github.com/corona10/go@v0.0.0-20180224231303-7a218942be57/src/cmd/compile/internal/ssa/bits_bootstrap.go (about)

     1  // Copyright 2018 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  // +build !go1.9
     6  
     7  package ssa
     8  
     9  const deBruijn64 = 0x03f79d71b4ca8b09
    10  
    11  var deBruijn64tab = [64]byte{
    12  	0, 1, 56, 2, 57, 49, 28, 3, 61, 58, 42, 50, 38, 29, 17, 4,
    13  	62, 47, 59, 36, 45, 43, 51, 22, 53, 39, 33, 30, 24, 18, 12, 5,
    14  	63, 55, 48, 27, 60, 41, 37, 16, 46, 35, 44, 21, 52, 32, 23, 11,
    15  	54, 26, 40, 15, 34, 20, 31, 10, 25, 14, 19, 9, 13, 8, 7, 6,
    16  }
    17  
    18  // TrailingZeros64 returns the number of trailing zero bits in x; the result is 64 for x == 0.
    19  func TrailingZeros64(x uint64) int {
    20  	if x == 0 {
    21  		return 64
    22  	}
    23  	return int(deBruijn64tab[(x&-x)*deBruijn64>>(64-6)])
    24  }