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16200c7333
- changes tests to check that the real and imaginary part of the go complex division result is equal to the result gcc produces for c99 - changes complex division code to satisfy new complex division test - adds float functions isNan, isFinite, isInf, abs and copysign in the runtime package Fixes #14644. name old time/op new time/op delta Complex128DivNormal-4 21.8ns ± 6% 13.9ns ± 6% -36.37% (p=0.000 n=20+20) Complex128DivNisNaN-4 14.1ns ± 1% 15.0ns ± 1% +5.86% (p=0.000 n=20+19) Complex128DivDisNaN-4 12.5ns ± 1% 16.7ns ± 1% +33.79% (p=0.000 n=19+20) Complex128DivNisInf-4 10.1ns ± 1% 13.0ns ± 1% +28.25% (p=0.000 n=20+19) Complex128DivDisInf-4 11.0ns ± 1% 20.9ns ± 1% +90.69% (p=0.000 n=16+19) ComplexAlgMap-4 86.7ns ± 1% 86.8ns ± 2% ~ (p=0.804 n=20+20) Change-Id: I261f3b4a81f6cc858bc7ff48f6fd1b39c300abf0 Reviewed-on: https://go-review.googlesource.com/37441 Reviewed-by: Robert Griesemer <gri@golang.org>
28 lines
1.2 KiB
Go
28 lines
1.2 KiB
Go
// Copyright 2015 The Go Authors. All rights reserved.
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// Use of this source code is governed by a BSD-style
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// license that can be found in the LICENSE file.
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package runtime
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// fastlog2 implements a fast approximation to the base 2 log of a
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// float64. This is used to compute a geometric distribution for heap
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// sampling, without introducing dependencies into package math. This
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// uses a very rough approximation using the float64 exponent and the
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// first 25 bits of the mantissa. The top 5 bits of the mantissa are
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// used to load limits from a table of constants and the rest are used
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// to scale linearly between them.
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func fastlog2(x float64) float64 {
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const fastlogScaleBits = 20
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const fastlogScaleRatio = 1.0 / (1 << fastlogScaleBits)
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xBits := float64bits(x)
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// Extract the exponent from the IEEE float64, and index a constant
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// table with the first 10 bits from the mantissa.
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xExp := int64((xBits>>52)&0x7FF) - 1023
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xManIndex := (xBits >> (52 - fastlogNumBits)) % (1 << fastlogNumBits)
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xManScale := (xBits >> (52 - fastlogNumBits - fastlogScaleBits)) % (1 << fastlogScaleBits)
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low, high := fastlog2Table[xManIndex], fastlog2Table[xManIndex+1]
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return float64(xExp) + low + (high-low)*float64(xManScale)*fastlogScaleRatio
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}
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