...
Run Format

Source file src/runtime/fastlog2.go

  // Copyright 2015 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.
  
  package runtime
  
  import "unsafe"
  
  // fastlog2 implements a fast approximation to the base 2 log of a
  // float64. This is used to compute a geometric distribution for heap
  // sampling, without introducing dependencies into package math. This
  // uses a very rough approximation using the float64 exponent and the
  // first 25 bits of the mantissa. The top 5 bits of the mantissa are
  // used to load limits from a table of constants and the rest are used
  // to scale linearly between them.
  func fastlog2(x float64) float64 {
  	const fastlogScaleBits = 20
  	const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)
  
  	xBits := float64bits(x)
  	// Extract the exponent from the IEEE float64, and index a constant
  	// table with the first 10 bits from the mantissa.
  	xExp := int64((xBits>>52)&0x7FF) - 1023
  	xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
  	xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)
  
  	low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
  	return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
  }
  
  // float64bits returns the IEEE 754 binary representation of f.
  // Taken from math.Float64bits to avoid dependencies into package math.
  func float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(&f)) }
  

View as plain text