Re: GNULIB mktime.c compilation failure on UNICOS 9.0 Cray Y-MP EL

[Paul Eggert]

Thanks for reporting that.  Does the following patch fix things on
your friend's Cray?

2004-11-10  Paul Eggert  <eggert@cs.ucla.edu>

        * mktime.c (SHR): New macro, which is a portable substitute for >>
        that should work even on Crays.
        Problem reported by Mark D. Baushke.
        (TIME_T_MIDPOINT, ydhms_diff, __mktime_internal): Use it.

--- mktime.c.~1.43.~    2004-10-29 15:55:02 -0700
+++ mktime.c    2004-11-10 00:16:56 -0800
@@ -45,6 +45,18 @@
 # define mktime my_mktime
 #endif /* DEBUG */
 
+/* Divide A by 2**B, truncating towards minus infinity.  This is a
+   substitute for A >> B that works portably even if A is negative.  A
+   and B should be free of side effects, and 0 <= B <= 30.  ISO C99
+   says that A >> B has implementation-defined behavior if A is
+   negative, and some implementations (e.g., UNICOS 9.0 on a Cray Y-MP
+   EL) don't shift right in the usual way, so SHR falls back on
+   division in cases like these.  */
+#define SHR(a, b)      \
+  (-1 >> 1 == -1       \
+   ? (a) >> (b)                \
+   : (a) / (1 << (b)) - ((a) % (1 << (b)) < 0))
+
 /* The extra casts work around common compiler bugs.  */
 #define TYPE_SIGNED(t) (! ((t) 0 < (t) -1))
 /* The outer cast is needed to work around a bug in Cray C 5.0.3.0.
@@ -59,14 +71,13 @@
 #ifndef TIME_T_MAX
 # define TIME_T_MAX TYPE_MAXIMUM (time_t)
 #endif
-#define TIME_T_MIDPOINT (((TIME_T_MIN + TIME_T_MAX) >> 1) + 1)
+#define TIME_T_MIDPOINT (SHR (TIME_T_MIN + TIME_T_MAX, 1) + 1)
 
 /* Verify a requirement at compile-time (unlike assert, which is runtime).  */
 #define verify(name, assertion) struct name { char a[(assertion) ? 1 : -1]; }
 
 verify (time_t_is_integer, (time_t) 0.5 == 0);
 verify (twos_complement_arithmetic, -1 == ~1 + 1);
-verify (right_shift_propagates_sign, -1 >> 1 == -1);
 /* The code also assumes that signed integer overflow silently wraps
    around, but this assumption can't be stated without causing a
    diagnostic on some hosts.  */
@@ -132,12 +143,12 @@ ydhms_diff (long int year1, long int yda
 
   /* Compute intervening leap days correctly even if year is negative.
      Take care to avoid integer overflow here.  */
-  int a4 = (year1 >> 2) + (TM_YEAR_BASE >> 2) - ! (year1 & 3);
-  int b4 = (year0 >> 2) + (TM_YEAR_BASE >> 2) - ! (year0 & 3);
+  int a4 = SHR (year1, 2) + SHR (TM_YEAR_BASE, 2) - ! (year1 & 3);
+  int b4 = SHR (year0, 2) + SHR (TM_YEAR_BASE, 2) - ! (year0 & 3);
   int a100 = a4 / 25 - (a4 % 25 < 0);
   int b100 = b4 / 25 - (b4 % 25 < 0);
-  int a400 = a100 >> 2;
-  int b400 = b100 >> 2;
+  int a400 = SHR (a100, 2);
+  int b400 = SHR (b100, 2);
   int intervening_leap_days = (a4 - b4) - (a100 - b100) + (a400 - b400);
 
   /* Compute the desired time in time_t precision.  Overflow might
@@ -321,14 +332,16 @@ __mktime_internal (struct tm *tp,
       int LOG2_YEARS_PER_BIENNIUM = 1;
 
       int approx_requested_biennia =
-       ((year_requested >> LOG2_YEARS_PER_BIENNIUM)
-        - ((EPOCH_YEAR - TM_YEAR_BASE) >> LOG2_YEARS_PER_BIENNIUM)
-        + (mday >> ALOG2_DAYS_PER_BIENNIUM)
-        + (hour >> ALOG2_HOURS_PER_BIENNIUM)
-        + (min >> ALOG2_MINUTES_PER_BIENNIUM)
-        + (LEAP_SECONDS_POSSIBLE ? 0 : sec >> ALOG2_SECONDS_PER_BIENNIUM));
+       (SHR (year_requested, LOG2_YEARS_PER_BIENNIUM)
+        - SHR (EPOCH_YEAR - TM_YEAR_BASE, LOG2_YEARS_PER_BIENNIUM)
+        + SHR (mday, ALOG2_DAYS_PER_BIENNIUM)
+        + SHR (hour, ALOG2_HOURS_PER_BIENNIUM)
+        + SHR (min, ALOG2_MINUTES_PER_BIENNIUM)
+        + (LEAP_SECONDS_POSSIBLE
+           ? 0
+           : SHR (sec, ALOG2_SECONDS_PER_BIENNIUM)));
 
-      int approx_biennia = t0 >> ALOG2_SECONDS_PER_BIENNIUM;
+      int approx_biennia = SHR (t0, ALOG2_SECONDS_PER_BIENNIUM);
       int diff = approx_biennia - approx_requested_biennia;
       int abs_diff = diff < 0 ? - diff : diff;
 
@@ -346,7 +359,7 @@ __mktime_internal (struct tm *tp,
          /* Overflow occurred.  Try repairing it; this might work if
             the time zone offset is enough to undo the overflow.  */
          time_t repaired_t0 = -1 - t0;
-         approx_biennia = repaired_t0 >> ALOG2_SECONDS_PER_BIENNIUM;
+         approx_biennia = SHR (repaired_t0, ALOG2_SECONDS_PER_BIENNIUM);
          diff = approx_biennia - approx_requested_biennia;
          abs_diff = diff < 0 ? - diff : diff;
          if (overflow_threshold < abs_diff)