linux-mm / page-reclaim / db9cc7d6f95e7d89b0ce57e785cfd9d67a7505d8 / . / arch / parisc / include / asm / hash.h

/* SPDX-License-Identifier: GPL-2.0 */ | |

#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 */ |