Date: Thu, 17 Oct 2002 10:01:10 -0400
Message-Id: <200210171401.g9HE1AR24630@envy.delorie.com>
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From: DJ Delorie <dj AT delorie DOT com>
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In-reply-to: <2.7.9.2472D.H44KS3@pauzner.dnttm.ru> (uue@pauzner.dnttm.ru)
Subject: Re: libc' ctime.c optimizations
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If you assume gettimeofday() will be called multiple times per hour,
it's faster to subtract off the minutes and seconds when storing
prev_sec, so that you don't have to keep doing it over and over.
Thus, you cache the tv_sec for the beginning of the hour, not for the
last time used.

> +  /* cache expensive mktime() calls, until next hour */
> +  if (last_env_changed == __environ_changed &&
> +      prev.tm_hour == tmblk.tm_hour &&
> +      prev.tm_mday == tmblk.tm_mday &&
> +      prev.tm_mon  == tmblk.tm_mon &&
> +      prev.tm_year == tmblk.tm_year)
> +  {
> +    tv->tv_sec = prev_sec + (tmblk.tm_sec - prev.tm_sec)
> +                          + (tmblk.tm_min - tmblk.tm_min)*60;
> +  }
> +  else
> +  {
> +    tv->tv_sec = mktime(&tmblk);
> +
> +    prev_sec = tv->tv_sec;
> +    prev = tmblk;
> +    last_env_changed = __environ_changed;
> +  }