math: new powf

from https://github.com/ARM-software/optimized-routines,
commit 04884bd04eac4b251da4026900010ea7d8850edc

POWF_SCALE != 1.0 case only matters if TOINT_INTRINSICS is set, which
is currently not supported for any target.

SNaN is not supported, it would require an issignalingf
implementation.

code size change: -816 bytes.
benchmark on x86_64 before, after, speedup:

-Os:
  powf rthruput:  95.14 ns/call 20.04 ns/call 4.75x
   powf latency: 137.00 ns/call 34.98 ns/call 3.92x
-O3:
  powf rthruput:  92.48 ns/call 13.67 ns/call 6.77x
   powf latency: 131.11 ns/call 35.15 ns/call 3.73x

1 files changed, 26 insertions, 0 deletions

diff --git a/src/math/powf_data.h b/src/math/powf_data.h
new file mode 100644
index 00000000..5b136e28
--- /dev/null
+++ b/src/math/powf_data.h
@@ -0,0 +1,26 @@
+/*
+ * Copyright (c) 2017-2018, Arm Limited.
+ * SPDX-License-Identifier: MIT
+ */
+#ifndef _POWF_DATA_H
+#define _POWF_DATA_H
+
+#include "libm.h"
+#include "exp2f_data.h"
+
+#define POWF_LOG2_TABLE_BITS 4
+#define POWF_LOG2_POLY_ORDER 5
+#if TOINT_INTRINSICS
+#define POWF_SCALE_BITS EXP2F_TABLE_BITS
+#else
+#define POWF_SCALE_BITS 0
+#endif
+#define POWF_SCALE ((double)(1 << POWF_SCALE_BITS))
+extern hidden const struct powf_log2_data {
+	struct {
+		double invc, logc;
+	} tab[1 << POWF_LOG2_TABLE_BITS];
+	double poly[POWF_LOG2_POLY_ORDER];
+} __powf_log2_data;
+
+#endif