1 files changed, 129 insertions, 0 deletions
diff --git a/math/exp10.c b/math/exp10.c
new file mode 100644
index 000000000000..0fbec4c694ca
--- /dev/null
+++ b/math/exp10.c
@@ -0,0 +1,129 @@
+/*
+ * Double-precision 10^x function.
+ *
+ * Copyright (c) 2023, Arm Limited.
+ * SPDX-License-Identifier: MIT OR Apache-2.0 WITH LLVM-exception
+ */
+
+#include "math_config.h"
+
+#define N (1 << EXP_TABLE_BITS)
+#define IndexMask (N - 1)
+#define OFlowBound 0x1.34413509f79ffp8 /* log10(DBL_MAX).  */
+#define UFlowBound -0x1.5ep+8 /* -350.  */
+#define SmallTop 0x3c6 /* top12(0x1p-57).  */
+#define BigTop 0x407   /* top12(0x1p8).  */
+#define Thresh 0x41    /* BigTop - SmallTop.  */
+#define Shift __exp_data.shift
+#define C(i) __exp_data.exp10_poly[i]
+
+static double
+special_case (uint64_t sbits, double_t tmp, uint64_t ki)
+{
+  double_t scale, y;
+
+  if (ki - (1ull << 16) < 0x80000000)
+    {
+      /* The exponent of scale might have overflowed by 1.  */
+      sbits -= 1ull << 52;
+      scale = asdouble (sbits);
+      y = 2 * (scale + scale * tmp);
+      return check_oflow (eval_as_double (y));
+    }
+
+  /* n < 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 lo = scale - y + scale * tmp;
+      double_t 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.  */
+      force_eval_double (opt_barrier_double (0x1p-1022) * 0x1p-1022);
+    }
+  y = 0x1p-1022 * y;
+
+  return check_uflow (y);
+}
+
+/* Double-precision 10^x approximation. Largest observed error is ~0.513 ULP.  */
+double
+exp10 (double x)
+{
+  uint64_t ix = asuint64 (x);
+  uint32_t abstop = (ix >> 52) & 0x7ff;
+
+  if (unlikely (abstop - SmallTop >= Thresh))
+    {
+      if (abstop - SmallTop >= 0x80000000)
+	/* Avoid spurious underflow for tiny x.
+	   Note: 0 is common input.  */
+	return x + 1;
+      if (abstop == 0x7ff)
+	return ix == asuint64 (-INFINITY) ? 0.0 : x + 1.0;
+      if (x >= OFlowBound)
+	return __math_oflow (0);
+      if (x < UFlowBound)
+	return __math_uflow (0);
+
+      /* Large x is special-cased below.  */
+      abstop = 0;
+    }
+
+  /* Reduce x: z = x * N / log10(2), k = round(z).  */
+  double_t z = __exp_data.invlog10_2N * x;
+  double_t kd;
+  int64_t ki;
+#if TOINT_INTRINSICS
+  kd = roundtoint (z);
+  ki = converttoint (z);
+#else
+  kd = eval_as_double (z + Shift);
+  kd -= Shift;
+  ki = kd;
+#endif
+
+  /* r = x - k * log10(2), r in [-0.5, 0.5].  */
+  double_t r = x;
+  r = __exp_data.neglog10_2hiN * kd + r;
+  r = __exp_data.neglog10_2loN * kd + r;
+
+  /* exp10(x) = 2^(k/N) * 2^(r/N).
+     Approximate the two components separately.  */
+
+  /* s = 2^(k/N), using lookup table.  */
+  uint64_t e = ki << (52 - EXP_TABLE_BITS);
+  uint64_t i = (ki & IndexMask) * 2;
+  uint64_t u = __exp_data.tab[i + 1];
+  uint64_t sbits = u + e;
+
+  double_t tail = asdouble (__exp_data.tab[i]);
+
+  /* 2^(r/N) ~= 1 + r * Poly(r).  */
+  double_t r2 = r * r;
+  double_t p = C (0) + r * C (1);
+  double_t y = C (2) + r * C (3);
+  y = y + r2 * C (4);
+  y = p + r2 * y;
+  y = tail + y * r;
+
+  if (unlikely (abstop == 0))
+    return special_case (sbits, y, ki);
+
+  /* Assemble components:
+     y  = 2^(r/N) * 2^(k/N)
+       ~= (y + 1) * s.  */
+  double_t s = asdouble (sbits);
+  return eval_as_double (s * y + s);
+}