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Source file src/runtime/internal/sys/intrinsics.go

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

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