usr: libc: Add math.h + musl math impl portexpt

Signed-off-by: Ian Moffett <ian@osmora.org>

1 files changed, 16 insertions, 0 deletions

diff --git a/lib/libc/src/musl-math/__expo2f.c b/lib/libc/src/musl-math/__expo2f.c
new file mode 100644
index 0000000..5163e41
--- /dev/null
+++ b/lib/libc/src/musl-math/__expo2f.c
@@ -0,0 +1,16 @@
+#include "libm.h"
+
+/* k is such that k*ln2 has minimal relative error and x - kln2 > log(FLT_MIN) */
+static const int k = 235;
+static const float kln2 = 0x1.45c778p+7f;
+
+/* expf(x)/2 for x >= log(FLT_MAX), slightly better than 0.5f*expf(x/2)*expf(x/2) */
+float __expo2f(float x)
+{
+	float scale;
+
+	/* note that k is odd and scale*scale overflows */
+	SET_FLOAT_WORD(scale, (uint32_t)(0x7f + k/2) << 23);
+	/* exp(x - k ln2) * 2**(k-1) */
+	return expf(x - kln2) * scale * scale;
+}