1 /* SPDX-License-Identifier: GPL-2.0 */
2 #ifndef _ASM_HASH_H
3 #define _ASM_HASH_H
4
5 /*
6 * HP-PA only implements integer multiply in the FPU. However, for
7 * integer multiplies by constant, it has a number of shift-and-add
8 * (but no shift-and-subtract, sigh!) instructions that a compiler
9 * can synthesize a code sequence with.
10 *
11 * Unfortunately, GCC isn't very efficient at using them. For example
12 * it uses three instructions for "x *= 21" when only two are needed.
13 * But we can find a sequence manually.
14 */
15
16 #define HAVE_ARCH__HASH_32 1
17
18 /*
19 * This is a multiply by GOLDEN_RATIO_32 = 0x61C88647 optimized for the
20 * PA7100 pairing rules. This is an in-order 2-way superscalar processor.
21 * Only one instruction in a pair may be a shift (by more than 3 bits),
22 * but other than that, simple ALU ops (including shift-and-add by up
23 * to 3 bits) may be paired arbitrarily.
24 *
25 * PA8xxx processors also dual-issue ALU instructions, although with
26 * fewer constraints, so this schedule is good for them, too.
27 *
28 * This 6-step sequence was found by Yevgen Voronenko's implementation
29 * of the Hcub algorithm at http://spiral.ece.cmu.edu/mcm/gen.html.
30 */
31 static inline u32 __attribute_const__ __hash_32(u32 x)
32 {
33 u32 a, b, c;
34
35 /*
36 * Phase 1: Compute a = (x << 19) + x,
37 * b = (x << 9) + a, c = (x << 23) + b.
38 */
39 a = x << 19; /* Two shifts can't be paired */
40 b = x << 9; a += x;
41 c = x << 23; b += a;
42 c += b;
43 /* Phase 2: Return (b<<11) + (c<<6) + (a<<3) - c */
44 b <<= 11;
45 a += c << 3; b -= c;
46 return (a << 3) + b;
47 }
48
49 #if BITS_PER_LONG == 64
50
51 #define HAVE_ARCH_HASH_64 1
52
53 /*
54 * Finding a good shift-and-add chain for GOLDEN_RATIO_64 is tricky,
55 * because available software for the purpose chokes on constants this
56 * large. (It's mostly designed for compiling FIR filter coefficients
57 * into FPGAs.)
58 *
59 * However, Jason Thong pointed out a work-around. The Hcub software
60 * (http://spiral.ece.cmu.edu/mcm/gen.html) is designed for *multiple*
61 * constant multiplication, and is good at finding shift-and-add chains
62 * which share common terms.
63 *
64 * Looking at 0x0x61C8864680B583EB in binary:
65 * 0110000111001000100001100100011010000000101101011000001111101011
66 * \______________/ \__________/ \_______/ \________/
67 * \____________________________/ \____________________/
68 * you can see the non-zero bits are divided into several well-separated
69 * blocks. Hcub can find algorithms for those terms separately, which
70 * can then be shifted and added together.
71 *
72 * Dividing the input into 2, 3 or 4 blocks, Hcub can find solutions
73 * with 10, 9 or 8 adds, respectively, making a total of 11 for the
74 * whole number.
75 *
76 * Using just two large blocks, 0xC3910C8D << 31 in the high bits,
77 * and 0xB583EB in the low bits, produces as good an algorithm as any,
78 * and with one more small shift than alternatives.
79 *
80 * The high bits are a larger number and more work to compute, as well
81 * as needing one extra cycle to shift left 31 bits before the final
82 * addition, so they are the critical path for scheduling. The low bits
83 * can fit into the scheduling slots left over.
84 */
85
86
87 /*
88 * This _ASSIGN(dst, src) macro performs "dst = src", but prevents GCC
89 * from inferring anything about the value assigned to "dest".
90 *
91 * This prevents it from mis-optimizing certain sequences.
92 * In particular, gcc is annoyingly eager to combine consecutive shifts.
93 * Given "x <<= 19; y += x; z += x << 1;", GCC will turn this into
94 * "y += x << 19; z += x << 20;" even though the latter sequence needs
95 * an additional instruction and temporary register.
96 *
97 * Because no actual assembly code is generated, this construct is
98 * usefully portable across all GCC platforms, and so can be test-compiled
99 * on non-PA systems.
100 *
101 * In two places, additional unused input dependencies are added. This
102 * forces GCC's scheduling so it does not rearrange instructions too much.
103 * Because the PA-8xxx is out of order, I'm not sure how much this matters,
104 * but why make it more difficult for the processor than necessary?
105 */
106 #define _ASSIGN(dst, src, ...) asm("" : "=r" (dst) : "" (src), ##__VA_ARGS__)
107
108 /*
109 * Multiply by GOLDEN_RATIO_64 = 0x0x61C8864680B583EB using a heavily
110 * optimized shift-and-add sequence.
111 *
112 * Without the final shift, the multiply proper is 19 instructions,
113 * 10 cycles and uses only 4 temporaries. Whew!
114 *
115 * You are not expected to understand this.
116 */
117 static __always_inline u32 __attribute_const__
118 hash_64(u64 a, unsigned int bits)
119 {
120 u64 b, c, d;
121
122 /*
123 * Encourage GCC to move a dynamic shift to %sar early,
124 * thereby freeing up an additional temporary register.
125 */
126 if (!__builtin_constant_p(bits))
127 asm("" : "=q" (bits) : "" (64 - bits));
128 else
129 bits = 64 - bits;
130
131 _ASSIGN(b, a*5); c = a << 13;
132 b = (b << 2) + a; _ASSIGN(d, a << 17);
133 a = b + (a << 1); c += d;
134 d = a << 10; _ASSIGN(a, a << 19);
135 d = a - d; _ASSIGN(a, a << 4, "X" (d));
136 c += b; a += b;
137 d -= c; c += a << 1;
138 a += c << 3; _ASSIGN(b, b << (7+31), "X" (c), "X" (d));
139 a <<= 31; b += d;
140 a += b;
141 return a >> bits;
142 }
143 #undef _ASSIGN /* We're a widely-used header file, so don't litter! */
144
145 #endif /* BITS_PER_LONG == 64 */
146
147 #endif /* _ASM_HASH_H */
148
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