github.com/gocuntian/go@v0.0.0-20160610041250-fee02d270bf8/src/runtime/internal/sys/intrinsics.go (about)

     1  // Copyright 2016 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  package sys
     6  
     7  // Using techniques from http://supertech.csail.mit.edu/papers/debruijn.pdf
     8  
     9  const deBruijn64 = 0x0218a392cd3d5dbf
    10  
    11  var deBruijnIdx64 = [64]byte{
    12  	0, 1, 2, 7, 3, 13, 8, 19,
    13  	4, 25, 14, 28, 9, 34, 20, 40,
    14  	5, 17, 26, 38, 15, 46, 29, 48,
    15  	10, 31, 35, 54, 21, 50, 41, 57,
    16  	63, 6, 12, 18, 24, 27, 33, 39,
    17  	16, 37, 45, 47, 30, 53, 49, 56,
    18  	62, 11, 23, 32, 36, 44, 52, 55,
    19  	61, 22, 43, 51, 60, 42, 59, 58,
    20  }
    21  
    22  const deBruijn32 = 0x04653adf
    23  
    24  var deBruijnIdx32 = [32]byte{
    25  	0, 1, 2, 6, 3, 11, 7, 16,
    26  	4, 14, 12, 21, 8, 23, 17, 26,
    27  	31, 5, 10, 15, 13, 20, 22, 25,
    28  	30, 9, 19, 24, 29, 18, 28, 27,
    29  }
    30  
    31  const deBruijn16 = 0x09af
    32  
    33  var deBruijnIdx16 = [16]byte{
    34  	0, 1, 2, 5, 3, 9, 6, 11,
    35  	15, 4, 8, 10, 14, 7, 13, 12,
    36  }
    37  
    38  const deBruijn8 = 0x17
    39  
    40  var deBruijnIdx8 = [8]byte{
    41  	0, 1, 2, 4, 7, 3, 6, 5,
    42  }
    43  
    44  // Ctz64 counts trailing (low-order) zeroes,
    45  // and if all are zero, then 64.
    46  func Ctz64(x uint64) uint64 {
    47  	x &= -x                      // isolate low-order bit
    48  	y := x * deBruijn64 >> 58    // extract part of deBruijn sequence
    49  	y = uint64(deBruijnIdx64[y]) // convert to bit index
    50  	z := (x - 1) >> 57 & 64      // adjustment if zero
    51  	return y + z
    52  }
    53  
    54  // Ctz32 counts trailing (low-order) zeroes,
    55  // and if all are zero, then 32.
    56  func Ctz32(x uint32) uint32 {
    57  	x &= -x                      // isolate low-order bit
    58  	y := x * deBruijn32 >> 27    // extract part of deBruijn sequence
    59  	y = uint32(deBruijnIdx32[y]) // convert to bit index
    60  	z := (x - 1) >> 26 & 32      // adjustment if zero
    61  	return y + z
    62  }
    63  
    64  // Ctz16 counts trailing (low-order) zeroes,
    65  // and if all are zero, then 16.
    66  func Ctz16(x uint16) uint16 {
    67  	x &= -x                      // isolate low-order bit
    68  	y := x * deBruijn16 >> 12    // extract part of deBruijn sequence
    69  	y = uint16(deBruijnIdx16[y]) // convert to bit index
    70  	z := (x - 1) >> 11 & 16      // adjustment if zero
    71  	return y + z
    72  }
    73  
    74  // Ctz8 counts trailing (low-order) zeroes,
    75  // and if all are zero, then 8.
    76  func Ctz8(x uint8) uint8 {
    77  	x &= -x                    // isolate low-order bit
    78  	y := x * deBruijn8 >> 5    // extract part of deBruijn sequence
    79  	y = uint8(deBruijnIdx8[y]) // convert to bit index
    80  	z := (x - 1) >> 4 & 8      // adjustment if zero
    81  	return y + z
    82  }
    83  
    84  // Bswap64 returns its input with byte order reversed
    85  // 0x0102030405060708 -> 0x0807060504030201
    86  func Bswap64(x uint64) uint64 {
    87  	c8 := uint64(0x00ff00ff00ff00ff)
    88  	a := x >> 8 & c8
    89  	b := (x & c8) << 8
    90  	x = a | b
    91  	c16 := uint64(0x0000ffff0000ffff)
    92  	a = x >> 16 & c16
    93  	b = (x & c16) << 16
    94  	x = a | b
    95  	c32 := uint64(0x00000000ffffffff)
    96  	a = x >> 32 & c32
    97  	b = (x & c32) << 32
    98  	x = a | b
    99  	return x
   100  }
   101  
   102  // Bswap32 returns its input with byte order reversed
   103  // 0x01020304 -> 0x04030201
   104  func Bswap32(x uint32) uint32 {
   105  	c8 := uint32(0x00ff00ff)
   106  	a := x >> 8 & c8
   107  	b := (x & c8) << 8
   108  	x = a | b
   109  	c16 := uint32(0x0000ffff)
   110  	a = x >> 16 & c16
   111  	b = (x & c16) << 16
   112  	x = a | b
   113  	return x
   114  }