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From: Rich Felker <dalias@libc.org>
To: "Mattias Andrée" <maandree@kth.se>
Cc: musl@lists.openwall.com
Subject: Re: [musl] [PATCH] Remove unnecessary if in __secs_to_tm
Date: Sun, 28 Feb 2021 12:06:15 -0500	[thread overview]
Message-ID: <20210228170614.GF32655@brightrain.aerifal.cx> (raw)
In-Reply-To: <20210228150912.1532943-1-maandree@kth.se>

On Sun, Feb 28, 2021 at 04:09:12PM +0100, Mattias Andrée wrote:
> Since years divisible by 100 but not by 400 are not leap years,
> q_cycles can at most be 24 (DAYS_PER_100Y / DAYS_PER_4Y == 24).
> ---
>  src/time/__secs_to_tm.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/src/time/__secs_to_tm.c b/src/time/__secs_to_tm.c
> index 093d9021..2d0c0b2c 100644
> --- a/src/time/__secs_to_tm.c
> +++ b/src/time/__secs_to_tm.c
> @@ -44,8 +44,7 @@ int __secs_to_tm(long long t, struct tm *tm)
>  	remdays -= c_cycles * DAYS_PER_100Y;
>  
>  	q_cycles = remdays / DAYS_PER_4Y;
> -	if (q_cycles == 25) q_cycles--;
> -	remdays -= q_cycles * DAYS_PER_4Y;
> +	remdays %= DAYS_PER_4Y;
>  
>  	remyears = remdays / 365;
>  	if (remyears == 4) remyears--;

I think you're right about the condition being impossible -- it looks
like the error in thinking was that, while 400Y and 4Y are strictly
larger than 4*100Y and 4*1Y respectively, 100Y is smaller than 25*4Y.

However, changing the -= to %= is not desirable. The point of the -=
has nothing to do with the edge case that can't happen; it's to avoid
a modulo operation. Since the divisor is a constant though maybe the
compiler can generate the same code for both, anyway..?

Rich

  reply	other threads:[~2021-02-28 17:06 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2021-02-28 15:09 Mattias Andrée
2021-02-28 17:06 ` Rich Felker [this message]
2021-02-28 17:24   ` Mattias Andrée
2021-02-28 17:34     ` Rich Felker
2021-02-28 19:22 ` [musl] [PATCH v2 1/2] " Mattias Andrée
2021-02-28 19:22   ` [musl] [PATCH v2 2/2] Use modulo instead of mul+sub " Mattias Andrée
2021-02-28 19:37     ` Szabolcs Nagy
2021-02-28 19:52       ` Mattias Andrée
2021-02-28 19:58       ` Jon Chesterfield
2021-03-01 19:26         ` Markus Wichmann
2021-02-28 19:27 ` [musl] [PATCH v3 1/2] Remove unnecessary if " Mattias Andrée
2021-02-28 19:27   ` [musl] [PATCH v3 2/2] Use modulo instead of mul+sub " Mattias Andrée

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