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diff --git a/libgo/go/math/frexp.go b/libgo/go/math/frexp.go
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+++ b/libgo/go/math/frexp.go
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+// Copyright 2009 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 math
+
+// Frexp breaks f into a normalized fraction
+// and an integral power of two.
+// It returns frac and exp satisfying f == frac × 2**exp,
+// with the absolute value of frac in the interval [½, 1).
+//
+// Special cases are:
+//	Frexp(±0) = ±0, 0
+//	Frexp(±Inf) = ±Inf, 0
+//	Frexp(NaN) = NaN, 0
+func Frexp(f float64) (frac float64, exp int) {
+	// TODO(rsc): Remove manual inlining of IsNaN, IsInf
+	// when compiler does it for us
+	// special cases
+	switch {
+	case f == 0:
+		return f, 0 // correctly return -0
+	case f < -MaxFloat64 || f > MaxFloat64 || f != f: // IsInf(f, 0) || IsNaN(f):
+		return f, 0
+	}
+	f, exp = normalize(f)
+	x := Float64bits(f)
+	exp += int((x>>shift)&mask) - bias + 1
+	x &^= mask << shift
+	x |= (-1 + bias) << shift
+	frac = Float64frombits(x)
+	return
+}