1 files changed, 21 insertions, 0 deletions
diff --git a/src/math/ilogb.c b/src/math/ilogb.c
new file mode 100644
index 00000000..c5915a0c
--- /dev/null
+++ b/src/math/ilogb.c
@@ -0,0 +1,21 @@
+#include <limits.h>
+#include "libm.h"
+
+int ilogb(double x)
+{
+	union dshape u = {x};
+	int e = u.bits>>52 & 0x7ff;
+
+	if (!e) {
+		u.bits <<= 12;
+		if (u.bits == 0)
+			return FP_ILOGB0;
+		/* subnormal x */
+		// FIXME: scale up subnormals with a *0x1p53 or find top set bit with a better method
+		for (e = -0x3ff; u.bits < (uint64_t)1<<63; e--, u.bits<<=1);
+		return e;
+	}
+	if (e == 0x7ff)
+		return u.bits<<12 ? FP_ILOGBNAN : INT_MAX;
+	return e - 0x3ff;
+}