gen_const_ppc64le.go

Documentation: hash/crc32

     1  // Copyright 2017 The Go Authors. All rights reserved.
     2  // Use of this source code is governed by a BSD-style
     3  // license that can be found in the LICENSE file.
     4  
     5  //go:build ignore
     6  
     7  // Generate the constant table associated with the poly used by the
     8  // vpmsumd crc32 algorithm.
     9  //
    10  // go run gen_const_ppc64le.go
    11  //
    12  // generates crc32_table_ppc64le.s
    13  
    14  // The following is derived from code written by Anton Blanchard
    15  // <anton@au.ibm.com> found at https://github.com/antonblanchard/crc32-vpmsum.
    16  // The original is dual licensed under GPL and Apache 2.  As the copyright holder
    17  // for the work, IBM has contributed this new work under the golang license.
    18  
    19  // This code was written in Go based on the original C implementation.
    20  
    21  // This is a tool needed to generate the appropriate constants needed for
    22  // the vpmsum algorithm.  It is included to generate new constant tables if
    23  // new polynomial values are included in the future.
    24  
    25  package main
    26  
    27  import (
    28  	"bytes"
    29  	"fmt"
    30  	"os"
    31  )
    32  
    33  var blocking = 32 * 1024
    34  
    35  func reflect_bits(b uint64, nr uint) uint64 {
    36  	var ref uint64
    37  
    38  	for bit := uint64(0); bit < uint64(nr); bit++ {
    39  		if (b & uint64(1)) == 1 {
    40  			ref |= (1 << (uint64(nr-1) - bit))
    41  		}
    42  		b = (b >> 1)
    43  	}
    44  	return ref
    45  }
    46  
    47  func get_remainder(poly uint64, deg uint, n uint) uint64 {
    48  
    49  	rem, _ := xnmodp(n, poly, deg)
    50  	return rem
    51  }
    52  
    53  func get_quotient(poly uint64, bits, n uint) uint64 {
    54  
    55  	_, div := xnmodp(n, poly, bits)
    56  	return div
    57  }
    58  
    59  // xnmodp returns two values, p and div:
    60  // p is the representation of the binary polynomial x**n mod (x ** deg + "poly")
    61  // That is p is the binary representation of the modulus polynomial except for its highest-order term.
    62  // div is the binary representation of the polynomial x**n / (x ** deg + "poly")
    63  func xnmodp(n uint, poly uint64, deg uint) (uint64, uint64) {
    64  
    65  	var mod, mask, high, div uint64
    66  
    67  	if n < deg {
    68  		div = 0
    69  		return poly, div
    70  	}
    71  	mask = 1<<deg - 1
    72  	poly &= mask
    73  	mod = poly
    74  	div = 1
    75  	deg--
    76  	n--
    77  	for n > deg {
    78  		high = (mod >> deg) & 1
    79  		div = (div << 1) | high
    80  		mod <<= 1
    81  		if high != 0 {
    82  			mod ^= poly
    83  		}
    84  		n--
    85  	}
    86  	return mod & mask, div
    87  }
    88  
    89  func main() {
    90  	w := new(bytes.Buffer)
    91  
    92  	// Standard: https://go.dev/s/generatedcode
    93  	fmt.Fprintln(w, `// Code generated by "go run gen_const_ppc64le.go"; DO NOT EDIT.`)
    94  	fmt.Fprintln(w)
    95  	fmt.Fprintln(w, `#include "textflag.h"`)
    96  
    97  	// These are the polynomials supported in vector now.
    98  	// If adding others, include the polynomial and a name
    99  	// to identify it.
   100  
   101  	genCrc32ConstTable(w, 0xedb88320, "IEEE")
   102  	genCrc32ConstTable(w, 0x82f63b78, "Cast")
   103  	genCrc32ConstTable(w, 0xeb31d82e, "Koop")
   104  	b := w.Bytes()
   105  
   106  	err := os.WriteFile("crc32_table_ppc64le.s", b, 0666)
   107  	if err != nil {
   108  		fmt.Fprintf(os.Stderr, "can't write output: %s\n", err)
   109  	}
   110  }
   111  
   112  func genCrc32ConstTable(w *bytes.Buffer, poly uint32, polyid string) {
   113  
   114  	ref_poly := reflect_bits(uint64(poly), 32)
   115  	fmt.Fprintf(w, "\n\t/* Reduce %d kbits to 1024 bits */\n", blocking*8)
   116  	j := 0
   117  	for i := (blocking * 8) - 1024; i > 0; i -= 1024 {
   118  		a := reflect_bits(get_remainder(ref_poly, 32, uint(i)), 32) << 1
   119  		b := reflect_bits(get_remainder(ref_poly, 32, uint(i+64)), 32) << 1
   120  
   121  		fmt.Fprintf(w, "\t/* x^%d mod p(x)%s, x^%d mod p(x)%s */\n", uint(i+64), "", uint(i), "")
   122  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%016x\n", polyid, j*8, b)
   123  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%016x\n", polyid, (j+1)*8, a)
   124  
   125  		j += 2
   126  		fmt.Fprintf(w, "\n")
   127  	}
   128  
   129  	for i := (1024 * 2) - 128; i >= 0; i -= 128 {
   130  		a := reflect_bits(get_remainder(ref_poly, 32, uint(i+32)), 32)
   131  		b := reflect_bits(get_remainder(ref_poly, 32, uint(i+64)), 32)
   132  		c := reflect_bits(get_remainder(ref_poly, 32, uint(i+96)), 32)
   133  		d := reflect_bits(get_remainder(ref_poly, 32, uint(i+128)), 32)
   134  
   135  		fmt.Fprintf(w, "\t/* x^%d mod p(x)%s, x^%d mod p(x)%s, x^%d mod p(x)%s, x^%d mod p(x)%s */\n", i+128, "", i+96, "", i+64, "", i+32, "")
   136  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%08x%08x\n", polyid, j*8, c, d)
   137  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%08x%08x\n", polyid, (j+1)*8, a, b)
   138  
   139  		j += 2
   140  		fmt.Fprintf(w, "\n")
   141  	}
   142  
   143  	fmt.Fprintf(w, "GLOBL ·%sConst(SB),RODATA,$4336\n", polyid)
   144  	fmt.Fprintf(w, "\n\t/* Barrett constant m - (4^32)/n */\n")
   145  	fmt.Fprintf(w, "DATA ·%sBarConst(SB)/8,$0x%016x\n", polyid, reflect_bits(get_quotient(ref_poly, 32, 64), 33))
   146  	fmt.Fprintf(w, "DATA ·%sBarConst+8(SB)/8,$0x0000000000000000\n", polyid)
   147  	fmt.Fprintf(w, "DATA ·%sBarConst+16(SB)/8,$0x%016x\n", polyid, reflect_bits((uint64(1)<<32)|ref_poly, 33)) // reflected?
   148  	fmt.Fprintf(w, "DATA ·%sBarConst+24(SB)/8,$0x0000000000000000\n", polyid)
   149  	fmt.Fprintf(w, "GLOBL ·%sBarConst(SB),RODATA,$32\n", polyid)
   150  }
   151
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