Implicit assumptions (was: Re: Some fun with -O2)

[Walter von Entferndt]

At Freitag, 15. Januar 2021, 21:57:14 CET, Mark Millard wrote:
> The rationale vintage that I have a copy of is available at:
> 
> http://www.open-std.org/jtc1/sc22/wg14/www/C99RationaleV5.10.pdf
> 
Thank you very much.  Now I found that "The result of shifting by a bit count 
greater than or equal to the word's size is undefined behavior in C and C++" 
(https://en.wikipedia.org/wiki/Bitwise_operation#C-family ; likewise http://
www.open-std.org/jtc1/sc22/wg14/www/docs/n1570.pdf).  So we'll have to go back 
to the loop-solution with the multiply-by-2:
--- check_mktime.c.patch ---
--- check_mktime.c.orig 2021-01-15 03:19:33.962253000 +0100
+++ check_mktime.c      2021-01-15 23:22:01.951385000 +0100
@@ -3,6 +3,10 @@
 # include <sys/time.h>
 # include <time.h>
 # include <unistd.h>
+# include <stdlib.h>
+# include <stdio.h>    /* printf() */
+# include <inttypes.h> /* format spec PRIX64: ll/l + X on 32/64-bit arch */
+# include <limits.h>   /* CHAR_BIT */

 /* Work around redefinition to rpl_putenv by other config tests.  */
 #undef putenv
@@ -106,9 +110,15 @@
   time_t t, delta;
   int i, j;

-  for (time_t_max = 1; 0 < time_t_max; time_t_max *= 2)
-    continue;
-  time_t_max--;
+  /* portably compute the maximum of a signed type:
+   * NOTE: << is undefined if the shift width >= word length
+   * i.e. shifting a 64-bit type by 62 on a 32-bit machine: undef
+   */
+  for (i = t = 1; i < CHAR_BIT*sizeof t - 1; i++)
+    t *= 2;
+  time_t_max = t|(t - 1);
+  printf ("time_t_max = 0x%"PRIX64"\n", time_t_max);
+
   delta = time_t_max / 997; /* a suitable prime number */
   for (i = 0; i < N_STRINGS; i++)
     {
-- 
=|o)	"Stell' Dir vor es geht und keiner kriegt's hin." (Wolfgang Neuss)
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