Index: lib/Headers/tgmath.h
===================================================================
--- lib/Headers/tgmath.h	(revision 151167)
+++ lib/Headers/tgmath.h	(working copy)
@@ -976,6 +976,23 @@
 #undef log2
 #define log2(__x) __tg_log2(__tg_promote1((__x))(__x))
 
+// logb
+
+static float
+    _TG_ATTRS
+    __tg_logb(float __x) {return logbf(__x);}
+
+static double
+    _TG_ATTRS
+    __tg_logb(double __x) {return logb(__x);}
+
+static long double
+    _TG_ATTRS
+    __tg_logb(long double __x) {return logbl(__x);}
+
+#undef logb
+#define logb(__x) __tg_logb(__tg_promote1((__x))(__x))
+
 // lrint
 
 static long