1 files changed, 121 insertions, 0 deletions
diff --git a/libc-top-half/musl/src/math/exp2.c b/libc-top-half/musl/src/math/exp2.c
new file mode 100644
index 0000000..e0ff54b
--- /dev/null
+++ b/libc-top-half/musl/src/math/exp2.c
@@ -0,0 +1,121 @@
+/*
+ * Double-precision 2^x function.
+ *
+ * Copyright (c) 2018, Arm Limited.
+ * SPDX-License-Identifier: MIT
+ */
+
+#include <math.h>
+#include <stdint.h>
+#include "libm.h"
+#include "exp_data.h"
+
+#define N (1 << EXP_TABLE_BITS)
+#define Shift __exp_data.exp2_shift
+#define T __exp_data.tab
+#define C1 __exp_data.exp2_poly[0]
+#define C2 __exp_data.exp2_poly[1]
+#define C3 __exp_data.exp2_poly[2]
+#define C4 __exp_data.exp2_poly[3]
+#define C5 __exp_data.exp2_poly[4]
+
+/* Handle cases that may overflow or underflow when computing the result that
+   is scale*(1+TMP) without intermediate rounding.  The bit representation of
+   scale is in SBITS, however it has a computed exponent that may have
+   overflown into the sign bit so that needs to be adjusted before using it as
+   a double.  (int32_t)KI is the k used in the argument reduction and exponent
+   adjustment of scale, positive k here means the result may overflow and
+   negative k means the result may underflow.  */
+static inline double specialcase(double_t tmp, uint64_t sbits, uint64_t ki)
+{
+	double_t scale, y;
+
+	if ((ki & 0x80000000) == 0) {
+		/* k > 0, the exponent of scale might have overflowed by 1.  */
+		sbits -= 1ull << 52;
+		scale = asdouble(sbits);
+		y = 2 * (scale + scale * tmp);
+		return eval_as_double(y);
+	}
+	/* k < 0, need special care in the subnormal range.  */
+	sbits += 1022ull << 52;
+	scale = asdouble(sbits);
+	y = scale + scale * tmp;
+	if (y < 1.0) {
+		/* Round y to the right precision before scaling it into the subnormal
+		   range to avoid double rounding that can cause 0.5+E/2 ulp error where
+		   E is the worst-case ulp error outside the subnormal range.  So this
+		   is only useful if the goal is better than 1 ulp worst-case error.  */
+		double_t hi, lo;
+		lo = scale - y + scale * tmp;
+		hi = 1.0 + y;
+		lo = 1.0 - hi + y + lo;
+		y = eval_as_double(hi + lo) - 1.0;
+		/* Avoid -0.0 with downward rounding.  */
+		if (WANT_ROUNDING && y == 0.0)
+			y = 0.0;
+		/* The underflow exception needs to be signaled explicitly.  */
+		fp_force_eval(fp_barrier(0x1p-1022) * 0x1p-1022);
+	}
+	y = 0x1p-1022 * y;
+	return eval_as_double(y);
+}
+
+/* Top 12 bits of a double (sign and exponent bits).  */
+static inline uint32_t top12(double x)
+{
+	return asuint64(x) >> 52;
+}
+
+double exp2(double x)
+{
+	uint32_t abstop;
+	uint64_t ki, idx, top, sbits;
+	double_t kd, r, r2, scale, tail, tmp;
+
+	abstop = top12(x) & 0x7ff;
+	if (predict_false(abstop - top12(0x1p-54) >= top12(512.0) - top12(0x1p-54))) {
+		if (abstop - top12(0x1p-54) >= 0x80000000)
+			/* Avoid spurious underflow for tiny x.  */
+			/* Note: 0 is common input.  */
+			return WANT_ROUNDING ? 1.0 + x : 1.0;
+		if (abstop >= top12(1024.0)) {
+			if (asuint64(x) == asuint64(-INFINITY))
+				return 0.0;
+			if (abstop >= top12(INFINITY))
+				return 1.0 + x;
+			if (!(asuint64(x) >> 63))
+				return __math_oflow(0);
+			else if (asuint64(x) >= asuint64(-1075.0))
+				return __math_uflow(0);
+		}
+		if (2 * asuint64(x) > 2 * asuint64(928.0))
+			/* Large x is special cased below.  */
+			abstop = 0;
+	}
+
+	/* exp2(x) = 2^(k/N) * 2^r, with 2^r in [2^(-1/2N),2^(1/2N)].  */
+	/* x = k/N + r, with int k and r in [-1/2N, 1/2N].  */
+	kd = eval_as_double(x + Shift);
+	ki = asuint64(kd); /* k.  */
+	kd -= Shift; /* k/N for int k.  */
+	r = x - kd;
+	/* 2^(k/N) ~= scale * (1 + tail).  */
+	idx = 2 * (ki % N);
+	top = ki << (52 - EXP_TABLE_BITS);
+	tail = asdouble(T[idx]);
+	/* This is only a valid scale when -1023*N < k < 1024*N.  */
+	sbits = T[idx + 1] + top;
+	/* exp2(x) = 2^(k/N) * 2^r ~= scale + scale * (tail + 2^r - 1).  */
+	/* Evaluation is optimized assuming superscalar pipelined execution.  */
+	r2 = r * r;
+	/* Without fma the worst case error is 0.5/N ulp larger.  */
+	/* Worst case error is less than 0.5+0.86/N+(abs poly error * 2^53) ulp.  */
+	tmp = tail + r * C1 + r2 * (C2 + r * C3) + r2 * r2 * (C4 + r * C5);
+	if (predict_false(abstop == 0))
+		return specialcase(tmp, sbits, ki);
+	scale = asdouble(sbits);
+	/* Note: tmp == 0 or |tmp| > 2^-65 and scale > 2^-928, so there
+	   is no spurious underflow here even without fma.  */
+	return eval_as_double(scale + scale * tmp);
+}