File ac-func-mktime.diff of Package autoconf

2006-12-22  Paul Eggert  <eggert@cs.ucla.edu>

* lib/autoconf/functions.m4 (AC_FUNC_MKTIME):
	Include <limits.h>, and use its INT_MAX to rewrite the
	j loop so that it does not overflow 'int'.  Problem reported by
	Ralf Wildenhues in
	<http://lists.gnu.org/archive/html/bug-gnulib/2006-12/msg00084.html>.
	Play it safe by shifting left by 1 rather than multiplying by 2,
	as GCC is less likely to optimize this away when the value
	is signed (when it assumes overflow leads to undefined behavior).
	Also, don't assume time_t uses two's complement.

diff --git a/lib/autoconf/functions.m4 b/lib/autoconf/functions.m4
index 185376e..a34c9f0 100644
--- a/lib/autoconf/functions.m4
+++ b/lib/autoconf/functions.m4
@@ -984,6 +984,7 @@ AC_CACHE_CHECK([for working mktime], ac_cv_func_working_mktime,
 # endif
 #endif
 
+#include <limits.h>
 #include <stdlib.h>
 
 #ifdef HAVE_UNISTD_H
@@ -1132,12 +1133,15 @@ main ()
      isn't worth using anyway.  */
   alarm (60);
 
-  for (time_t_max = 1; 0 < time_t_max; time_t_max *= 2)
-    continue;
-  time_t_max--;
-  if ((time_t) -1 < 0)
-    for (time_t_min = -1; (time_t) (time_t_min * 2) < 0; time_t_min *= 2)
-      continue;
+  for (;;)
+    {
+      t = (time_t_max << 1) + 1;
+      if (t <= time_t_max)
+	break;
+      time_t_max = t;
+    }
+  time_t_min = - ((time_t) ~ (time_t) 0 == (time_t) -1) - time_t_max;
+
   delta = time_t_max / 997; /* a suitable prime number */
   for (i = 0; i < N_STRINGS; i++)
     {
@@ -1152,10 +1156,12 @@ main ()
 	     && mktime_test ((time_t) (60 * 60 * 24))))
 	return 1;
 
-      for (j = 1; 0 < j; j *= 2)
+      for (j = 1; ; j <<= 1)
 	if (! bigtime_test (j))
 	  return 1;
-      if (! bigtime_test (j - 1))
+	else if (INT_MAX / 2 < j)
+	  break;
+      if (! bigtime_test (INT_MAX))
 	return 1;
     }
   return ! (irix_6_4_bug () && spring_forward_gap () && year_2050_test ());