AUDITSYSCALL
+ select HAVE_ARCH_HASH
select HAVE_ARCH_SECCOMP_FILTER
select ARCH_NO_COHERENT_DMA_MMAP
new file mode 100644
@@ -0,0 +1,164 @@
+#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 are out of order and don't need such careful
+ * scheduling.
+ *
+ * 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
+
+#if HAVE_ARCH_HASH_64 == 1
+/*
+ * Multiply by GOLDEN_RATIO_64. This number factors as:
+ * 0x0x61C8864680B583EB
+ * = 7046029254386353131
+ * = 3 * 3 * 53 * 2237 * 22739 * 290394721
+ * = 9 * 2695958579 * 290394721
+ * = 2695958579 * 2613552489
+ *
+ * While the Hcub software crashes on numbers too close to 64 bits,
+ * and finds crappy solutions for numbers much more than 32 bits, it
+ * can find decent sequences for those two values. So cascade the two.
+ *
+ * Here's an alternative sequence for step 2, based on
+ * a 6-shift 6-add sequence for 290394721 (sign-flipped so the one
+ * small shift is in an add) followed by a multiply by 9:
+ * a = x - (x << 7);
+ * b = (a << 16) + a - (x << 20);
+ * c = (x << 3) + b;
+ * x = (a << 10) - (c << 5) + b;
+ * x *= 9;
+ * ... however, it's not clear which is better.
+ *
+ * The asm("" : "=r" () : "0" (...)) statements are simply assignment
+ * statments, but they stop GCC from doing stupid things.
+ * Any time GCC sees two consecutive shifts, it insists on trying to
+ * merge them, even at the expense of another temporary and going out
+ * of short shift range.
+ * Likewise, it tries to combine adds and subtracts without considering
+ * that PA has shift-and-add but not shift-and-subtract. So "x = -x;
+ * a = (a << 1) + x" is the same two instructions as "a <<= 1; a -= x",
+ * but the latter is a 2-cycle dependency chain on a, while the former
+ * is only one.
+ */
+static __always_inline u32 __attribute_const__
+hash_64(u64 x, unsigned int bits)
+{
+ u64 a, b, c;
+
+ /* Step 1: Multiply by 2695958579 */
+ /* 6 shifts + 6 adds, one is small enough for shladd */
+ /* 6 cycle dependency chain */
+ a = (x << 20) + x;
+ b = x << 29;
+ a += (x << 4);
+ x <<= 11;
+ b = a - b;
+ a += x;
+ b *= 3;
+ x = (a << 12) + b;
+
+ /* Step 2: Multiply by 2613552489 */
+ /* 7 shift + 7 adds, three are small enough for shladd */
+ /* 6 cycle dependency chain (via a) */
+ a = (x << 19) + x;
+ asm("" : "=r" (c) : "0" (x << 9));
+ asm("" : "=r" (b) : "0" (a + (c << 3)));
+ asm("" : "=r" (x) : "0" (-x));
+ asm("" : "=r" (a) : "0" (a << 7));
+ b = a - b;
+ a = (a << 1) + x;
+ c -= b;
+ a = (a << 1) + b;
+ x = (a << 3) + c;
+
+ return x >> (64 - bits);
+}
+#else /* HAVE_ARCH_HASH_64 != 1 */
+/*
+ * If we don't care about matching the generic function, here's an
+ * alternative hash function; Thomas Wang's 64-to-32 bit hash function.
+ * https://web.archive.org/web/2011/http://www.concentric.net/~Ttwang/tech/inthash.htm
+ * http://burtleburtle.net/bob/hash/integer.html
+ *
+ * This algorithm concentrates the entropy in the low bits of the output,
+ * so they are returned.
+ *
+ * The code is smaller than the multiply, but each instruction (there
+ * are usually 2 per line) is sequentially dependent, so it's also a
+ * 12-cycle dependency chain.
+ */
+static __always_inline u32 __attribute_const__
+hash_64(u64 x, unsigned int bits)
+{
+ u64 y;
+
+ if (!__builtin_constant_p(bits))
+ asm("mtsarcm %1" : "=q" (bits) : "r" (bits));
+
+ x = ~x + (x << 18);
+ x ^= x >> 31;
+ y = x * 5; /* GCC uses 3 instructions for "x *= 21" */
+ x += y << 2;
+ x ^= x >> 11;
+ x += x << 6;
+ x ^= x >> 22;
+
+ if (__builtin_constant_p(bits)) {
+ x = x >> (64 - bits) << (64 - bits);
+ } else {
+ asm("depdi,z 0x1F,%%sar,63,%0" : "=r" (y) : "q" (bits));
+ x &= ~y;
+ }
+
+ return x;
+}
+
+#endif /* HAVE_ARCH_HASH_64 */
+#endif /* BITS_PER_LONG == 64 */
+
+#endif /* _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. __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. My tools can't find good shift-and-add sequence for the large multiplier, but the multiplier can be factored and two sequences for smaller multipliers cascaded. An alternative implementation is included, but not enabled by default: Thomas Wang's 64-to-32-bit hash. This is more compact than the multiply, but has the same length dependency chain. Signed-off-by: George Spelvin <linux@sciencehorizons.net> Cc: Helge Deller <deller@gmx.de> Cc: linux-parisc@vger.kernel.org --- Definitely an interesting architecture. How did I do? This should pass the self-test in test_hash.ko. arch/parisc/Kconfig | 1 + arch/parisc/include/asm/hash.h | 164 +++++++++++++++++++++++++++++++++++++++++ 2 files changed, 165 insertions(+) create mode 100644 arch/parisc/include/asm/hash.h