Re: lib/50009: strptime small enhancement

[Brian Ginsbach <ginsbach%netbsd.org@localhost>]

The following reply was made to PR lib/50009; it has been noted by GNATS.

From: Brian Ginsbach <ginsbach%netbsd.org@localhost>
To: matthew green <mrg%eterna.com.au@localhost>
Cc: gnats-bugs%NetBSD.org@localhost, lib-bug-people%netbsd.org@localhost,
	gnats-admin%netbsd.org@localhost, netbsd-bugs%netbsd.org@localhost, devnexen%gmail.com@localhost
Subject: Re: lib/50009: strptime small enhancement
Date: Thu, 9 Jul 2015 12:45:07 +0000

 On Mon, Jun 29, 2015 at 02:42:18PM +1000, matthew green wrote:
 > 
 > >  On Sun, Jun 28, 2015 at 07:50:00PM +0000, devnexen%gmail.com@localhost wrote:
 > >   > +#ifndef ISLEAPYEAR
 > >   > +#define ISLEAPYEAR(y) ((y % 400) == 0 && (y % 4) == 0 && (y % 100) != 0)
 > >   > +#endif
 > >  
 > >  ...this is wrong...
 > 
 > indeed, sys/clock.h has this:
 > 
 > 63 /*
 > 64  * This inline avoids some unnecessary modulo operations
 > 65  * as compared with the usual macro:
 > 66  *   ( ((year % 4) == 0 &&
 > 67  *      (year % 100) != 0) ||
 > 68  *     ((year % 400) == 0) )
 > 69  * It is otherwise equivalent.
 > 70  */
 > 71 static inline int
 > 72 is_leap_year(uint64_t year)
 > 73 {
 > 74         if ((year & 3) != 0)
 > 75                 return 0;
 > 76
 > 77         if (__predict_false((year % 100) != 0))
 > 78                 return 1;
 > 79
 > 80         return __predict_false((year % 400) == 0);
 > 81 }
 > 
 > which should probably be used instead, for netbsd, or at
 > least the same expression in the comment.
 > 
 > (the macro above also doesn't work properly for various
 > inputs, like a good macro should.)
 > 
 > 
 
 Is there some specific reason why the sys/clock.h version should be preferred over
 the macros in libc/time/tzfile.h which are used throughout the rest of the the
 libc/time code?
 
 154 #define isleap(y) (((y) % 4) == 0 && (((y) % 100) != 0 || ((y) % 400) == 0))
 155 
 156 /*
 157 ** Since everything in isleap is modulo 400 (or a factor of 400), we know that
 158 **      isleap(y) == isleap(y % 400)
 159 ** and so
 160 **      isleap(a + b) == isleap((a + b) % 400)
 161 ** or
 162 **      isleap(a + b) == isleap(a % 400 + b % 400)
 163 ** This is true even if % means modulo rather than Fortran remainder
 164 ** (which is allowed by C89 but not C99).
 165 ** We use this to avoid addition overflow problems.
 166 */
 167 
 168 #define isleap_sum(a, b)        isleap((a) % 400 + (b) % 400)
 
 Wouldn't it be more consistent to keep using the same macro(s) throughout all the
 libc/time code?
 
 Brian