github.com/geraldss/go/src@v0.0.0-20210511222824-ac7d0ebfc235/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  // +build !386
     6  
     7  // TODO finish intrinsifying 386, deadcode the assembly, remove build tags, merge w/ intrinsics_common
     8  // TODO replace all uses of CtzXX with TrailingZerosXX; they are the same.
     9  
    10  package sys
    11  
    12  // Using techniques from http://supertech.csail.mit.edu/papers/debruijn.pdf
    13  
    14  const deBruijn64ctz = 0x0218a392cd3d5dbf
    15  
    16  var deBruijnIdx64ctz = [64]byte{
    17  	0, 1, 2, 7, 3, 13, 8, 19,
    18  	4, 25, 14, 28, 9, 34, 20, 40,
    19  	5, 17, 26, 38, 15, 46, 29, 48,
    20  	10, 31, 35, 54, 21, 50, 41, 57,
    21  	63, 6, 12, 18, 24, 27, 33, 39,
    22  	16, 37, 45, 47, 30, 53, 49, 56,
    23  	62, 11, 23, 32, 36, 44, 52, 55,
    24  	61, 22, 43, 51, 60, 42, 59, 58,
    25  }
    26  
    27  const deBruijn32ctz = 0x04653adf
    28  
    29  var deBruijnIdx32ctz = [32]byte{
    30  	0, 1, 2, 6, 3, 11, 7, 16,
    31  	4, 14, 12, 21, 8, 23, 17, 26,
    32  	31, 5, 10, 15, 13, 20, 22, 25,
    33  	30, 9, 19, 24, 29, 18, 28, 27,
    34  }
    35  
    36  // Ctz64 counts trailing (low-order) zeroes,
    37  // and if all are zero, then 64.
    38  func Ctz64(x uint64) int {
    39  	x &= -x                       // isolate low-order bit
    40  	y := x * deBruijn64ctz >> 58  // extract part of deBruijn sequence
    41  	i := int(deBruijnIdx64ctz[y]) // convert to bit index
    42  	z := int((x - 1) >> 57 & 64)  // adjustment if zero
    43  	return i + z
    44  }
    45  
    46  // Ctz32 counts trailing (low-order) zeroes,
    47  // and if all are zero, then 32.
    48  func Ctz32(x uint32) int {
    49  	x &= -x                       // isolate low-order bit
    50  	y := x * deBruijn32ctz >> 27  // extract part of deBruijn sequence
    51  	i := int(deBruijnIdx32ctz[y]) // convert to bit index
    52  	z := int((x - 1) >> 26 & 32)  // adjustment if zero
    53  	return i + z
    54  }
    55  
    56  // Ctz8 returns the number of trailing zero bits in x; the result is 8 for x == 0.
    57  func Ctz8(x uint8) int {
    58  	return int(ntz8tab[x])
    59  }
    60  
    61  // Bswap64 returns its input with byte order reversed
    62  // 0x0102030405060708 -> 0x0807060504030201
    63  func Bswap64(x uint64) uint64 {
    64  	c8 := uint64(0x00ff00ff00ff00ff)
    65  	a := x >> 8 & c8
    66  	b := (x & c8) << 8
    67  	x = a | b
    68  	c16 := uint64(0x0000ffff0000ffff)
    69  	a = x >> 16 & c16
    70  	b = (x & c16) << 16
    71  	x = a | b
    72  	c32 := uint64(0x00000000ffffffff)
    73  	a = x >> 32 & c32
    74  	b = (x & c32) << 32
    75  	x = a | b
    76  	return x
    77  }
    78  
    79  // Bswap32 returns its input with byte order reversed
    80  // 0x01020304 -> 0x04030201
    81  func Bswap32(x uint32) uint32 {
    82  	c8 := uint32(0x00ff00ff)
    83  	a := x >> 8 & c8
    84  	b := (x & c8) << 8
    85  	x = a | b
    86  	c16 := uint32(0x0000ffff)
    87  	a = x >> 16 & c16
    88  	b = (x & c16) << 16
    89  	x = a | b
    90  	return x
    91  }