FCMPD	F2, F3
	BLT	nearzero	// fabs(x) < NearZero, return 1 + x
	// argument reduction, x = k*ln2 + r,  |r| <= 0.5*ln2
	// computed as r = hi - lo for extra precision.
	FMOVD	$Log2e, F2
	FMOVD	$0.5, F3
	FNMSUBD	F0, F3, F2, F4	// Log2e*x - 0.5
	FMADDD	F0, F3, F2, F3	// Log2e*x + 0.5
	FCMPD	$0.0, F0
	FCSELD	LT, F4, F3, F3	// F3 = k
	FCVTZSD	F3, R1		// R1 = int(k)
	SCVTFD	R1, F3		// F3 = float64(int(k))
	FMOVD	$Ln2Hi, F4	// F4 = Ln2Hi

		return 0
	case -NearZero < x && x < NearZero:
		return 1 + x
	}

	// reduce; computed as r = hi - lo for extra precision.
	var k int
	switch {
	case x < 0:
		k = int(Log2e*x - 0.5)
	case x > 0:
		k = int(Log2e*x + 0.5)
	}
	hi := x - float64(k)*Ln2Hi
	lo := float64(k) * Ln2Lo

	// compute
	return expmulti(hi, lo, k)
}

// Exp2 returns 2**x, the base-2 exponential of x.
//

// You can use the software without any obligation."
//
// This code is a simplified version of the original.

#define LN2 0.6931471805599453094172321214581766 // log_e(2)
#define LOG2E 1.4426950408889634073599246810018920 // 1/LN2
#define LN2U 0.69314718055966295651160180568695068359375 // upper half LN2
#define LN2L 0.28235290563031577122588448175013436025525412068e-12 // lower half LN2

	SqrtPhi = 1.27201964951406896425242246173749149171560804184009624861664038 // https://oeis.org/A139339

	Ln2    = 0.693147180559945309417232121458176568075500134360255254120680009 // https://oeis.org/A002162
	Log2E  = 1 / Ln2
	Ln10   = 2.30258509299404568401799145468436420760110148862877297603332790 // https://oeis.org/A002392
	Log10E = 1 / Ln10
)

// Floating-point limit values.

	for i := 0; i < len(vf); i++ {
		a := Abs(vf[i])
		if f := Log2(a); !veryclose(log2[i], f) {
			t.Errorf("Log2(%g) = %g, want %g", a, f, log2[i])
		}
	}
	if f := Log2(E); f != Log2E {
		t.Errorf("Log2(%g) = %g, want %g", E, f, Log2E)
	}
	for i := 0; i < len(vflogSC); i++ {
		if f := Log2(vflogSC[i]); !alike(logSC[i], f) {
			t.Errorf("Log2(%g) = %g, want %g", vflogSC[i], f, logSC[i])
		}
	}

		{"Lgamma", Func, 0},
		{"Ln10", Const, 0},
		{"Ln2", Const, 0},
		{"Log", Func, 0},
		{"Log10", Func, 0},
		{"Log10E", Const, 0},
		{"Log1p", Func, 0},
		{"Log2", Func, 0},
		{"Log2E", Const, 0},
		{"Logb", Func, 0},
		{"Max", Func, 0},
		{"MaxFloat32", Const, 0},
		{"MaxFloat64", Const, 0},
		{"MaxInt", Const, 17},
		{"MaxInt16", Const, 0},
		{"MaxInt32", Const, 0},

pkg log/syslog (linux-arm), type Writer struct
pkg math, const E ideal-float
pkg math, const Ln10 ideal-float
pkg math, const Ln2 ideal-float
pkg math, const Log10E ideal-float
pkg math, const Log2E ideal-float
pkg math, const MaxFloat32 ideal-float
pkg math, const MaxFloat64 ideal-float
pkg math, const MaxInt16 ideal-int
pkg math, const MaxInt32 ideal-int
pkg math, const MaxInt64 ideal-int

pkg math, const Log10E = 0.434294  // 10000000000000000000000000000000000000000000000000000000000000/23025850929940456840179914546843642076011014886287729760333279
pkg math, const Log2E = 1.4427  // 1000000000000000000000000000000000000000000000000000000000000000/693147180559945309417232121458176568075500134360255254120680009
pkg math, const MaxFloat32 = 3.40282e+38  // 340282346638528859811704183484516925440