Re: bug#54764: encode-time: make DST and TIMEZONE fields of the list argument optional ones

[Bernhard Voelker <mail@bernhard-voelker.de>]

On 4/20/22 20:19, Paul Eggert wrote:
> diff --git a/lib/gettime-res.c b/lib/gettime-res.c
> index 611f83ad27..bb4d0b191d 100644
> --- a/lib/gettime-res.c
> +++ b/lib/gettime-res.c
> @@ -53,6 +53,8 @@ gettime_res (void)
>
>    long int hz = TIMESPEC_HZ;
>    long int r = hz * res.tv_sec + res.tv_nsec;
> +  struct timespec earlier;
> +  earlier.tv_nsec = -1;
>
>    /* On some platforms, clock_getres (CLOCK_REALTIME, ...) yields a
>       too-large resolution, under the mistaken theory that it should
> @@ -61,9 +63,22 @@ gettime_res (void)
>       resolution.  Work around the problem with high probability by
>       trying clock_gettime several times and observing the resulting
>       bounds on resolution.  */
> -  for (int i = 0; 1 < r && i < 32; i++)
> +  int nsamples = 32;
> +  for (int i = 0; 1 < r && i < nsamples; i++)
>      {
> -      struct timespec now = current_timespec ();
> +      /* If successive timestamps disagree the clock resolution must
> +         be small, so exit the inner loop to check this sample.
> +         Otherwise, arrange for the outer loop to exit but continue
> +         the inner-loop search for a differing timestamp sample.  */
> +      struct timespec now;
> +      for (;; i = nsamples)
> +        {
> +          now = current_timespec ();
> +          if (earlier.tv_nsec != now.tv_nsec || earlier.tv_sec != now.tv_sec)
> +            break;
> +        }
> +      earlier = now;
> +
>        r = gcd (r, now.tv_nsec ? now.tv_nsec : hz);
>      }

lib/gettime-res.c: In function 'gettime_res':
lib/gettime-res.c:77:46: error: 'earlier.tv_sec' may be used uninitialized in this function \
[-Werror=maybe-uninitialized]
   77 |           if (earlier.tv_nsec != now.tv_nsec || earlier.tv_sec != now.tv_sec)
      |               ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~


We know that earlier.tv_sec is set when tv_nsec is set, but the compiler does not,
obviously.  Considering the nested loops, I'd say initializing tv_sec doesn't
harm performance-wise.

Have a nice day,
Berny