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

Documentation: runtime/internal/sys

  // Copyright 2016 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 !386
  
  package sys
  
  // Using techniques from http://supertech.csail.mit.edu/papers/debruijn.pdf
  
  const deBruijn64 = 0x0218a392cd3d5dbf
  
  var deBruijnIdx64 = [64]byte{
  	0, 1, 2, 7, 3, 13, 8, 19,
  	4, 25, 14, 28, 9, 34, 20, 40,
  	5, 17, 26, 38, 15, 46, 29, 48,
  	10, 31, 35, 54, 21, 50, 41, 57,
  	63, 6, 12, 18, 24, 27, 33, 39,
  	16, 37, 45, 47, 30, 53, 49, 56,
  	62, 11, 23, 32, 36, 44, 52, 55,
  	61, 22, 43, 51, 60, 42, 59, 58,
  }
  
  const deBruijn32 = 0x04653adf
  
  var deBruijnIdx32 = [32]byte{
  	0, 1, 2, 6, 3, 11, 7, 16,
  	4, 14, 12, 21, 8, 23, 17, 26,
  	31, 5, 10, 15, 13, 20, 22, 25,
  	30, 9, 19, 24, 29, 18, 28, 27,
  }
  
  // Ctz64 counts trailing (low-order) zeroes,
  // and if all are zero, then 64.
  func Ctz64(x uint64) int {
  	x &= -x                      // isolate low-order bit
  	y := x * deBruijn64 >> 58    // extract part of deBruijn sequence
  	i := int(deBruijnIdx64[y])   // convert to bit index
  	z := int((x - 1) >> 57 & 64) // adjustment if zero
  	return i + z
  }
  
  // Ctz32 counts trailing (low-order) zeroes,
  // and if all are zero, then 32.
  func Ctz32(x uint32) int {
  	x &= -x                      // isolate low-order bit
  	y := x * deBruijn32 >> 27    // extract part of deBruijn sequence
  	i := int(deBruijnIdx32[y])   // convert to bit index
  	z := int((x - 1) >> 26 & 32) // adjustment if zero
  	return i + z
  }
  
  // Bswap64 returns its input with byte order reversed
  // 0x0102030405060708 -> 0x0807060504030201
  func Bswap64(x uint64) uint64 {
  	c8 := uint64(0x00ff00ff00ff00ff)
  	a := x >> 8 & c8
  	b := (x & c8) << 8
  	x = a | b
  	c16 := uint64(0x0000ffff0000ffff)
  	a = x >> 16 & c16
  	b = (x & c16) << 16
  	x = a | b
  	c32 := uint64(0x00000000ffffffff)
  	a = x >> 32 & c32
  	b = (x & c32) << 32
  	x = a | b
  	return x
  }
  
  // Bswap32 returns its input with byte order reversed
  // 0x01020304 -> 0x04030201
  func Bswap32(x uint32) uint32 {
  	c8 := uint32(0x00ff00ff)
  	a := x >> 8 & c8
  	b := (x & c8) << 8
  	x = a | b
  	c16 := uint32(0x0000ffff)
  	a = x >> 16 & c16
  	b = (x & c16) << 16
  	x = a | b
  	return x
  }
  

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