Index: convert.c
===================================================================
--- convert.c	(revision 121335)
+++ convert.c	(working copy)
@@ -451,6 +451,37 @@
 	}
     }
 
+  /* Convert (int)logb(d) -> ilogb(d).  */
+  if (optimize
+      && flag_unsafe_math_optimizations
+      && integer_type_node
+      && (outprec > TYPE_PRECISION (integer_type_node)
+	  || (outprec == TYPE_PRECISION (integer_type_node)
+	      && !TYPE_UNSIGNED (type))))
+    {
+      tree s_expr = strip_float_extensions (expr);
+      tree s_intype = TREE_TYPE (s_expr);
+      const enum built_in_function fcode = builtin_mathfn_code (s_expr);
+      tree fn = 0;
+       
+      switch (fcode)
+	{
+	CASE_FLT_FN (BUILT_IN_LOGB):
+	  fn = mathfn_built_in (s_intype, BUILT_IN_ILOGB);
+	  break;
+
+	default:
+	  break;
+	}
+
+      if (fn)
+        {
+	  tree arglist = TREE_OPERAND (s_expr, 1);
+	  tree newexpr = build_function_call_expr (fn, arglist);
+	  return convert_to_integer (type, newexpr);
+	}
+    }
+
   switch (TREE_CODE (intype))
     {
     case POINTER_TYPE:
Index: testsuite/gcc.dg/builtins-63.c
===================================================================
--- testsuite/gcc.dg/builtins-63.c	(revision 0)
+++ testsuite/gcc.dg/builtins-63.c	(revision 0)
@@ -0,0 +1,39 @@
+/* { dg-do link } */
+/* { dg-options "-O2 -ffast-math" } */
+
+extern int ilogbf (float);
+extern float logbf (float);
+extern int ilogb (double);
+extern double logb (double);
+extern int ilogbl (long double);
+extern long double logbl (long double);
+
+extern void link_error(void);
+
+void testf(float x)
+{
+  if ((int) logbf (x) != ilogbf (x))
+    link_error ();
+}
+
+void test(double x)
+{
+  if ((int) logb (x) != ilogb (x))
+    link_error ();
+}
+
+void testl(long double x)
+{
+  if ((int) logbl (x) != ilogbl (x))
+    link_error ();
+}
+
+int main()
+{
+  testf (2.0f);
+  test (2.0);
+  testl (2.0l);
+
+  return 0;
+}
+