fastlog2.go

Documentation: runtime

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