github.com/q45/go@v0.0.0-20151101211701-a4fb8c13db3f/src/runtime/fastlog2.go (about)

     1  // Copyright 2015 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 runtime
     6  
     7  import "unsafe"
     8  
     9  // fastlog2 implements a fast approximation to the base 2 log of a
    10  // float64. This is used to compute a geometric distribution for heap
    11  // sampling, without introducing dependences into package math. This
    12  // uses a very rough approximation using the float64 exponent and the
    13  // first 25 bits of the mantissa. The top 5 bits of the mantissa are
    14  // used to load limits from a table of constants and the rest are used
    15  // to scale linearly between them.
    16  func fastlog2(x float64) float64 {
    17  	const fastlogScaleBits = 20
    18  	const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)
    19  
    20  	xBits := float64bits(x)
    21  	// Extract the exponent from the IEEE float64, and index a constant
    22  	// table with the first 10 bits from the mantissa.
    23  	xExp := int64((xBits>>52)&0x7FF) - 1023
    24  	xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
    25  	xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)
    26  
    27  	low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
    28  	return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
    29  }
    30  
    31  // float64bits returns the IEEE 754 binary representation of f.
    32  // Taken from math.Float64bits to avoid dependences into package math.
    33  func float64bits(f float64) uint64 { return *(*uint64)(unsafe.Pointer(&f)) }