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-rw-r--r--src/runtime/fastlog2.go27
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diff --git a/src/runtime/fastlog2.go b/src/runtime/fastlog2.go
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+// 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
+
+// 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
+}