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Source file src/hash/crc32/gen_const_ppc64le.go

Documentation: hash/crc32

  // Copyright 2017 The Go Authors. All rights reserved.
  // Use of this source code is governed by a BSD-style
  // license that can be found in the LICENSE file.
  
  // +build ignore
  
  // Generate the constant table associated with the poly used by the
  // vpmsumd crc32 algorithm.
  //
  // go run gen_const_ppc64le.go
  //
  // generates crc32_table_ppc64le.s
  
  // The following is derived from code written by Anton Blanchard
  // <anton@au.ibm.com> found at https://github.com/antonblanchard/crc32-vpmsum.
  // The original is dual licensed under GPL and Apache 2.  As the copyright holder
  // for the work, IBM has contributed this new work under the golang license.
  
  // This code was written in Go based on the original C implementation.
  
  // This is a tool needed to generate the appropriate constants needed for
  // the vpmsum algorithm.  It is included to generate new constant tables if
  // new polynomial values are included in the future.
  
  package main
  
  import (
  	"bytes"
  	"fmt"
  	"io/ioutil"
  )
  
  var blocking = 32 * 1024
  
  func reflect_bits(b uint64, nr uint) uint64 {
  	var ref uint64
  
  	for bit := uint64(0); bit < uint64(nr); bit++ {
  		if (b & uint64(1)) == 1 {
  			ref |= (1 << (uint64(nr-1) - bit))
  		}
  		b = (b >> 1)
  	}
  	return ref
  }
  
  func get_remainder(poly uint64, deg uint, n uint) uint64 {
  
  	rem, _ := xnmodp(n, poly, deg)
  	return rem
  }
  
  func get_quotient(poly uint64, bits, n uint) uint64 {
  
  	_, div := xnmodp(n, poly, bits)
  	return div
  }
  
  // xnmodp returns two values, p and div:
  // p is the representation of the binary polynomial x**n mod (x ** deg + "poly")
  // That is p is the binary representation of the modulus polynomial except for its highest-order term.
  // div is the binary representation of the polynomial x**n / (x ** deg + "poly")
  func xnmodp(n uint, poly uint64, deg uint) (uint64, uint64) {
  
  	var mod, mask, high, div uint64
  
  	if n < deg {
  		div = 0
  		return poly, div
  	}
  	mask = 1<<deg - 1
  	poly &= mask
  	mod = poly
  	div = 1
  	deg--
  	n--
  	for n > deg {
  		high = (mod >> deg) & 1
  		div = (div << 1) | high
  		mod <<= 1
  		if high != 0 {
  			mod ^= poly
  		}
  		n--
  	}
  	return mod & mask, div
  }
  
  func main() {
  	w := new(bytes.Buffer)
  
  	fmt.Fprintf(w, "// autogenerated: do not edit!\n")
  	fmt.Fprintf(w, "// generated from crc32/gen_const_ppc64le.go\n")
  	fmt.Fprintln(w)
  	fmt.Fprintf(w, "#include \"textflag.h\"\n")
  
  	// These are the polynomials supported in vector now.
  	// If adding others, include the polynomial and a name
  	// to identify it.
  
  	genCrc32ConstTable(w, 0xedb88320, "IEEE")
  	genCrc32ConstTable(w, 0x82f63b78, "Cast")
  	genCrc32ConstTable(w, 0xeb31d82e, "Koop")
  	b := w.Bytes()
  
  	err := ioutil.WriteFile("crc32_table_ppc64le.s", b, 0666)
  	if err != nil {
  		fmt.Printf("can't write output: %s\n", err)
  	}
  }
  
  func genCrc32ConstTable(w *bytes.Buffer, poly uint32, polyid string) {
  
  	ref_poly := reflect_bits(uint64(poly), 32)
  	fmt.Fprintf(w, "\n\t/* Reduce %d kbits to 1024 bits */\n", blocking*8)
  	j := 0
  	for i := (blocking * 8) - 1024; i > 0; i -= 1024 {
  		a := reflect_bits(get_remainder(ref_poly, 32, uint(i)), 32) << 1
  		b := reflect_bits(get_remainder(ref_poly, 32, uint(i+64)), 32) << 1
  
  		fmt.Fprintf(w, "\t/* x^%d mod p(x)%s, x^%d mod p(x)%s */\n", uint(i+64), "", uint(i), "")
  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%016x\n", polyid, j*8, b)
  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%016x\n", polyid, (j+1)*8, a)
  
  		j += 2
  		fmt.Fprintf(w, "\n")
  	}
  
  	for i := (1024 * 2) - 128; i >= 0; i -= 128 {
  		a := reflect_bits(get_remainder(ref_poly, 32, uint(i+32)), 32)
  		b := reflect_bits(get_remainder(ref_poly, 32, uint(i+64)), 32)
  		c := reflect_bits(get_remainder(ref_poly, 32, uint(i+96)), 32)
  		d := reflect_bits(get_remainder(ref_poly, 32, uint(i+128)), 32)
  
  		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, "")
  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%08x%08x\n", polyid, j*8, c, d)
  		fmt.Fprintf(w, "DATA ·%sConst+%d(SB)/8,$0x%08x%08x\n", polyid, (j+1)*8, a, b)
  
  		j += 2
  		fmt.Fprintf(w, "\n")
  	}
  
  	fmt.Fprintf(w, "GLOBL ·%sConst(SB),RODATA,$4336\n", polyid)
  	fmt.Fprintf(w, "\n /* Barrett constant m - (4^32)/n */\n")
  	fmt.Fprintf(w, "DATA ·%sBarConst(SB)/8,$0x%016x\n", polyid, reflect_bits(get_quotient(ref_poly, 32, 64), 33))
  	fmt.Fprintf(w, "DATA ·%sBarConst+8(SB)/8,$0x0000000000000000\n", polyid)
  	fmt.Fprintf(w, "DATA ·%sBarConst+16(SB)/8,$0x%016x\n", polyid, reflect_bits((uint64(1)<<32)|ref_poly, 33)) // reflected?
  	fmt.Fprintf(w, "DATA ·%sBarConst+24(SB)/8,$0x0000000000000000\n", polyid)
  	fmt.Fprintf(w, "GLOBL ·%sBarConst(SB),RODATA,$32\n", polyid)
  }
  

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