https://gcc.gnu.org/ml/gcc-patches/2009-06/msg01249.html
--- gcc/gcc/toplev.c    (revision 148539)
+++ gcc/gcc/toplev.c    (revision 148540)
@@ -532,11 +532,11 @@ read_integral_parameter (const char *p,
   return atoi (p);
 }

-/* When compiling with a recent enough GCC, we use the GNU C "extern inline"
-   for floor_log2 and exact_log2; see toplev.h.  That construct, however,
-   conflicts with the ISO C++ One Definition Rule.   */
+#if GCC_VERSION < 3004

-#if GCC_VERSION < 3004 || !defined (__cplusplus)
+/* The functions floor_log2 and exact_log2 are defined as inline
+   functions in toplev.h if GCC_VERSION >= 3004.  The definitions here
+   are used for older versions of gcc.  */

 /* Given X, an unsigned number, return the largest int Y such that 2**Y <= X.
    If X is 0, return -1.  */
@@ -549,9 +549,6 @@ floor_log2 (unsigned HOST_WIDE_INT x)
   if (x == 0)
     return -1;

-#ifdef CLZ_HWI
-  t = HOST_BITS_PER_WIDE_INT - 1 - (int) CLZ_HWI (x);
-#else
   if (HOST_BITS_PER_WIDE_INT > 64)
     if (x >= (unsigned HOST_WIDE_INT) 1 << (t + 64))
       t += 64;
@@ -568,7 +565,6 @@ floor_log2 (unsigned HOST_WIDE_INT x)
     t += 2;
   if (x >= ((unsigned HOST_WIDE_INT) 1) << (t + 1))
     t += 1;
-#endif

   return t;
 }
@@ -581,14 +577,10 @@ exact_log2 (unsigned HOST_WIDE_INT x)
 {
   if (x != (x & -x))
     return -1;
-#ifdef CTZ_HWI
-  return x ? CTZ_HWI (x) : -1;
-#else
   return floor_log2 (x);
-#endif
 }

-#endif /*  GCC_VERSION < 3004 || !defined (__cplusplus)  */
+#endif /* GCC_VERSION < 3004 */

 /* Handler for fatal signals, such as SIGSEGV.  These are transformed
    into ICE messages, which is much more user friendly.  In case the
--- gcc/gcc/toplev.h    (revision 148539)
+++ gcc/gcc/toplev.h    (revision 148540)
@@ -169,14 +169,17 @@ extern void decode_d_option       (const char
 extern bool fast_math_flags_set_p  (void);
 extern bool fast_math_flags_struct_set_p (struct cl_optimization *);

+/* Inline versions of the above for speed.  */
+#if GCC_VERSION < 3004
+
 /* Return log2, or -1 if not exact.  */
 extern int exact_log2                  (unsigned HOST_WIDE_INT);

 /* Return floor of log2, with -1 for zero.  */
 extern int floor_log2                  (unsigned HOST_WIDE_INT);

-/* Inline versions of the above for speed.  */
-#if GCC_VERSION >= 3004
+#else /* GCC_VERSION >= 3004 */
+
 # if HOST_BITS_PER_WIDE_INT == HOST_BITS_PER_LONG
 #  define CLZ_HWI __builtin_clzl
 #  define CTZ_HWI __builtin_ctzl
@@ -188,17 +191,18 @@ extern int floor_log2                  (
 #  define CTZ_HWI __builtin_ctz
 # endif

-extern inline int
+static inline int
 floor_log2 (unsigned HOST_WIDE_INT x)
 {
   return x ? HOST_BITS_PER_WIDE_INT - 1 - (int) CLZ_HWI (x) : -1;
 }

-extern inline int
+static inline int
 exact_log2 (unsigned HOST_WIDE_INT x)
 {
   return x == (x & -x) && x ? (int) CTZ_HWI (x) : -1;
 }
+
 #endif /* GCC_VERSION >= 3004 */

 /* Functions used to get and set GCC's notion of in what directory